<%BANNER%>

Real-Time Particle Detection Using Sub-Threshold Laser Induced Breakdown Spectroscopy

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PAGE 1

1 REAL-TIME PARTICLE DETEC TION USING SU B-THRESHOLD LASER INDUCED BREAKDOWN DETECTION By WILLIAM PAUL MASON A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORI DA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2007

PAGE 2

2 Copyright 2007 by William Paul Mason

PAGE 3

3 For Constantine Panageotes: Fasooli, fasooli, ghiomeeze tor sakooli (A bean, a bean, it fills the bag.) For Jane Woodward Mason and family: Somewhere ages and ages hence, Two roads diverged in a wood and I, I chose the one less traveled by, And that has made all the difference. Robert Frost

PAGE 4

4 ACKNOWLEDGMENTS I wish to thank my colleagues in the lab and particularly Ben Smith, Nicolo Omenetto, and Ron Whiddon for many stimulating conversations, for the patience of oak, and for sharing insight into the language of nature. There are more things in heaven and earth, Hora tio, than are dreamt of in your philosophies. Shakespeare, Hamlet Act I. Scene V Special thanks go to Ryan Mohney and Dan Shelby for long hours in the lab and valuable discussions. Thanks to Sue, Sal, and Lily for strong prayers. And God saw the light, that it was good: and God divided the light from the darkness. Genesis 1:4

PAGE 5

5 TABLE OF CONTENTS page ACKNOWLEDGMENTS...............................................................................................................4 LIST OF TABLES................................................................................................................. ..........6 LIST OF FIGURES................................................................................................................ .........7 LIST OF ABBREVIATIONS..........................................................................................................9 ABSTRACT....................................................................................................................... ............11 CHAPTER 1 INTRODUCTION..................................................................................................................12 2 PARTICLE DETECTION......................................................................................................28 3 EXPERIMENTAL SETUP AND DEVELOPMENT............................................................38 4 RESULTS........................................................................................................................ .......52 5 CONCLUSION..................................................................................................................... ..58 APPENDIX LASER CHARACTERISTICS................................................................................59 LIST OF REFERENCES............................................................................................................. ..60 BIOGRAPHICAL SKETCH.........................................................................................................64

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6 LIST OF TABLES Table page 1-1 Comparison of particle counting techniques......................................................................23 2-1 Particle mode versus diameter...........................................................................................30 2-2 Particle composition versus diameter................................................................................30

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7 LIST OF FIGURES Figure page 1-1 Laser energy diagram....................................................................................................... ..23 1-2 Passive Q-switching........................................................................................................ ...24 1-3 Active mode locking........................................................................................................ ..24 1-4 Cavity dumping............................................................................................................. .....24 1-5 Chirped pulse amplification...............................................................................................25 1-6 Rayleigh and Raman scatter...............................................................................................25 1-7 Multiphoton excitation..................................................................................................... ..26 1-8 Cascade electron ionization...............................................................................................26 2-1 Threshold irradian ce versus pulse width............................................................................35 2-2 Size range of aerosol physics.............................................................................................36 2-3 Scanning mobility particle sizer.........................................................................................37 2-4 Tapered element oscillating microbalance.........................................................................37 2-5 Bernoulli effect on particle focusing..................................................................................37 3-1 Laser and detection setup.................................................................................................. .46 3-2 Effect of the HEPA filter.................................................................................................. .46 3-3 Note baseline shift........................................................................................................ ......47 3-4 Drying tower and overall experimental layout..................................................................48 3-5 Electronics cart........................................................................................................... ........48 3-6 Target chamber and PMT..................................................................................................49 3-7 Comparison of laser pulse and emission lifetime..............................................................49 3-8 Oscilloscope trace of laser pulse........................................................................................50 3-9 Illustration of near single-mode operation.........................................................................50

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8 3-10 Diagram to accompany scatter calculation, using a nominal 500 nm particle close to the PMT........................................................................................................................ .....51 4-1 03 November outdoor air background...............................................................................53 4-2 03 November outdoor air with HEPA filter.......................................................................53 4-3 50 nm Gelman filter........................................................................................................ ...54 4-4 20 nm Gelman filter........................................................................................................ ...54 4-5 Short tube background.................................................................................................... .55 4-6 Long tube background.....................................................................................................55 4-7 24-hour time series........................................................................................................ .....56 4-8 24-hour time series........................................................................................................ .....56 4-9 24-hour time series........................................................................................................ .....57 4-10 40-hour time series....................................................................................................... ......57

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9 LIST OF ABBREVIATIONS a Particle radius Aij Transition probability c From Latin circa meaning nearby or approximately c Speed of light, 3 x 108 meters / second C Constant incorporating partition function Q(T) cw Continuous Wave d Beam diameter D Unfocused laser diameter or particle diffusion coefficient Do Original droplet diameter DIAL DIfferential Absorption LADAR Ei Energy of upper level f Lens focal length gi Statistical weight of upper level h Plancks constant, 6.6 x 10-34 kg m2 / s HEPA High Efficiency Particulate-Air Iij Intensity of LIBS spectral line Io Incident intensity. K Evaporation constant L Internal diameter of tubing (4 mm) or resonator cavity length Laser wavelength or mean free path (m) ij Transition wavelength LADAR LAser Detection And Ranging LPM Liters Per Minute LTE Local Thermodynamic Equilibrium me Electron rest mass M Molecular species Dynamic fluid viscosity or Dipole moment MASER Microwave Amplification by S timulated Emission of Radiation n Number of modes Nc Critical electron density for LTE

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10 n Number of particle s at large distance ns Nanosecond (10-9 second) P Pressure p Momentum Fluid density (1.168 kg/m3 for air at STP) p Particle density Vapor density at r. pw Power (watts) pdf Probability distribution function ps Picosecond (10-12 second) r Focal spot radius Absorption cross section (m2) or particle diameter (m) Tc Critical temperature Pulse width trt Round trip travel time in laser resonator us Mean fluid velocity = 13.26 m/s in these experiments v Kinematic fluid viscosity fsr Free spectral range Z Degree of ionization

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11 Abstract of Thesis Presen ted to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science REAL-TIME PARTICLE DETEC TION USING SU B-THRESHOLD LASER INDUCED BREAKDOWN SPECTROSCOPY By William Paul Mason May 2007 Chair: Nicolo Omenetto Major Department: Chemistry Ambient aerosols play an important role in a variety of processes ranging from semiconductor fabrication and industrial emission m onitoring to chemical warfare and the global climate. Though interest in particle detec tion is not new, advanc ing technology provides heretofore unavailable methods of particle detec tion. This work lays a historical foundation for optical development, compares related particle detection techniques, desc ribes related work, and exercises the potential of the PowerChip laser in real time aer osol monitoring. Following the work of Kwabena Amponsah-Manager and David Hahn, this effort relates the versatility and robustness of the PowerChip laser with test chamber and electronics for real-time aerosol monitoring. For particles w ith finite absorption at 1.06 m, the PowerChip laser shows itself a stable and reliable laser source. With engine ering development such as weatherization and battery power, this instrument could beco me field portable and remotely operable.

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12 CHAPTER 1 INTRODUCTION The inflationary growth of lasers and optic al spectroscopy comes only after millennia of development. The first optical element offici ally recognized is the Assyrian Layard/Nimrud lens, dated to 700 BC [BBC, 1999]. However, Robe rt Temple suggests that lenses existed as early as 3300 BC in Egypt, evidenced by micros copic carving on a knife handle, and in 2500 BC by crystal lenses found in the facial wrappings of a mummy [Temple, 2000]. Euclid (325-265 BC) produced the first known writings on optics [Euclid], and both the Greeks and Romans used crystal and water-filled glass sphe res as lenses [Pliny]. The triu mph of Archimedes against the armies of Marcellus is oft-quoted as the first us e of optics [Djiksterhuis, 1987]. The veracity of the claim has been debated for centuries, but Aristophanes ( c 448 c 385 BC) does mention the use of a burning glass to start fires in his play The Clouds [Aristophanes, 423 BC]. Ptolemy (AD c 90 c 168) measured the changes in the path of li ght from air to water, air to glass, and water to glass [Hecht, 2001]. Ibn al-Haitlam, known in the West as Alhazen (965040), developed the first comprehensive alternative to Greek theory. He knew that light traveled in rays composed of colors and that bodies do not emit visible light, the eyes only receive reflected light, unless from a source such as a lamp or the s un. Further, he realized that light had a large but finite velocity and that re fraction occurs because light has different velocities in different media [Lindberg, 1976]. The crystal lenses of 11t h century Sweden were comparable in quality to aspheric lenses of the 1950s [Schmidt, et al ., 1999]. Rene Descartes developed a theory of light as a wave traveling through the plenum, a forerunner to the luminiferous aether proposed by Robert Hooke in 1678. Isaac Newton championed th e particle nature of light, but allowed that particles of light could create wa ves in the aether to explain diffraction (having been recently demonstrated by Francesco Grimaldi and late r by Thomas Young). In 1845, Michael Faraday

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13 found that the polarizatio n of a beam of light would change in a magnetic field as it passed through a polarizing medium. In 1847, he proposed that light was an elec tromagnetic vibration requiring no medium of propaga tion. Influenced by Faraday, Ja mes Clerk Maxwell ultimately derived the equations describing electromagnetic waves [Maxwell, 1865]. However, the failure of the Michelson-Morley experiment to detect the aether, and the inab ility to explain blackbody radiation, Einsteins photo electric effect, and the constant sp eed of light regardless of reference frame remained troublesome issues for the wave theory of light. Max Planck in 1900 resolved the blackbody problem by proposing the quantization of energy, giving credence to the notion of light as particles, called photons The packets of discrete ener gy were called quanta. Thus was born quantum mechanics. Einstein explained the photoelectric effect by suggesting that the energy of ejected electrons was a function of th e wavelength of incoming light, while the number of electrons ejected was a func tion of the incoming photon flux (photons per unit time and area). The invariance of the speed of light was also so lved by Einstein with his Special Theory of Relativity. From the time of Galileo, velocities were considered relative to the velocity of the observer. Einsteins theory s uggested that this was not trul y the case [Thornton and Rex, 2002]. Einstein also derived the so-c alled Einstein coefficients for stimulated absorption and emission, the first indication that coherent emission may be possible. In short, if a population inversion can develop with sufficient pumpi ng, coherent radiation will result (Figure 1). Denoting by subscripts one and two the ground state and first electronic excited state, respectively, the Einstein coefficients describe both stimulated (B) and spontaneous (A) emission and absorption. The overall energy balance for a lasing system in a steady state is given by the change in number of excited atoms through time: 21 1 12 1 12 2) ( B N B N A dt dN 2 21 2) ( N A N (1.1)

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14 1 1 ) 8 ( ) (3 T k hBe c h (1.2) However, A12N1 can be neglected by the Boltzmann distribution, assuming the laser is operating at room temperature. Once the laser achieves steady state (less than one second), the number of atoms brought to the excited state by pumping is balanced by the de-excitation of atoms through both spontaneous and stimulated em ission. Einsteins derivation of stimulated emission, the B21 term, is what initially gave rise to the idea of a laser. Further development by Nikolay Bosov and Alexander Prokhov laid the theoretical framework to build a device for coherent produc tion of microwaves, now known as the MASER. Charles Townes, J. P. Gordon, and H. J. Zeiger built the first maser at Columbia University in 1953. Townes and Arthur Shawlow went on to de scribe the theoretical basis for the LASER (using visible light instead of microwaves). Theodore Maiman built the first laser in 1960 [Maiman, 1960]. The advantages of lasers over other sources are manifold, including monochromaticity, coherence, shor t pulse length, and extremely hi gh intensities. A host of laser types exist, from CO2 gas lasers occupying entire buildings to solid state laser pointers, each with concomitant advantages and disa dvantages. Lasers may be broa dly defined as continuous wave (cw) or pulsed. While cw lasers are useful fo r applications such as welding and have been developed to the megawatt level, they are large a nd costly to operate. Pulsed lasers offer the advantage of short pulsewidths (femtoseconds) and high pulse energies (gigawatts). The femtosecond regime offers a new class of phys ics under active investigation by many groups. Unfortunately, femtosecond lasers remain prohibitively expensive. The JDS Uniphase (now Teem Photonics) PowerChip laser, a 500 ps solid state system, offers a compact, rugged, and cost effective compromise.

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15 The generation of short pulses is done primarily by three methods: Mode locking, Qswitching, and chirped pulse amplification. Mode locking was the first technique developed and takes advantage of the relation: bandwidth N tc p1 1 2 (1.3) Active and passive mode-locking methods are pos sible, such as electro-optic modulation (active) or saturable absorbers (passive). Consid er the modulator as a weak shutter. By timing it to coincide with the rou nd trip time of the cavity, c L 2 (1.4) A standing wave develops in the cavity. On e packet of photons will bounce back and forth in the resonator, emitting regular pulses and re charged by the pump. The round trip time of flight in the laser cavity determines the inter-pulse separation. The pulse duration t obeys the following relation: t = -1, (1.5) for the gain bandwidth. For a laser with n output modes, 2 n L (1.6) L c 2 (1.7) One obtains short pulses by decreasing cavity length or increasing the number of output modes oscillating in phase. Thus the PowerChip has very short pulses because the cavity length is short the round trip time of f light in the laser cavity determin es the inter-pulse separation. Since the number of modes that can oscillate de pends on the Doppler width of the transition and the cavity length, optical modulat ors inside the resonator can cause active mode locking.

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16 Saturable absorbers achieve passive mode lockin g, as with the PowerChip. To produce more energetic pulses, Q-switching is used to contain the intensity of emitted photons. Here the energy in the resonator builds up to a threshol d determined by optical switches or nonlinear crystals. However, active Q-sw itching requires significant electrica l power and is difficult in practice. In passive Q-switchi ng, the pulse repetition freque ncy can be changed simply by changing the pump power, which changes the amount of time needed to reach a threshold in the passive switching medium. Passive mode-locking is possibl e using a Kerr lens that absorbs low intensity light while passing high intensity transient pu lses, leading to mode-locking. Passive Q-switching depends on the saturable ab sorber becoming transm issive at a certain threshold of photon intensity and dumping the photons in one large pulse. The setup is the same as for active Q-switching, sans drive electronics (Figure 2). Electro-optic devices can also be used to dump the cavity all at once, a form of Qswitching. For example, Pockels cells will change the path of light in response to a change in applied voltage across certain faces of the nonlin ear crystal (e.g. potassium dihydrogen phthalate, or KDP). Thus, the population inversion can be built up and then dumped at will (Figure 3). Active Q-switching is brought about by controlli ng a saturable absorber, which controls the quality of the resonator and therefore the tran smissivity of the resonator. This change in transmittivity is tantamount to a change in the quality of the resonator, hence the name Q (Quality) -switching [McClung a nd Hellwarth, 1962]. The pump creates a population inversion with only a small number of phot ons circulating. When the signal is given, the saturable absorber becomes transparent and the excited atom s relax, falling into phase with the small beam of lasing photons to create a giant pulse that exits the resonator all at once (Figure 4).

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17 Chirped pulse amplification is common with femtosecond lasers and essentially stretches the pulse in time so that it can be amplified w ithout damaging the optical system. The amplified pulse is then recompressed to exit the resonato r (Figure 5). Once the pulse leaves the laser, several processes can take place. Overall, they fall into three categories: absorption, scatter, and transmission. The normalized energy distri bution equals unity: ab sorption + scatter + transmission = 1. Working backwards, transmission is the trivial case. The laser must have a backstop of some sort to remove excess photons in the event that an absorbing or scattering body is not present. Scattering represents a more complicated picture. Perhap s the most well known form of scattering is Rayleigh, in which photons are elastically scattered in preferential di rections as a function of frequency to the fourth power, he nce blue light is scattered moreso than red: 42 c I Io Rayleigh (1.8) The primary direction of scatter is normal to the incident path, thus the sky overhead appears blue while sunrise and sunset appear red. In Rayleigh scat tering, the atom is excited to a virtual state lower in energy than the first exc ited electronic state and ra pidly de-excites, emitting an identical photon in a preferenti ally radial direction. Rayleigh scattering is dominant in particles of diameter less than or equa l to the incident wavelength (Figure 6). Raman scattering is the inelastic counterp art to Rayleigh scatte ring and also involves excitation to virtual states. Since the transiti on probabilities are much smaller than Rayleigh scatter, Raman has very low amplitude. Despite this low intensity, Raman scattering has many important applications such as non-destructive ar tifact testing. Now Mie scattering applies primarily to particles of diameter equal to or larger than the incident wavelength and results in preferential scatter along the direction of transmission. Mie

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18 scattering is not highly wavelength dependent, so scattered light appears white, as in clouds and fog. So far the interactions described have dealt with uncharged particles. Consider now a photon interacting with a charged particle, say, an electron. This interaction is defined by Compton scatter, which does not typically occu r with visible wavelengt h photons because their energy is too low to overcome the atomic binding energy. However, X-ray photons have plenty of energy and can lose energy to electrons: h c h p (1.9) c m h p he (1.10) cos 1 c m he, (1.11) for = radiation scattering angle. By considering wave theory, can arise due to the Doppler effect [Ditchburn, 1991]. Thomson scatter involves th e interaction between a photon and a free charged particle, though only in the plane of polarizat ion of the incident photon. The magnitude of the oscillation varies as (cos ), where is the angle between the incident light and the observer. Such scattering can give rise to a polarization effect. Brillouin scatter occurs when light changes its vector due to density changes in its path. Such density changes can arise from acousti c modes (phonons), temperature gradients, or pressure gradients. Brillouin scattering occurs in a Pockels cell when using acoustic shutter frequencies to produce Q-switching.

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19 Photoacoustic scatter is the process wher ein light strikes a surface and creates sound waves. Though first noted by Alexander Grah am Bell [1881], it was not developed until the 1970s [Rosencwaig and Gersho, 1973]. In essen ce, the photon source is modulated at an acoustic frequency, say, 1 kHz. The electrom agnetic energy excites electrons, which rapidly transfer their energy to the phonons, resu lting in acoustic fr equency signals. LADAR is a comparatively new but burgeoning field. Aerosols at large distances (km) can be interrogated by scatter and absorption measurements. Information about particle concentration and composition b ecomes available upon comparison of retro-reflected scatter at one wavelength versus a differe nt wavelength, a technique know n as DIfferential Absorption LADAR or DIAL. While standoff analysis of aeroso ls is important and c ontinues to grow as a field, interesting physical processes arise when th e laser energy is increas ed to the point where plasma forms. Plasma composes 99% of the observable uni verse. Methods for analyzing plasma emission are well characterized, though plasma spectroscopy is still subject to significant background in many situations. Given this, consider now the application of lasers to particle detection by plasma formation, which amounts to looking at the final term, absorption. The basis for LIBS and the present particle detection sc heme rests on absorption of laser photons into particles to cause their ionization and emission. Two years after the first laser was built, F. Brench and L. Cross proposed the theory of LIBS [Brench and Cross, 1962]. In 1967, Moonke and Moenke-Blankenburg built the first LIBS instrument [Cremers and Radziemski, 1989]. Though the complete process of LIBS is still not fully understood, great advances in its application as an atomiza tion/ionization source have been made.

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20 Consider the absorption of photons into a pa rticle. Classically, one may imagine photons of light striking the surface of the particle like bullets hitting a ta rget. The larger the target, the more likely the bullets are to strike. The size of the target is quantified by the absorption cross section In practice, this is related to but not necessarily the same as the geometrical cross section of the particle. The absorption cross section in essence desc ribes the likelihood of interaction with a photon of give n wavelength. A more meaningful analogy is available in terms of resonance. The photon interacts with the outer shell electrons of the atom. The electrons can be seen as point masses vibrating on springs with spring constant k a measure of the strength of the electrons binding energy. Loosely bound electrons will have a small k and will interact with relatively low energy photons. Depending on the energy and number of photons, the electron will excite to a higher energy level and then relax through fluorescence, phosphorescence, or collisional de-excitation. In multiphoton excitation, comple te ionization is possible given sufficient photon flux (Figure 7). Since the density of free electrons in most materials at STP is negligibly small, initiation of cascade ionizati on requires some sort of catalyst such as multiphoton excitation in a laser. Multiphoton ionization: nh + M M+ + e(1.12) Once free, the electron is accelera ted by the electric field of th e laser, leading to cascade ionization wherein it collides with an atom or molecule and knoc ks loose another electron, both of which then accelerate in the el ectric field and repeat the pr ocess, forming a geometrically growing electron cascade [Radziemsk i and Cremers, 1989] (Figure 8). Again, for LIBS, photons interact with the el ectrons of the material. The electrons are excited to high temperatures in femtoseconds but transfer the ener gy to the phonon lattice, resulting in a shockwave and expl osive removal of material in a plasma state. In the ns-ps

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21 regime, the laser pulse continues to excite the ej ected material as it leaves the surface, heating and ionizing the ejecta to a plasma. This forms an optically opaque plasma with a temperature in excess of 10,000 K. The plasma temperature can be calculated by solving the following equation for temperature [R. Harmon, et al., 2005]: T k E ij ij i ijBe A g C I (1.13) For ultrashort pulse lasers (< 1 ps) intera cting with a solid surface, all the energy is deposited at once into the el ectron lattice, which transfers the energy to the phonon lattice, resulting in explosive re moval of material with virtually no melting, though some researchers have found extensive ionization [M artin, et al., 2002] (Figure 9). After about 1 s the shock wave decouples from the plasma, leaving the plasma in local thermodynamic equilibrium [Zeng, et al. 2006]. Thermodynamic equilibrium is defined as a zero gradient for all intensive properties of the sy stem (temperature, chemical activity, pressure, etc.). For a gas, this is tant amount to having a specific Maxwell-Bo ltzmann distribution. Such a condition is virtually impossible to achieve in LIBS plasma it is a non-equilibrium phenomenon. However, the approximation suffi ces for many instances. LTE implies that, though the system parameters vary across space a nd time, they vary slowly enough to permit the assumption of thermodynamic equilibrium a bout any given point instantaneously. Thus, the 58 J of the PowerChip may or may not allow LTE. One may use the Griem criterion to determine if LTE exists [Yueh et al., 2000]: 3 2 2 1 17 310 545 30 ) ( nm Z K T x cm Nc c (1.14) Given a plasma in LTE, the problem of spatial measurement arises. Though the present effort does not involve LIBS per se an understanding of the processe s at work is instructive.

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22 When an intense ultrashort laser field propagate s inside a dielectric me dium, it induces a strong polarization field and high density of electrons and holes. This space-time dependent problem is intricate because it involves nonlinear effect s such as multiphoton excitation, free carrier absorption, photoemission, electron-phonon interacti on, exciton generation, and carrier-carrier interaction, all in the presence of a high intensity field [Audebert et al., 1994]. Additionally for natural substances, the situation is further complicated by inhomogeneous sample composition and surface irregularities [Harmon, et al., 2005]. Variation in composition manifests as variation in laser-target coupling convolved with surface roughness variation. Given such difficulties, why bothe r with LIBS at all? Though LIBS is often touted for little or no sample prep, caution must be exerci sed in some cases. For example, inhomogeneous matrices can present spectral complexity and create difficulty in interpretion. Further, heavy surface contamination such as grease or dirt can cause wide variation in signal intensities and interference. Surface films and surface roughness can also create skewed results. Bearing these caveats in mind, particle counting and the natural extension to LI BS offer many advantages such as light weight, solid state electronics, no vacuum requirement, and real-time analysis. Comparison to other techniques shows the usef ulness of LIBS as summarized in Table 1: Armed now with a conceptual understanding of plasma forma tion and LIBS, the properties of particles may be addressed. For small par ticles, some transmission and scatter may occur, depending on the shape and composition of the particle. For silicon dioxide (SiO2), we assume that transmission and reflection are negligible. Because the pulse duration is so long (500 ps) relative to the time it takes to eject material from the bulk (femtoseconds), the initial part of the pulse excites electrons a nd breaks up the particle into clouds of molecules which continue to be irradiated by the later part of the pulse to form a plasma. The emission from this plasma,

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23 Bremsstrahlung, fluorescence, and recombination, can be used for simple particle detection by the photomultiplier tube. The photomultiplier tube is composed of a material that emits electrons when struck by photons of sufficie nt energy. In the case of th e Hamamatsu R 647, the material responds to photons with wavele ngths between 300 and 650 nm. The response is controlled in part by the applied voltage, a sensit ivity selector of sorts. The price of high sensitivity is an increase in false hits from, for example, cosmic particles or noise fluctuations that are amplified by the high voltage required for sensitive measurements. Particle detection is desired in a host of applicat ions, from semiconductor fabrication to chemical/biological warfare agent detection and global climate modeling. The following chapter details particle characte rization and detection. Table 1-1: Comparison of Pa rticle Counting Techniques. Technique Sensitivity Range Portability Ease of Use Efficiency Data Complexity LIBS Low ModerateHigh Moderate Low High MS High High Low Low Low High CPC High Low Moderate Moderate High Low Filtration Moderate ModerateModerate Low High Moderate Scatter High Low High Moderate Low High Figure 1-1: Laser Energy Diagram Singlet Ground State Singlet Excited State Excited Triplet Deexcited Triplet Vibrational States Internal Conversion (Vibrational Relaxation) Fast Intersystem Crossing (Singlet to Triplet) Excitation Photon Lasing Photon Fast Intersystem Crossing Energy

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24 Figure1-2: Passive Q-Switching Figure 1-3: Active Mode locking Figure 1-4: Cavity Dumping 100% reflector Gain Pockels Cell 100% reflector 100% reflector Gain Medium Saturable Absorber 99% reflector 100% reflecto r Gain Acousto-Optic Modulator 99% reflecto r

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25 Figure 1-5: Chirped Pulse Amplification Image Credit: http://www.nsu.ru/psj/lector/lotov/terawatt/cpa.gif Figure 1-6: Rayleigh and Raman Scatter. Energy Virtual State

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26 Figure 1-7: Multiphoton Excitation Figure 1-8: Cascad e electron ionization Positive ion Virtual State Energy Continuum h h Two electrons Neutral molecule Incoming electron

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27 Figure 1-9: Laser-material interaction. Pulse strikes surface Material ejected Photons excite ejected matter Brehmsstrahlung emission LIBS (> 1 s)

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28 CHAPTER 2 PARTICLE DETECTION The term aerosol derives from the term for hydrosol, meaning a solid (colloid) suspended in liquid (solution). Aerosols are important to many aspects of life, including health, global climate, and military weapons. Epidemiological studies found associations between particulate air pollution and human health [S chwartz, et al., 1996]. People for some time argued that air pollution only sped up the inevitable, killing onl y those who would soon die anyway. However, though mortality increases as air pollution incr eases, it is not follo wed by a deficit when pollution decreases, implying that pollution not only harvests from the vulnerable pool, but recruits new people into the pool. [Zanobetti, et al., 2002]. Aerosols also have an impact on the environm ent. Their primary effect is to alter the scatter and absorption of solar ra diation, leading to either wa rming or cooling depending on the fraction scattered versus absorbed. The seconda ry effect manifests by altering the scattering properties and longevity of clouds [Penner, et al., 2001]. Without aerosols in the atmosphere, very few clouds would form [Zal absky, 1974]. As aerosol number increases in a cloud, water in the cloud is spread over many more droplets, each of which is proportionally smaller. Clouds with smaller droplets reflect more light and last longer, since it takes more time for droplets to coalesce and fall. According to Raes, et al. [2000], primary particles can be emitted directly into the atmosphere as particles (primary process) or formed in the atmosphere from gas-to-particle conversion (secondary process). Atmospheric aerosols range in size from a few nm to m in diameter. Once airborne, part icles evolve in size and com position through condensation, evaporation, coagulation, chemical reaction, or activation within supersaturated water vapor to

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29 form cloud and fog droplets. Particles smaller than one m range from 10 10,000 cm-3, while particles with diameters greater than 1 m are typically < 10 cm-3. A primary aerosol is emitted into the atmosphere as a particle, whereas secondary aerosols are formed in the atmosphere by gas particle conversion [Raes, et al., 2000]. Particle formation can be categorized by diameter ( ): > 1 m = primary formation < 1 m = secondary formation Strong overlap exists for particles of diameter 0.1 1.0 m. Combustion soot is typically 5 20 nm, but coagulates rapidl y to form fractal aggregates which collapse to more stable structures of tens of nm due to the capill ary forces of condensing vapors. Shah et al. [2005] f ound that lubricating oil was a prim ary fraction in diesel emission, and increased while the engine was accelerating, versus cruising. The Kelvin effect plays an important role in particle formation. The equilibrium vapor pressure over a spherical particle increases with decreasing radius of curvature; hence equilibrium vapor pressure above molecular cluste rs formed by random collisions is much larger than that above a film or flat surface. Conseque ntly, molecular clusters tend to evaporate. Small particles (<1 m) diffuse to the Earths surface, a pr ocess that becomes less efficient with increasing For 0.1 < < 1 m, dry removal is very slow, so these particles tend to accumulate in the atmosphere. They are removed mostly by cloud activation and precipitation [Willeke and Baron, 1993]. Particle behavior and compos ition are weak functions of the particle diameter (Tables 2 and 3).

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30 Table 2-1: Particle mode versus diameter. Diameter (um) Mode <0.01 Nucleation 0.01< <0.1 Aitken 0.1< <1 Accumulation >1 um Coarse Table 2-2: Particle co mposition versus diameter. Diameter (um) Composition <0.1 Carbon 0.1< <2.0 Sulfate, nitrate, heavy metal >2 Geologic material, pollen To develop a new particle detector, consid eration of previous de vices is important. Important work on the detection of aerosols began with Aitken in the 19th century [Aitken, 1923], who determined that most atmospheric aero sols were less than 10 0 nm in diameter and ranged from hundreds to tens of millions per mL depending on the cleanliness of the air. Interestingly, the Wilson cloud chamber was deve loped as a result of Wilson being moved by the sighting of a Brocken specter wh ile working at the meteorological observatory atop Ben Nevis in Scotland. So struck was he that he studied cl oud formation and condensati on in the laboratory. The result was the cloud chamber, which is also one of the most sensitive particle counters for aerosol measurements. Particle detection methods encompass a range of responses: Aerodynamic particle sizer measures the ve locity of particles in accelerating air flow using two laser beams and sca tter detectors at various angles. Photoacoustic spectroscopy chopped laser illuminates ambient air. Particles absorb energy from the beam and transfer it as heat to surrounding air. The expansion of heated gas produces a sound wave at the same frequency as the chopper. This acoustic signal is detect ed by microphone and is proportional to the amount of light absorbed.

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31 Electrical aerosol analyzer collects part icles according to size dependent mobility in electric field, then detected by deposition of charge on an electrometer. Differential mobility particle sizer clas sifies particles by their mobility in an electrical field and counts them with a condensation nuclei count er in a range of size bins. Scanning mobility particle sizer a comp lex version of the Differential Mobility Analyzer (Figure 11), including a radioac tive ionization source and Condensation Particle Counter. Because the Condensa tion Nuclei Counter (CNC) cannot classify particles by size, it is combined with the DMA to give both particle size and number. One drawback of the SMPS is that it can take up to 300 s to obtain size distributions, since the partic les need time to form a hydra tion shell in order to be detected in the chamber. Further, CNCs are overwhelmed by particle concentrations greater than class 100 0 environments [Particle Measurement Systems website, 2006]. Electrical low pressure impactor of fers real time size distribution and concentration measurem ent from 30 nm to 10 m. The electric current carried by charged particles into each impactor stage is measured by a sensitive electrometer. One anticipates the difficulty of measur ing charged particles at low pressure (requiring some measure of pumping), a nd interference effects from electronic noise. However, it has the advantage of being able to measure rapid changes in both particle size a nd concentration. Tapered element oscillating microbalance operates by changing the frequency of oscillation as mass accumulates on the cantilever (Figure 11). As particles accumulate the frequency changes as m k The balance lasts for about 3 weeks, with a hour equilibration time before data can be taken. The classic methods of atomic absorption/em ission spectroscopy are well-characterized, but suffer heavy background emission and comparativ ely low sensitivity. Scattering, absorption, and emission techniques such as Rayleigh, Ra man, Fourier-Transform Infrared Absorption, LIBS, LAser Detection And Rangi ng (LADAR), filter impaction, and mass spectrometry offer a host of techniques for particle detection. Wh ile each technique has some advantage and some disadvantage, LIBS offers a few important adva ntages over other techni ques. Mass spectrometry provides high resolution and good limits of detec tion, but requires the us e of vacuum pumps, adding to the cost and bulk of the system, and experiences peak broadeni ng due to excess kinetic

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32 energy [Tolocka, 2004]. Light scat tering techniques are effective on ly to the point at which the particle diameter equals the wavelength. For particles smaller than the wavelength used (below ~300 nm), such techniques are less reliable [Maynard, 2000]. Techniques like LADAR offer remote detection, but have poor sensitivity and mi nimum detectable particle sizes (~300 nm). CNCs are sensitive, but require periodic refilling with alcohol and are easily saturated in dirty environments. LIBS provides a good detectable size range and good portabili ty, for the price of sensitivity. However, particle focusing could ameliorate this problem [Wu, 2006]. Research has shown a bimodal pa rticle distribution in the atmo sphere, while other research has shown that particles in th e accumulation region (around 0.1 m in diameter and smaller) are most harmful to humans. These are also the mo st difficult to count cont inuously and the most difficult to filter. Filtration and detection met hods are many and wide ranging, from Raman and fluorescence to condensation nuc lei counters and impaction. Having considered particle counting, particle transport bears remark. Particle transport both through the air and through tubing is complex. Physical phenomena such as thermophoresis, turbulence, and adhesion are fa ctors. Velocity focusing, diffusion, and Brownian motion also play into the scheme (Fi gure 12). Brownian motion of particles results from collision with other partic les whose velocity is proportional to the square root of temperature. Equating kinetic energy and the th ermal energy gives a relation between velocity and temperature: T k mvB2 3 2 12 (2.1) T k vB3 (2.2)

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33 Because their mass is so small, Brownian par ticles do not settle from a given volume; they are kept afloat by the thermal motion of the par ticles around them. In other words, they are perpetually diffusing at a rate given by: m T kB (2.3) Thermophoresis is brought about by temperat ure gradients in a gi ven volume. Higher temperature will increase the volume between particles, causing cooler particles to move to cooler regions of the gradient. Note that energy must be added to the system to maintain the gradient. Similarly, eddy and turbulence focusing arise from pressure gradients, similar to thermophoresis. In addition to various transport phenomena, particles may adsorb to surfaces by the following two processes: Electrostriction may arise between particles with a charge or a strong permanent dipole moment. In liquids, such charges will be solvat ed, but in the gas phase electrostriction can be a significant factor. Electrostricti on between two or more particles, usually of opposite sign, is a phenomenon known as accretion. Simple friction can play a role, such as in HEPA filters, where particles are mechanically trapped by small gaps in a medium. Diffusion and adhesion coexist in a dynamic equilibrium, suggesting that a sudden shift in some relevant parameter (e.g. temperature or pressure) could shift the particle equilibrium and result in a pul se of free particles or a sudden shift in average particle diameter. Also, particle concentration d ecreases with increasing tube length. The rate of decrease depends on several factors, includi ng type of tubing [Willeke and Baron, 1993]. Carranza, et al. [2001] found that particle transport efficiency wa s greater than 95% for particles from 0.1 to ~1.5 m in diameter for their system.

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34 Deliquescence, a sharp rise in liquid water content at ~ 55% relative humidity, is a result of the hygroscopicity of particle s and is relevant for many salts such as calcium chloride and magnesium chloride. Because th e particles are strongly hygroscopic, under high humidity they can absorb enough moisture to dissolve themselves. This could be a factor in situations where particle solutions are created and then dried to fo rm aggregates. The ability of the tower to dry such salts may be questioned. Clearly a host of physico-chemical processes are at work. Desirous to further develop particle detection capabilities with the PowerChi p laser, several experiments were performed. Note here that M.D. Cheng [2003] failed to produce meaningful results from the reference standards while testing the sub-th reshold setup similar to the pres ent study. The present work may add further strengthen the overall effort of particle detection w ith lasers. Though this effort recapitulates much of Cheng, Hahn, and Amponsah -Managers work, the use of the PowerChip laser and the discussion of par ticle distribution (Poisson vers us diffusion mediated) may lend insight into the investigation. LIBS offers some advantages because it ca n both count particles and characterize them by constituents. Several particle counting syst ems also provide spectral analysis (e.g. Hahn [1998] and Cheng [2000]). Cheng proposed th e use of sub-threshold breakdown for aerosol detection (akin to sub-threshold br eakdown in liquids). That is, the laser power is set just below the breakdown threshold of air. When a particle ente rs the laser focus, the breakdown threshold of air is decreased, causing plasma formation. The tacit assumption is made that the absorption cross section of the partic le is greater than that of the bac kground carrier gas. The advantage of using sub-threshold pulses is a reduction in background noise, sin ce a plasma forms only in the presence of particles of nontrivi al absorption cross-section. Following this line of work, we

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35 sought to further characterize a nd expand the capabilities of such a system. The next chapter describes those efforts, including initial development, enhanced chamber setup, aerosol nebulizer and flow gas, and long term measurements. Figure 2-1: Threshold irra diance versus pulse width.

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36 Figure 2-2: Size range of aerosol physics. Radial DMA High Voltage Power Mixing Chamber Detector Krypton 85 Charger Aerosol Flow Pump

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37 Figure 2-3: Scanning Mob ility Particle Sizer. Figure 2-4: Tapered Element Oscillating Microbalance. Figure 2-5: Ber noulli Effect on particle focusing. Gas particles bouncing against a surface at low velocity Gas particles bouncing against a surface at high velocity Electronic driver/recorder

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38 CHAPTER 3 EXPERIMENTAL SETUP AND DEVELOPMENT Manager [2005] worked on particle detection with the JDS MicroChip laser and nebulized solutions of simple salts. Particle focusing occurred by nebulizing and desolvating particles, then passing the air flow through a narrow pipett e tip. The stream was interrogated by a 5 kHz MicroChip laser at 50 J / pulse. Results were promising but inconclusive. Inspired by the work of Smith, Hahn, Omenetto, Amponsah-Manager, a nd Cheng, the present study investigated the feasibility of the PowerChip laser for ambient aerosol monitoring at 1 kHz. The PowerChip laser offers the advantage of relatively high repeti tion rate, short pulse width, solid state passive Q-switc hing, air cooling, and short cav ity length, yielding essentially single mode output. Again, all of these properties play well into a field portable design as well as complimenting the LIBS detection. The PowerChip laser is amenable to particle detection because its short cavity length al lows longitudinal mode spacing gr eater than the gain bandwidth; it enjoys virtually single mode operation with no frequency beating (Figure 16). rt fsr 1 (3.1) While Amponsah-Manager [2005] worked mostly with solids, some particle sampling was done. This work extends his initial investigatio n. The test chamber (Figure 17) was built and used in all subsequent experiments. Several m odifications were made over time to improve its performance. Light tightness is critical si nce the PMT is sensitive and low background is desirable. Tightness was improved through the add ition of a laser beam tube, black electrical tape around the housing seam, and an aluminum gua rd ring over the laser entry port. A mirror placed opposite the PMT (in the bottom of the chamber) reflected light towards the PMT to increase signal. In the future, an elliptical mirro r with one focus at the laser focus and the other

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39 focus on the PMT might prove a better choice. La stly, the chamber was wrapped in black felt to further reduce stray room light. Note that a beam expander would improve the focus characteristics (sharper focus), though this might push the system into breakdown regardless of whether a particle was present and would decrease the effective plasma volume. Depending on the desired experimental setup, a variable diameter beam expande r could serve as a simple power control. In the limit of perfect system function, particles be low a given diameter (all other parameters, e.g. absorptivity, being equal) coul d be made invisible to the system. If the absorption cross section is too small to create a plasma, the particle will pass undetected. Above some minimum diameter, the cross-section will be large enough to form plasma. The signal passed through an SR 570 amplifie r, boxcar, SR 245 signal processor, and out to a computer for data storage and display (Figure 16). Initial tests demonstrated the effectiveness of HEPA filters at reducing the numb er of events detected in ambient air (Figure 14). The system measured the emission of light from plasma formed when a particle was struck by the laser pulse and converted to plasma. The number of photons expected for nominal 500 nm diameter, 2% (weight) silic on dioxide particles is given by: mg cm g L cm g g x 2 1 1 1000 100 2 10 1003 3 6 silica in 100 L solution. mL particles x g x particle g9 1410 9 2 10 6 1 002 0 sec 10 8 4 sec 60 min 1 mi n 1 0 10 9 26 9particles x mL mL particles x Ambient air traveled through th e target chamber by laboratory exhaust flow at roughly 0.2 LPM as measured by a rotometer. If the PMT de tected sufficient photons the signal continued

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40 through preamplifier, boxcar, and postamplifier before readout on a stripchart or storage in computer memory. Data processing included co unting the number of events greater than a certain threshold. Initially this thres hold was set at 3 standard deviations (3 ) from the mean, but minor (< 0.05 V) step fluctuations in the back ground forced a change to the less sensitive but more trustworthy measure of 0.2 V as threshold (Figure 15). This was well above any baseline fluctuations. A power supply regulator (SOL A MCR 1000, Mini/Micro Computer Regulator, catalogue # 63-13-210-05) was installed to eliminat e background shifts, but was unsuccessful at doing so. The threshold remained at 0.2 V. Computer sampling rate: Expect: shots x hours hour shots710 64 8 24 sec 600 3 sec 000 1 Found: shots x files files710 98 4 sec 000 15 317 3 Ratio = 0.576 ~ 58% duty cycle. This is very repeatable for all runs, leading to the conclusion that it represents the computer writing data to memory, during which time it cannot simultaneously take data. The amount of light scattered by the particle into the PMT we expect to be negligible compared to the 0.2 V threshold. The following cal culation bears this out. Because the particle diameter (500 nm) is less than the laser wavele ngth (1,064 nm), Rayleigh scatter dominates. Assume a 90o cone of emission circling the equator of the particle normal to the laser beam containing all of the scattered light. Further, assume that approximately one quarter ( sr 42) of the emission is detected by the PMT: For a 58 pulse mJ 1064 nm laser,

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41 = c = 1 14 6 810 83 2 10 064 1 10 3 s x m x s m x J x s x s m kg x h Eph19 1 14 2 3410 87 1 10 83 2 10 6 6 pulse photons x J x ph pulse J x14 19 610 1 3 10 87 1 10 58 sec Pulses Angle Gain Efficiency n attenuatio ty reflectivi pulse Ph gna l ExpectedSiView PMT filter particle = sec 10 9 4 sec 000 1 4 10 0 1 10 3 6 0 1 10 1 311 2 6 6 14 e x pulses x pulse ph x A x electrons x Coulomb electrons x rrent ExpectedCu7 18 1110 94 2 sec 10 66 1 sec 10 9 4 V A x ltage ExpectedVo7 14 50 10 94 27 This is well below the 0.2 V threshold for part icle hits. Using conservative values for the reflectivity, attenuation, efficiency, and hit rate, we are confident that we do not detect significant scattered light. Now consider the Reynolds and Knudsen numbers regarding fl ow characteristics: Reynolds Number = Re = L Us (3.2) 4 3480 10 78 1 004 0 26 13 168 15 3 s m kg x m s m m kg Re

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42 The results suggest that the flow is unstable. This finding prompted a change to a longer drying tube, permitting a lower drying gas flow ra te because the particles would have more time to dry. Knudsen Number Kn = ) ( 22PL T k LB (3.3) Kn = 5 9 2 9 2310 8 1 300 101 10 50 ) 10 100 ( 2 295 10 38 1 x Pa m x m x K x K J x Therefore continuum mech anics are valid (for Kn < 1), as expected. Consider now the time to to completely evaporate water: to = K Do [Hahn et al., 2001] (3.4) Then s s x s m x m x to71 10 14 7 10 5 3 ) 10 500 (5 9 2 9 The lower flow rates should also help redu ce effects such as velocity and eddy focusing, plus increase the dwell time within the PMT viewing angle. The particle dwell time can be calculated as follows: Dwell time = (Flow rate) (Interaction volume) = sec 10 5 1 2 0 2 0 min 1 sec 60 000 10 min 14 2 3 x cm cm cm This should provide plenty of time for the de tectors, which has a response time of a few nanoseconds. When the composition of particles is considered, the difficulty of matrix effects comes to bear. Defined as variations in laser-target coupling secondary to variable sample composition

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43 and surface characteristics [Harmon, et al., 2005], ma trix effects can yield transients in plasma emission that could affect the observed hit ra te. Indeed, Harmon found that surface roughness is a primary factor in experiment repeatability. Th e surfaces of aerosols are known to vary greatly from spherical to ne edle-like [Hinds, 1999]. A final consideration is the di stribution of particles, taken to follow a Poisson distribution P: ) ( x e x Px (3.5) The Poisson distribution assumes no correlations in either space or time, an assumption which may be called into question by point sour ce emission of particles, e.g. from a vehicle passing close to the detector. The diffusion equa tion may be a more appropriate model. The properties of the diffusion equation are complex, but a brief treatment follows. For a probability distribution function of a single par ticle P, use the heat equation: PT = D P (3.6) If the diffusion coefficient D is not constant but depends on P, then one gets the nonlinear diffusion equation. The random trajectory of a part icle subject to the particle diffusion equation is Brownian motion. To treat it, place a particle in R = 0 at t = 0 and find the pdf associated with R to be: P(R, T) = G(R, T) = ) 4 ( 2 32) 4 (DT Re Dt (3.7) For R2 = Rx 2 + Ry 2 + Rz 2 (3.8) At t = 0, P(R, T) is singular with a pdf for the particle at R = 0 given by the Dirac delta function. The solution of the diffusion equation subject to this initial condition is the Green Function G(R, T) given above. This tr eatment can be extended to a large number

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44 of particles by a decomposition of Green f unctions giving the time evolution of the particles. Such a decomposition can be gene ralized to any diffusiv e process like heat transfer or momentum diffusion, which is the phenomenon at th e origin of viscosity in liquids. If the Poisson distri bution does not hold, the diffusi on equation could serve as a new model [Willeke and Baron, 1993]. Having laid a theoretical founda tion, a review of the experi ments conducted is indicated. Many experimental series were run to determine th e effectiveness of particle detection with the PowerChip laser. The first was with a HEPA f ilter on ambient outdoor particles. The hit rate showed a marked decrease with in stallation of the filter, suggesting that the detection system was functioning nominally (Figure 14). A drying tower was as sembled and operated using compressed air (Figure 16). The time responses of the laser beam and the dete ctor were measured to verify that plasma and not scattered light was detected. Because th e plasma lifetime is long compared to the laser pulse (microseconds versus picos econds, respectively), we conclude that plasma, not scatter, is observed. Further, a calculation of expected scatter shows that it s hould produce a negligible signal at best (Figure 18). Calculation of Beam waist = o = m m x m x m x d f r 7 2 10 7 2 10 5 02 0 10 1064 2 26 3 9 Photon flux: = hc r Pw3 = s m x s m kg x m x m x s x J x8 2 34 3 6 9 12 610 3 10 6 6 10 7 2 10 1064 10 500 10 58=1040 photon The expected particle hit rate may be estimated as follows: For 2 % by weight silicon dioxide particle s of 500 nm diameter and density = 1.05 mL g

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45 0021 0 05 1 1000 10 100 100 26 mL g L mL L x g g g of silica in 100 L of solution. For the mass of one particle, find the volume: 3 14 3 7 310 55 6 10 250 3 4 3 4 cm x cm x r V Then find the mass of a single particle: g x mL g cm x14 3 1410 9 6 05 1 10 55 6 To give the total number of particles in 10 mL: particles x g x particle g10 1410 9 2 10 9 6 002 0 Calculate the flow rate using the nebulization rate: sec 10 5 mi n 10 3 mi n 100 0 10 10 9 26 8 10particles x particles x mL mL particles x Given a counting efficiency of 1:106, this amounts to roughly 5 hits per second. Experiments were performed using undilute d samples and approximately this rate was observed. A more sensitive dete ctor or higher partic le concentrations must be use to see significant hit rates.

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46 Table 3-1: Flow regimes. Reynolds number, Re Flow <2300 Laminar 23004000 Turbulent Figure 3-1: Laser and detect ion setup. [Reprinted w ith permission from Xihong Wu] Figure 3-2: Effect of the HEPA filter.

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47 0100002000030000400005000060000 0.0 0.5 1.0 1.5 2.0 2.5 3.0 VoltsLaser Shots Nubulizer 2 LPM II 26 April 2006 Figure 3-3: Note Baseline Shift.

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48 Figure 3-4: Drying tower and overall experimental layout. Figure 3-5: Electronics cart.

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49 Figure 3-6: Target chamber and PMT. Figure 3-7: Comparison of lase r pulse and emission lifetime.

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50 -0.00000010.00000000.00000010.00000020.00000030.00000040.0000005 -0.08 -0.06 -0.04 -0.02 0.00 0.02 Y Axis TitleX Axis Title Figure 3-8: Oscilloscope trace of laser pulse. Figure 3-9: Illust ration of near single-mode operation. Resonator Modes Pump Energy Pump Bandwidth Free spectral range

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51 Figure 3-10: Diagram to accompany scatter calcul ation, using a nominal 500 nm particle close to the PMT. Laser PMT

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52 CHAPTER 4 RESULTS Experimental results over the past 18 months served to ch aracterize and improve system performance, find environmental trends, and determin e limits of detection for particle size. Note that the limit of detection for particle concen tration is poorly posed because detection with decreasing concentration simply implies a longer time interval between hits, assuming cpntinuous flow. The question is vali d and crucial for batch applications. Styrofoam was tested, but no evidence of in teraction was found, so silica powder was used. Long time-series data collection campaigns sought long term environmental effects, as in Tolacka et al. [2004] who found a bimodal partic le distribution over 24 ho urs due to increased human activity in the morning and evening. No such trends emerged in this study, excepting a clear correlation between rainfall and decreased particle counts. Interesting peaks were found, such as transient spikes one order of magnitude higher than surrounding peaks. No satisfactory explanation of their origin was found. The trials did establish the re liability of the PowerChip and associated electronics for potential field applications to particle mon itoring. A test for plasma current (MFP = 40 m) was unsuccessful. Amponsah-Manager [2005] reported a small current, but did not describe the setup used. The effect of adhesion in tubi ng was measurable (Figur es 19 and 20), but was neglected in the present study. Tests were conducted using Gelmann filte rs of varying pore size, though no clear correlations were found. This could be due to dirty filters, fluctuating background signal, or tubing effects.

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53 Figure 4-1: 03 November Outdoor Air Background .Figure 4-2: 03 November Ou tdoor Air with HEPA Filter.

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54 Figure 4-3: 50 nm Gelman Filter. Figure 4-4: 20 nm Gelman Filter.

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55 Figure 4-5: Short Tube Background. Figure 4-6: Long Tube Background.

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56 Figure 4-7: 24 hour time series. Figure 4-8: 24 hour time series.

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57 Figure 4-9: 24 hour time series. Figure 4-10: 40 hour time series.

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58 CHAPTER 5 CONCLUSION The PowerChip laser was used to form plasma on ambient aerosol particles which were detected by a photomultiplier tube and stored to a computer. A variety of subtleties arose in the development of the system, ranging from making th e detector housing lighttight to changing the drying tower to make the flow properties more laminar. A 5-day sampling campaign produced no clear environmental cycles and only one correlation; that between rainfall and decreased particle counts. However, PowerChip features such as high repetition rate, short pulsewidth, stable output, and nearly single-mode operation re nder it useful for many applications including real-time particle monitoring By taking long time-series data of ambien t air and studying the si ze and concentration dependence of particle counting, the PowerChip laser shows itse lf a steady and reliable source for plasma excitation. The PowerChip would be a strong candidate for continuous aerosol monitoring. Future work may include more precise de termination of minimum particle size and improvement in efficiency through, for example, particle focusing [Wu, 2006; Erdmann, et al, 2005] and combination of the la ser with an iCCD or other spectrometer to gain spectral identification along with particle counting. Interesting experi ments would include using the spectrometer to calculate the plasma temperature to correlat e the particle si ze versus breakdown energy from Weyl [1989]. Lastly particle beam focusing could dramatically improve particle transport efficiency and, concomitantly, detection efficiency.

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59 APPENDIX LASER CHARACTERISTICS Experimental Equipment JDS Uniphase PowerChip Laser 500 picosecond pulsewidth 56 J/pulse 1 kHz repetition rate Hamamatsu R647 PhotoMultiplier Tube (PMT) Diameter = 13 mm Wavelength Range = 300 650 nm Gain = 1.4 x 106 BG 38 filter glued to end of PMT to block laser light ~300 nm FWHM transmittance

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60 LIST OF REFERENCES Aitken, J., Collected scientific papers of John Aitken L. L. D., F. R. S., Ed. C. Knott, Cambridge University Press, London, 1923. Alfarra, M., Insights into atmospheric organic aerosols using an Aerosol Mass Spectrometer PhD. Dissertation, University of Manchester, Manchester, UK, 2004. Amponsah-Manager, K., N. Omenetto, B. Smith, I. Gornushkin, and J. Winefordner, J. Anal. At. Spectrom. 20 (2005) 544-551. Amponsah-Manager, K., PhD. Dissertation: MicroChip Lasers as sources for laserinduced breakdown spectroscopy: Plas ma characteristics and analytical performance University of Florida, Gainesville, Florida, 2005. Aristophanes, Clouds 423 BC. Translation by Ian Johnston, Malaspina UniversityCollege, Nanaimo, BC, Canada, 2006. Audebert, P., Ph. Daguzan, A. Dos Santos, J. Gauthier, J. Geindre, S. Guizard, G. Hamoniaux, K. Krastev, P. Mar tin, G. Petite, A. Antonetti, Phys. Rev. Lett. 73 (14), 03 October 1994. BBC, Thursday, 01 July 1999, David Whitehouse Bell, A. G., Phil. Mag ., 11 (510), (1881). Brench, F. and L. Cross, Appl. Spectrosc ., 16 (5), (1962). Carranza, J., B. Fisher, G. Yoder, D. Hahn, Spectrochim. Act. Part B 56 (2001) 851-864. Cheng, Meng-Dawn, Talanta 61 (2003) 127-137. Cheng, Meng-Dawn Fuel Processing Technology 65-66 (2000) 219-229. Ditchburn, R. W. Light Dover Publications, New York. 1991. Djiksterhuis, E. J, Archimedes Princeton University Press, Princeton, 1987. Erdmann, N., A. DellAcqua, P. Cavalli, C. Gruning, N. Omenetto, JP Putaud, F. Raes, R. Van Dingenen, Aerosol Science and Technology 39 (5), May 2005, 377-393. Einstein, A. Relativity: The Special and General Theory translated by R. Lawson, Pi Press, New York, 2005.

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61 Euclid of Alexandria, Optics, D. Joyce, translator, Clark University, Dept. of Math and Computer Sciences, Copyri ght 1998, accessed October 2006. http://aleph0.clarku.edu/~djoyce/java/elements/elements.html Gornushkin, I., K. Amponsah-Manager, B. Smith, N. Omenetto, J. Winefordner, App. Spectros. 58 (7), July 2004, 762-769. Hahn, D., J. Carranza, G. Arsenault, H. Johansen, K. Hencken, Rev. Sci. Inst ., 72 (9), September 2001. Hahn, David W., App. Phys. Lett. 72 (23), 08 June 1998, 2960-2962. Harmon, R., F. De Lucia, A. Miziolek, K. McNesby, R. Walters, and P. French, Geochemistry: Exploration, Environment, Analysis ; 5 (1), February 2005, 21-28. Hecht, E. Optics (4th ed.) Pearson Education, 2001. Hinds, W., Aerosol Technology: Properties, Behavior, and Measurement of Airborne Particles 2nd Edition, Wiley-Interscience, New York, January 1999. Lindberg, D. C., Theories of Vi sion from al-Kindi to Kepler University. of Chicago Press, Chicago, 1976. Maiman, T., Journal of the Optical Society of America 50, 11 (1960) 1134-1134. Martin, F., R. Mawassi, F. Vidal, I. Gallimberti, D. Comtois, H. Pepin, J. Kieffer, and H. Mercure, Appl. Spect 56 (11), 2002. Maxwell, J. C., Philosophical Transactions of the Royal Society of London 155 (1865) 459-512. Maynard, A., Phil. Trans. R. Soc. Lond. 358 (2000) 2593-2610. McClung, F.J. and Hellwarth, R.W. Journal of Applied Physics, 33 (3), (1962). Particle Measurement Systems, http://www.pmeasuring.com/support/papers/pa rticlemonitoring/air?gclid=CLD28dK5w4 gCFSVNVAodyQr2Lw 15 October 2006. Penner, J., D. Hegg, R. Leaitch, Env. Sci. Tech., 35 (15), August 1, 2001. Petrucci, G.; Farnsworth, P.; Cavalli, P.; Omenetto, N., Aerosol Science and Technology 33, (1-2), 105-121, (2000). Raes, F., R. Van Dingen, E. Vignati, J. Wilson, J.P. Putaud, J. Seinfeld, P. Adams, Atmospheric Environment 34 (2000).

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62 Radziemski, L. and D. Cremers, Laser Induced Plasmas: Physical, Chemical, and Biological Applications New York, M. Dekker, 1989. Rosencwaig, A. and A. Gersho, J. Appl. Phys. 47, 64-69, (1976). Schmidt, O., K. Wilms, B. Lingebach, Optometry and vision science 76(9), September 1999, 624-630. Schwartz, J., D. Dockery, L. Neas, Journal of the Air and Waste Management Association 46 (10), October 1996, 927-939. Secundus, Gaius Plinius (23-79), Naturalis Historia John Bostok and H. T. Riley, Eds, Book XXXVII, p. 6396 Perseus Digital Library Project Ed. Gregory R. Crane Tufts University. http://www.perseus.tufts.edu. 30 October 2006. Shah, S., D. Cocker, Aerosol Science and Technology 39, (2005) 519 526. Shawlow, Arthur and Charles Townes, Physical Review 112 (6), 15 December 1958, 1940-1949. Temple, Robert. The Crystal Sun Century Books, London, 2000. Thornton, S. and A. Rex, Modern Physics for Scientists and Engineers, 2nd Ed. Thompson Learning, Inc., United States, 2002. Tolacka, M., D. Lake, M. Johnston, A. Wexler, Atmospheric Environment 38 (2004) 3263 3273. Twomey, S., Atmospheric Aerosols Elsevier Scientific Pub lishing Company, New York, 1977. Weyl, G. Physics of laser-induced breakdown: an update in L. Radziemski and D. Cremers, Eds., Laser induced plasmas and applications M. Dekker, New York, 1989. Willeke, K. and P. Baron, Aerosol Measurement: Principles, Techniques, and Applications Van Nostrand Reinhold, New York, 1993. Wu, Xihong, Theoretical design, construction, and exper imental characterization of a versatile apparatus for detecting i ndividual aerosol particles PhD. Dissertation University of Florida, Gainesville, Florida, 2006. Yueh, Feng-Yu, J. Singh, H. Zhang, in Encyclopedia of Analytical Chemistry R. Myers, ed., Wiley, New York, 2000. Zalabsky, R. and S. Twomey, Bulletin of the American Meteorological Society 55 (6), (1974) 669-669.

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63 Zanobetti, A., J. Schwartz, E. Samoli, A. Gr yparis, G. Touloumi, R. Atkinson, A. Le Tertre, J. Bobros, M. Celko, A. Goren, B. Forsberg, P. Michelozzi, D. Rabczenko, R. Aranguez, and K. Katsouyanni, Epidemiology 13 (1), January 2002. Zeng, X., X. Mao, S. Mao, S. Wen, R. Grei f, and R. Russo, Applied Physics Letters 88, 061502 (2006).

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64 BIOGRAPHICAL SKETCH William Paul Mason was born in Gardner, Massachusetts on 15 May 1973. The youngest of five children, William graduated from Oa kmont Regional High School in 1991 and obtained a B.A. in outdoor leadership from Prescott Colleg e in 1995. After working as a mountain guide and EMT, he returned to Northern Arizona University to obtain a B.S. in environmental chemistry. Upon graduation he commissioned in the U.S. Air Force and served his first assignment at Kirtland AFB, New Mexi co as a scientist. From there, he was selected to obtain a masters degree and attended the University of Florida. When not studying, William enjoys outdoor activities ranging from skydiving to cave diving, and is a budding musician with bass guitar and violin.


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Title: Real-Time Particle Detection Using Sub-Threshold Laser Induced Breakdown Spectroscopy
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Title: Real-Time Particle Detection Using Sub-Threshold Laser Induced Breakdown Spectroscopy
Physical Description: Mixed Material
Copyright Date: 2008

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REAL-TIME PARTICLE DETECTION USING SUB-THRESHOLD
LASER INDUCED BREAKDOWN DETECTION
















By

WILLIAM PAUL MASON


A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE

UNIVERSITY OF FLORIDA

2007


























Copyright 2007

by

William Paul Mason


































For Constantine Panageotes:

Fasooli, fasooli, ghiomeeze tor sakooli.
(A bean, a bean, it fills the bag.)




For Jane Woodward Mason and family:

Somewhere ages and ages hence,
Two roads diverged in a wood and I,
I chose the one less traveled by,
And that has made all the difference.

Robert Frost










ACKNOWLEDGMENTS

I wish to thank my colleagues in the lab and particularly Ben Smith, Nicolo Omenetto, and Ron

Whiddon for many stimulating conversations, for the patience of oak, and for sharing insight into

the language of nature.


There are more things in heaven and earth, Horatio, than are dreamt of in your philosophies.
Shakespeare, Hamlet, Act I. Scene V



Special thanks go to Ryan Mohney and Dan Shelby for long hours in the lab and valuable

discussions. Thanks to Sue, Sal, and Lily for strong prayers.



And God saw the light, that it was good: and God divided the light from the darkness.
Genesis 1:4












TABLE OF CONTENTS





ACKNOWLEDGMENTS .............. ...............4.....


LIST OF TABLES ............. ...... ...............6...


LI ST OF FIGURE S .............. ...............7.....


LI ST OF AB BREVIAT IONS ............. ...... .__ ...............9...


AB S TRAC T ............._. .......... ..............._ 1 1..


CHAPTER


1 INTRODUCTION ................. ...............12.......... ......


2 PARTICLE DETECTION............... ...............2


3 EXPERIMENTAL SETUP AND DEVELOPMENT .............. ...............38....


4 RE SULT S .............. ...............52....


5 CONCLU SION................ ..............5


APPENDIX LASER CHARACTERISTICS............... ............5


LIST OF REFERENCES ................. ...............60........... ....


BIOGRAPHICAL SKETCH .............. ...............64....










LIST OF TABLES

Table page

1-1 Comparison of particle counting techniques. ................ ............... ......... ...._..23

2-1 Particle mode versus diameter. ............. ...............30.....

2-2 Particle composition versus diameter. ............. ...............30.....











LIST OF FIGURES


Figure page


1-1 Laser energy diagram............... ...............23

1-2 Passive Q-switching............... ..............2

1-3 Active mode locking ................. ...............24................

1-4 Cavity dumping............... ...............24

1-5 Chirped pulse amplification............... .............2

1-6 Rayleigh and Raman scatter ................. ...............25........... ...

1-7 Multiphoton excitation............... ...............2

1-8 Cascade electron ionization .............. ...............26....


2-1 Thre should irradi ance versus pulse wi dth ................. ...............35...........

2-2 Size range of aerosol physics. .............. ...............36....

2-3 Scanning mobility particle sizer ................. ...............37........... ...

2-4 Tapered element oscillating microbalance............... ..............3

2-5 Bernoulli effect on particle focusing............... ...............37

3-1 Laser and detection setup............... ...............46.

3-2 Effect of the HEPA filter. ................ ...............46....... ....

3-3 Note baseline shift. ................. ...............47....... ....


3-4 Drying tower and overall experimental layout. ............. ...............48.....

3-5 Electronics cart. ............ ............ ...............48...


3-6 Target chamber and PMT. ............. ...............49.....

3-7 Comparison of laser pulse and emission lifetime. ............. ...............49.....

3-8 Oscilloscope trace of laser pulse. ................. ...............50.___ ...

3-9 Illustration of near single-mode operation. .....__.....___ .........._ ................50












3-10 Diagram to accompany scatter calculation, using a nominal 500 nm particle close to
the PM T. ............. ...............51.....


4-1 03 November outdoor air background .............. ...............53...._._._....


4-2 03 November outdoor air with HEPA filter ................. ...............53..............


4-3 50 nm Gelman filter. .............. ...............54....


4-4 20 nm Gelman filter. .............. ...............54....


4-5 Short tube background. ............. ...... ...............55..


4-6 Long tube background. .............. ...............55....


4-7 24-hour time series. ............_...... ...............56..


4-8 24-hour time series. ............_...... ...............56..


4-9 24-hour time series. ............_...... ...............57..


4-10 40-hour time series. ............_...... ...............57..









LIST OF ABBREVIATIONS


a Particle radius

A, Transition probability
c From Latin circa, meaning nearby or approximately

c Speed of light, 3 x 10s meters / second
C Constant incorporating partition function Q(T)
cw Continuous Wave
d Beam diameter

D Unfocused laser diameter or particle diffusion coefficient
Do Original droplet diameter
DIAL DIfferential Absorption LADAR
Ei Energy of upper level
f Lens focal length

gi Statistical weight of upper level
h Planck' s constant, 6.6 x 10-34 kg m2 / S
HEPA High Efficiency Particulate-Air

I, Intensity of LIBS spectral line
Io Incident intensity.
K Evaporation constant
L Internal diameter of tubing (4 mm) or resonator cavity length
h Laser wavelength or mean free path (m)

hij Transition wavelength
LADAR LAser Detection And Ranging
LPM Liters Per Minute

LTE Local Thermodynamic Equilibrium
me Electron rest mass
M Molecular species

CL Dynamic fluid viscosity or Dipole moment
MASER Microwave Amplification by Stimulated Emission of Radiation
n Number of modes

No Critical electron density for LTE









n, Number of particles at large distance
ns Nanosecond (10-9 Second)
P Pressure

p Momentum
p Fluid density (1.168 kg/m3 for air at STP)
pP Particle density
pm Vapor density at r,.
pw Power (watts)
pdf Probability distribution function
ps Picosecond (10-12 Second)
r Focal spot radius
o Absorption cross section (m2) Or particle diameter (m)
To Critical temperature
AT Pulse width

trt Round trip travel time in laser resonator
us Mean fluid velocity = 13.26 m/s in these experiments
v Kinematic fluid viscosity
Avesr Free spectral range
Z Degree of ionization









Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Master of Science

REAL-TIME PARTICLE DETECTION USING SUB-THRESHOLD
LASER INDUCED BREAKDOWN SPECTROSCOPY

By

William Paul Mason

May 2007

Chair: Nicolo Omenetto
Major Department: Chemistry

Ambient aerosols play an important role in a variety of processes ranging from

semiconductor fabrication and industrial emission monitoring to chemical warfare and the global

climate. Though interest in particle detection is not new, advancing technology provides

heretofore unavailable methods of particle detection. This work lays a historical foundation for

optical development, compares related particle detection techniques, describes related work, and

exercises the potential of the PowerChip laser in real time aerosol monitoring. Following the

work of Kwabena Amponsah-Manager and David Hahn, this effort relates the versatility and

robustness of the PowerChip laser with test chamber and electronics for real-time aerosol

monitoring. For particles with finite absorption at 1.06 Cpm, the PowerChip laser shows itself a

stable and reliable laser source. With engineering development such as weatherization and

battery power, this instrument could become field portable and remotely operable.









CHAPTER 1
INTTRODUCTION

The inflationary growth of lasers and optical spectroscopy comes only after millennia of

development. The first optical element officially recognized is the Assyrian Layard/Nimrud

lens, dated to 700 BC [BBC, 1999]. However, Robert Temple suggests that lenses existed as

early as 3300 BC in Egypt, evidenced by microscopic carving on a knife handle, and in 2500 BC

by crystal lenses found in the facial wrappings of a mummy [Temple, 2000]. Euclid (325-265

BC) produced the first known writings on optics [Euclid], and both the Greeks and Romans used

crystal and water-fi11ed glass spheres as lenses [Pliny]. The triumph of Archimedes against the

armies of Marcellus is oft-quoted as the first use of optics [Djiksterhuis, 1987]. The veracity of

the claim has been debated for centuries, but Aristophanes (c. 448-c. 385 BC) does mention the

use of a burning glass to start fires in his play "The Clouds" [Aristophanes, 423 BC]. Ptolemy

(AD c. 90-c. 168) measured the changes in the path of light from air to water, air to glass, and

water to glass [Hecht, 2001]. Ibn al-Haitlam, known in the West as Alhazen (965-1040),

developed the first comprehensive alternative to Greek theory. He knew that light traveled in

rays composed of colors and that bodies do not emit visible light, the eyes only receive reflected

light, unless from a source such as a lamp or the sun. Further, he realized that light had a large

but finite velocity and that refraction occurs because light has different velocities in different

media [Lindberg, 1976]. The crystal lenses of 11Ith century Sweden were comparable in quality

to aspheric lenses of the 1950s [Schmidt, et al., 1999]. Rene Descartes developed a theory of

light as a wave traveling through the plenum, a forerunner to the luminiferous aether proposed by

Robert Hooke in 1678. Isaac Newton championed the particle nature of light, but allowed that

particles of light could create waves in the aether to explain diffraction (having been recently

demonstrated by Francesco Grimaldi and later by Thomas Young). In 1845, Michael Faraday









found that the polarization of a beam of light would change in a magnetic field as it passed

through a polarizing medium. In 1847, he proposed that light was an electromagnetic vibration

requiring no medium of propagation. Influenced by Faraday, James Clerk Maxwell ultimately

derived the equations describing electromagnetic waves [Maxwell, 1865]. However, the failure

of the Michelson-Morley experiment to detect the aether, and the inability to explain blackbody

radiation, Einstein's photoelectric effect, and the constant speed of light regardless of reference

frame remained troublesome issues for the wave theory of light. Max Planck in 1900 resolved

the blackbody problem by proposing the quantization of energy, giving credence to the notion of

light as particles, called photons. The packets of discrete energy were called quanta. Thus was

born quantum mechanics. Einstein explained the photoelectric effect by suggesting that the

energy of ej ected electrons was a function of the wavelength of incoming light, while the number

of electrons ej ected was a function of the incoming photon flux (photons per unit time and area).

The invariance of the speed of light was also solved by Einstein with his Special Theory of

Relativity. From the time of Galileo, velocities were considered relative to the velocity of the

observer. Einstein' s theory suggested that this was not truly the case [Thornton and Rex, 2002].

Einstein also derived the so-called "Einstein coefficients" for stimulated absorption and

emission, the first indication that coherent emission may be possible. In short, if a population

inversion can develop with sufficient pumping, coherent radiation will result (Figure 1).

Denoting by subscripts one and two the ground state and first electronic excited state,

respectively, the Einstein coefficients describe both stimulated (B) and spontaneous (A) emission

and absorption. The overall energy balance for a lasing system in a steady state is given by the

change in number of excited atoms through time:

dN,
= A,,N, + B,,p(v)N, B,, p(v)N, AN (1.1)










(8mb~ v) 1
p(V) = (1.2)
e kBT--

However, Al2N1 can be neglected by the Boltzmann distribution, assuming the laser is

operating at room temperature. Once the laser achieves steady state (less than one second), the

number of atoms brought to the excited state by pumping is balanced by the de-excitation of

atoms through both spontaneous and stimulated emission. Einstein's derivation of stimulated

emission, the B21 term, is what initially gave rise to the idea of a laser.

Further development by Nikolay Bosov and Alexander Prokhov laid the theoretical

framework to build a device for coherent production of microwaves, now known as the MASER.

Charles Townes, J. P. Gordon, and H. J. Zeiger built the first maser at Columbia University in

1953. Townes and Arthur Shawlow went on to describe the theoretical basis for the LASER

(using visible light instead of microwaves). Theodore Maiman built the first laser in 1960

[Maiman, 1960]. The advantages of lasers over other sources are manifold, including

monochromaticity, coherence, short pulse length, and extremely high intensities. A host of laser

types exist, from CO2 gaS lasers occupying entire buildings to solid state laser pointers, each with

concomitant advantages and disadvantages. Lasers may be broadly defined as continuous wave

(cw) or pulsed. While cw lasers are useful for applications such as welding and have been

developed to the megawatt level, they are large and costly to operate. Pulsed lasers offer the

advantage of short pulsewidths (femtoseconds) and high pulse energies gigawattss). The

femtosecond regime offers a new class of physics under active investigation by many groups.

Unfortunately, femtosecond lasers remain prohibitively expensive. The JDS Uniphase (now

Teem Photonics) PowerChip laser, a 500 ps solid state system, offers a compact, rugged, and

cost effective compromise.









The generation of short pulses is done primarily by three methods: Mode locking, Q-

switching, and chirped pulse amplification. Mode locking was the first technique developed and

takes advantage of the relation:

27r 1 1
At (1.3)
C Av B bnancidth

Active and passive mode-locking methods are possible, such as electro-optic modulation

(active) or saturable absorbers (passive). Consider the modulator as a weak shutter. By timing it

to coincide with the round trip time of the cavity,


r = 2L(1.4)


A standing wave develops in the cavity. One packet of photons will bounce back and forth

in the resonator, emitting regular pulses and recharged by the pump. The round trip time of

flight in the laser cavity determines the inter-pulse separation.

The pulse duration At obeys the following relation:

At = Av- (1.5)

for Av the gain bandwidth. For a laser with n output modes,

nAl
L = (1.6)


Av = (1.7)
2L

One obtains short pulses by decreasing cavity length or increasing the number of output

modes oscillating in phase. Thus the PowerChip has very short pulses because the cavity length

is short the round trip time of flight in the laser cavity determines the inter-pulse separation.

Since the number of modes that can oscillate depends on the Doppler width of the transition and

the cavity length, optical modulators inside the resonator can cause active mode locking.









Saturable absorbers achieve passive mode locking, as with the PowerChip. To produce more

energetic pulses, Q-switching is used to contain the intensity of emitted photons. Here the

energy in the resonator builds up to a threshold determined by optical switches or nonlinear

crystals. However, active Q-switching requires significant electrical power and is difficult in

practice. In passive Q-switching, the pulse repetition frequency can be changed simply by

changing the pump power, which changes the amount of time needed to reach a threshold in the

passive switching medium.

Passive mode-locking is possible using a Kerr lens that absorbs low intensity light while

passing high intensity transient pulses, leading to mode-locking.

Passive Q-switching depends on the saturable absorber becoming transmissive at a certain

threshold of photon intensity and dumping the photons in one large pulse. The setup is the same

as for active Q-switching, sans drive electronics (Figure 2).

Electro-optic devices can also be used to "dump" the cavity all at once, a form of Q-

switching. For example, Pockels cells will change the path of light in response to a change in

applied voltage across certain faces of the nonlinear crystal (e.g. potassium dihydrogen phthalate,

or KDP). Thus, the population inversion can be built up and then dumped at will (Figure 3).

Active Q-switching is brought about by controlling a saturable absorber, which controls

the quality of the resonator and therefore the transmissivity of the resonator. This change in

transmittivity is tantamount to a change in the quality of the resonator, hence the name Q

(Quality) -switching [McClung and Hellwarth, 1962]. The pump creates a population inversion

with only a small number of photons circulating. When the signal is given, the saturable

absorber becomes transparent and the excited atoms relax, falling into phase with the small beam

of lasing photons to create a giant pulse that exits the resonator all at once (Figure 4).









Chirped pulse amplification is common with femtosecond lasers and essentially stretches

the pulse in time so that it can be amplified without damaging the optical system. The amplified

pulse is then recompressed to exit the resonator (Figure 5). Once the pulse leaves the laser,

several processes can take place. Overall, they fall into three categories: absorption, scatter, and

transmission. The normalized energy distribution equals unity: absorption + scatter +

transmission = 1. Working backwards, transmission is the trivial case. The laser must have a

backstop of some sort to remove excess photons in the event that an absorbing or scattering body

is not present.

Scattering represents a more complicated picture. Perhaps the most well known form of

scattering is Rayleigh, in which photons are elastically scattered in preferential directions as a

function of frequency to the fourth power, hence blue light is scattered moreso than


red I~aylerg o~ (-) (1.8)


The primary direction of scatter is normal to the incident path, thus the sky overhead

appears blue while sunrise and sunset appear red. In Rayleigh scattering, the atom is excited to a

virtual state lower in energy than the first excited electronic state and rapidly de-excites, emitting

an identical photon in a preferentially radial direction. Rayleigh scattering is dominant in

particles of diameter less than or equal to the incident wavelength (Figure 6).

Raman scattering is the inelastic counterpart to Rayleigh scattering and also involves

excitation to virtual states. Since the transition probabilities are much smaller than Rayleigh

scatter, Raman has very low amplitude. Despite this low intensity, Raman scattering has many

important applications such as non-destructive artifact testing.

Now Mie scattering applies primarily to particles of diameter equal to or larger than the

incident wavelength and results in preferential scatter along the direction of transmission. Mie









scattering is not highly wavelength dependent, so scattered light appears white, as in clouds and

fog.

So far the interactions described have dealt with uncharged particles. Consider now a

photon interacting with a charged particle, say, an electron. This interaction is defined by

Compton scatter, which does not typically occur with visible wavelength photons because their

energy is too low to overcome the atomic binding energy. However, X-ray photons have plenty

of energy and can lose energy to electrons:

hv h
p = (1.9)
c i


A1 h (1.10)
p mec


A = h (1- csB), (1.11)
m~e

for 6 = radiation scattering angle. By considering wave theory, AL can arise due to the

Doppler effect [Ditchburn, 1991].

Thomson scatter involves the interaction between a photon and a free charged particle,

though only in the plane of polarization of the incident photon. The magnitude of the oscillation

varies as (cos a), where a is the angle between the incident light and the observer. Such

scattering can give rise to a polarization effect.

Brillouin scatter occurs when light changes its vector due to density changes in its path.

Such density changes can arise from acoustic modes (phonons), temperature gradients, or

pressure gradients. Brillouin scattering occurs in a Pockels cell when using acoustic shutter

frequencies to produce Q-switching.










Photoacoustic scatter is the process wherein light strikes a surface and creates sound

waves. Though first noted by Alexander Graham Bell [1881], it was not developed until the

1970s [Rosenewaig and Gersho, 1973]. In essence, the photon source is modulated at an

acoustic frequency, say, 1 k
transfer their energy to the phonons, resulting in acoustic frequency signals.

LADAR is a comparatively new but burgeoning field. Aerosols at large distances (km)

can be interrogated by scatter and absorption measurements. Information about particle

concentration and composition becomes available upon comparison of retro-reflected scatter at

one wavelength versus a different wavelength, a technique known as DIfferential Absorption

LADAR or DIAL. While standoff analysis of aerosols is important and continues to grow as a

field, interesting physical processes arise when the laser energy is increased to the point where

plasma forms.

Plasma composes 99% of the observable universe. Methods for analyzing plasma

emission are well characterized, though plasma spectroscopy is still subj ect to significant

background in many situations. Given this, consider now the application of lasers to particle

detection by plasma formation, which amounts to looking at the final term, absorption. The basis

for LIBS and the present particle detection scheme rests on absorption of laser photons into

particles to cause their ionization and emission.

Two years after the first laser was built, F. Brench and L. Cross proposed the theory of

LBES [Brench and Cross, 1962]. In 1967, Moonke and Moenke-Blankenburg built the first LIBS

instrument [Cremers and Radziemski, 1989]. Though the complete process of LIBS is still not

fully understood, great advances in its application as an atomization/ionization source have been

made.









Consider the absorption of photons into a particle. Classically, one may imagine photons

of light striking the surface of the particle like bullets hitting a target. The larger the target, the

more likely the bullets are to strike. The size of the target is quantified by the absorption cross

section o. In practice, this is related to but not necessarily the same as the geometrical cross

section of the particle. The absorption cross section in essence describes the likelihood of

interaction with a photon of given wavelength. A more meaningful analogy is available in terms

of resonance. The photon interacts with the outer shell electrons of the atom. The electrons can

be seen as point masses vibrating on springs with spring constant k a measure of the strength of

the electron's binding energy. Loosely bound electrons will have a small k and will interact with

relatively low energy photons. Depending on the energy and number of photons, the electron

will excite to a higher energy level and then relax through fluorescence, phosphorescence, or

collisional de-excitation. In multiphoton excitation, complete ionization is possible given

sufficient photon flux (Figure 7). Since the density of free electrons in most materials at STP is

negligibly small, initiation of cascade ionization requires some sort of catalyst such as

multiphoton excitation in a laser.

Multiphoton ionization: nhy + M M' + e- (1.12)

Once free, the electron is accelerated by the electric Hield of the laser, leading to cascade

ionization wherein it collides with an atom or molecule and knocks loose another electron, both

of which then accelerate in the electric Hield and repeat the process, forming a geometrically

growing electron cascade [Radziemski and Cremers, 1989] (Figure 8).

Again, for LBES, photons interact with the electrons of the material. The electrons are

excited to high temperatures in femtoseconds but transfer the energy to the phonon lattice,

resulting in a shockwave and explosive removal of material in a plasma state. In the ns-ps









regime, the laser pulse continues to excite the ej ected material as it leaves the surface, heating

and ionizing the ej ecta to a plasma. This forms an optically opaque plasma with a temperature in

excess of 10,000 K. The plasma temperature can be calculated by solving the following equation

for temperature [R. Harmon, et al., 2005]:


I ekB': (1.13)


For ultrashort pulse lasers (< 1 ps) interacting with a solid surface, all the energy is

deposited at once into the electron lattice, which transfers the energy to the phonon lattice,

resulting in explosive removal of material with virtually no melting, though some researchers

have found extensive ionization [Martin, et al., 2002] (Figure 9).

After about 1 Cps the shock wave decouples from the plasma, leaving the plasma in local

thermodynamic equilibrium [Zeng, et al. 2006]. Thermodynamic equilibrium is defined as a

zero gradient for all intensive properties of the system (temperature, chemical activity, pressure,

etc.). For a gas, this is tantamount to having a specific Maxwell-Boltzmann distribution. Such a

condition is virtually impossible to achieve in LIBS plasma it is a non-equilibrium

phenomenon. However, the approximation suffices for many instances. LTE implies that,

though the system parameters vary across space and time, they vary slowly enough to permit the

assumption of thermodynamic equilibrium about any given point instantaneously.

Thus, the 58 pIJ of the PowerChip may or may not allow LTE. One may use the Griem

criterion to determine if LTE exists [Yueh et al., 2000]:



No~lcm ) >> 30,545x10 I[To- /(K 7n) Z (1.14)

Given a plasma in LTE, the problem of spatial measurement arises. Though the present

effort does not involve LIB S per se, an understanding of the processes at work is instructive.









When an intense ultrashort laser field propagates inside a dielectric medium, it induces a strong

polarization field and high density of electrons and holes. This space-time dependent problem is

intricate because it involves nonlinear effects such as multiphoton excitation, free carrier

absorption, photoemission, electron-phonon interaction, exciton generation, and carrier-carrier

interaction, all in the presence of a high intensity field [Audebert, et al., 1994]. Additionally for

natural substances, the situation is further complicated by inhomogeneous sample composition

and surface irregularities [Harmon, et al., 2005]. Variation in composition manifests as variation

in laser-target coupling convolved with surface roughness variation.

Given such difficulties, why bother with LIBS at all? Though LBES is often touted for

little or no sample prep, caution must be exercised in some cases. For example, inhomogeneous

matrices can present spectral complexity and create difficulty in interpretion. Further, heavy

surface contamination such as grease or dirt can cause wide variation in signal intensities and

interference. Surface films and surface roughness can also create skewed results. Bearing these

caveats in mind, particle counting and the natural extension to LBES offer many advantages such

as light weight, solid state electronics, no vacuum requirement, and real-time analysis.

Comparison to other techniques shows the usefulness of LIBS as summarized in Table 1:

Armed now with a conceptual understanding of plasma formation and LIBS, the properties

of particles may be addressed. For small particles, some transmission and scatter may occur,

depending on the shape and composition of the particle. For silicon dioxide (SiO2), We aSSume

that transmission and reflection are negligible. Because the pulse duration is so long (500 ps)

relative to the time it takes to ej ect material from the bulk (femtoseconds), the initial part of the

pulse excites electrons and breaks up the particle into clouds of molecules which continue to be

irradiated by the later part of the pulse to form a plasma. The emission from this plasma,










Bremsstrahlung, fluorescence, and recombination, can be used for simple particle detection by

the photomultiplier tube. The photomultiplier tube is composed of a material that emits electrons

when struck by photons of sufficient energy. In the case of the Hamamatsu R 647, the material

responds to photons with wavelengths between 300 and 650 nm. The response is controlled in

part by the applied voltage, a sensitivity selector of sorts. The price of high sensitivity is an

increase in false hits from, for example, cosmic particles or noise fluctuations that are amplified

by the high voltage required for sensitive measurements.

Particle detection is desired in a host of applications, from semiconductor fabrication to

chemical/biological warfare agent detection and global climate modeling. The following chapter

details particle characterization and detection.

Table 1-1: Comparison of Particle Counting Techniques.


Technique
LIBS
MS
CPC
Filtration
Scatter


Sensitivity
Low
High
High
Moderate
High


Range Portability
Moderate High
High Low
Low Moderate
Moderate Moderate
Low High


Ease of
Use
Moderate
Low
Moderate
Low
Moderate


Data
Complexity
High
High
Low
Moderate
High


Efficiency
Low
Low
High
High
Low


Internal Conversion
(Vibrational Relaxation)

Fast Intersystem Crossing (Singlet to Triplet)
Excited Triplet


Lasing Photon


Deexcited Triplet

Fast Intersystem Crossing



iagram


En< rgy

Singlet Excited State




Excitation Photon


V~ibrtina Stte

Singlet Ground State

Figure 1-1: Laser Energy D








100% reflector


99% reflector


Gain Medium Saturable Absorber


Figurel-2: Passive Q-Switching


100% reflector


99% reflector


Gain


Acousto-Optic
Modulator


Figure 1-3: Active Mode locking


100% reflector


100% reflector


Gain


Pockels Cell


Figure 1-4: Cavity Dumping


~





Figure 1-5: Chirped Pulse Amplification

Image Credit: http ://www.nsu.ru/psj/lector/lotov/terawatt/cp~i


Energy


Virtual State


..... r----zr....


Figure 1-6:


Rayleigh and Raman Scatter.


I :
i STRETC=HER

i


i i


-$QO k





Energy


hv


""""""' Virtual State


hv


Figure 1-7: Multiphoton Excitation


O




Two
electrons


O




Incoming
electron


Neutral
molecule


Positive ion


Figure 1-8: Cascade electron ionization





















Material ejected


Pulse strikes surface


matter


Brehmsstrahlung emission LIBS (> 1 Cps)
Figure 1-9: Laser-material interaction.









CHAPTER 2
PARTICLE DETECTION


The term aerosol derives from the term for hydrosol, meaning a solid colloidd) suspended

in liquid (solution). Aerosols are important to many aspects of life, including health, global

climate, and military weapons. Epidemiological studies found associations between particulate

air pollution and human health [Schwartz, et al., 1996]. People for some time argued that air

pollution only sped up the inevitable, killing only those who would soon die anyway. However,

though mortality increases as air pollution increases, it is not followed by a deficit when

pollution decreases, implying that pollution not only harvests from the vulnerable pool, but

recruits new people into the pool. [Zanobetti, et al., 2002].

Aerosols also have an impact on the environment. Their primary effect is to alter the

scatter and absorption of solar radiation, leading to either warming or cooling depending on the

fraction scattered versus absorbed. The secondary effect manifests by altering the scattering

properties and longevity of clouds [Penner, et al., 2001]. Without aerosols in the atmosphere,

very few clouds would form [Zalabsky, 1974]. As aerosol number increases in a cloud, water in

the cloud is spread over many more droplets, each of which is proportionally smaller. Clouds

with smaller droplets reflect more light and last longer, since it takes more time for droplets to

coalesce and fall.

According to Raes, et al. [2000], primary particles can be emitted directly into the

atmosphere as particles (primary process) or formed in the atmosphere from gas-to-particle

conversion (secondary process). Atmospheric aerosols range in size from a few nm to Cpm in

diameter. Once airborne, particles evolve in size and composition through condensation,

evaporation, coagulation, chemical reaction, or activation within supersaturated water vapor to









form cloud and fog droplets. Particles smaller than one Cpm range from 10 10,000 cm-3, while

particles with diameters greater than 1 Cpm are typically < 10 cm-3

A primary aerosol is emitted into the atmosphere as a particle, whereas secondary aerosols

are formed in the atmosphere by gas+ particle conversion [Raes, et al., 2000].

Particle formation can be categorized by diameter (cp):

cp > 1 Cpm = primary formation

cp < 1 Cpm = secondary formation

Strong overlap exists for particles of diameter 0. 1 1.0 pm.

Combustion soot is typically 5 20 nm, but coagulates rapidly to form fractal aggregates

which collapse to more stable structures of tens of nm due to the capillary forces of condensing

vapors. Shah et al. [2005] found that lubricating oil was a primary fraction in diesel emission,

and increased while the engine was accelerating, versus cruising.

The Kelvin effect plays an important role in particle formation. The equilibrium vapor

pressure over a spherical particle increases with decreasing radius of curvature; hence

equilibrium vapor pressure above molecular clusters formed by random collisions is much larger

than that above a film or flat surface. Consequently, molecular clusters tend to evaporate. Small

particles (<1 Cpm) diffuse to the Earth's surface, a process that becomes less efficient with

increasing cp. For 0. 1 < cp < 1 Cpm, dry removal is very slow, so these particles tend to accumulate

in the atmosphere. They are removed mostly by cloud activation and precipitation [Willeke and

Baron, 1993].

Particle behavior and composition are weak functions of the particle diameter (Tables 2

and 3).









Table 2-1: Particle mode versus diameter.
Diameter (um) Mode
<0.01 Nucleation
0.01<(p<0.1 Aitken
0.1<(p<1 Accumulation
cp>1 um Coarse


Table 2-2: Particle composition versus diameter.
Diameter (um) Composition

(p<0.1 Carbon
0.1<(p<2.0 Sulfate, nitrate, heavy metal

(p>2 Geologic material, pollen



To develop a new particle detector, consideration of previous devices is important.

Important work on the detection of aerosols began with Aitken in the 19th century [Aitken,

1923], who determined that most atmospheric aerosols were less than 100 nm in diameter and

ranged from hundreds to tens of millions per mL depending on the cleanliness of the air.

Interestingly, the Wilson cloud chamber was developed as a result of Wilson being moved by the

sighting of a Brocken specter while working at the meteorological observatory atop Ben Nevis in

Scotland. So struck was he that he studied cloud formation and condensation in the laboratory.

The result was the cloud chamber, which is also one of the most sensitive particle counters for

aerosol measurements. Particle detection methods encompass a range of responses:

Aerodynamic particle sizer measures the velocity of particles in accelerating air
flow using two laser beams and scatter detectors at various angles.

Photoacoustic spectroscopy chopped laser illuminates ambient air. Particles
absorb energy from the beam and transfer it as heat to surrounding air. The
expansion of heated gas produces a sound wave at the same frequency as the
chopper. This acoustic signal is detected by microphone and is proportional to the
amount of light absorbed.










Electrical aerosol analyzer collects particles according to size dependent mobility
in electric Hield, then detected by deposition of charge on an electrometer.

Differential mobility particle sizer classifies particles by their mobility in an
electrical Hield and counts them with a condensation nuclei counter in a range of
size bins.

Scanning mobility particle sizer a complex version of the Differential Mobility
Analyzer (Figure 11), including a radioactive ionization source and Condensation
Particle Counter. Because the Condensation Nuclei Counter (CNC) cannot classify
particles by size, it is combined with the DMA to give both particle size and
number. One drawback of the SMPS is that it can take up to 300 s to obtain size
distributions, since the particles need time to form a hydration shell in order to be
detected in the chamber. Further, CNCs are overwhelmed by particle
concentrations greater than class 1000 environments [Particle Measurement
Systems website, 2006].

Electrical low pressure impactor offers real time size distribution and
concentration measurement from 30 nm to 10 pm. The electric current carried by
charged particles into each impactor stage is measured by a sensitive electrometer.
One anticipates the difficulty of measuring charged particles at low pressure
(requiring some measure of pumping), and interference effects from electronic
noise. However, it has the advantage of being able to measure rapid changes in
both particle size and concentration.

Tapered element oscillating microbalance operates by changing the frequency
of oscillation as mass accumulates on the cantilever (Figure 11). As particles

accumulate the frequency co changes as co, = kI- The balance lasts for about 3
\m
weeks, with a V/2 hour equilibration time before data can be taken.

The classic methods of atomic absorption/emission spectroscopy are well-characterized,

but suffer heavy background emission and comparatively low sensitivity. Scattering, absorption,

and emission techniques such as Rayleigh, Raman, Fourier-Transform Infrared Absorption,

LBES, LAser Detection And Ranging (LADAR), fi1ter impaction, and mass spectrometry offer a

host of techniques for particle detection. While each technique has some advantage and some

disadvantage, LBES offers a few important advantages over other techniques. Mass spectrometry

provides high resolution and good limits of detection, but requires the use of vacuum pumps,

adding to the cost and bulk of the system, and experiences peak broadening due to excess kinetic









energy [Tolocka, 2004]. Light scattering techniques are effective only to the point at which the

particle diameter equals the wavelength. For particles smaller than the wavelength used (below

~300 nm), such techniques are less reliable [Maynard, 2000]. Techniques like LADAR offer

remote detection, but have poor sensitivity and minimum detectable particle sizes (~300 nm).

CNCs are sensitive, but require periodic refilling with alcohol and are easily saturated in dirty

environments. LIBS provides a good detectable size range and good portability, for the price of

sensitivity. However, particle focusing could ameliorate this problem [Wu, 2006].

Research has shown a bimodal particle distribution in the atmosphere, while other research

has shown that particles in the accumulation region (around 0.1 Cpm in diameter and smaller) are

most harmful to humans. These are also the most difficult to count continuously and the most

difficult to filter. Filtration and detection methods are many and wide ranging, from Raman and

fluorescence to condensation nuclei counters and impaction.

Having considered particle counting, particle transport bears remark. Particle transport

both through the air and through tubing is complex. Physical phenomena such as

thermophoresis, turbulence, and adhesion are factors. Velocity focusing, diffusion, and

Brownian motion also play into the scheme (Figure 12). Brownian motion of particles results

from collision with other particles whose velocity is proportional to the square root of

temperature. Equating kinetic energy and the thermal energy gives a relation between velocity

and temperature:

1,3
-my k,T (2.1)
2 2

v =~i (2.2)









Because their mass is so small, Brownian particles do not settle from a given volume; they

are kept "afloat" by the thermal motion of the particles around them. In other words, they are

perpetually diffusing at a rate given by:


kgT
Vm (2.3)


Thermophoresis is brought about by temperature gradients in a given volume. Higher

temperature will increase the volume between particles, causing cooler particles to move to

cooler regions of the gradient. Note that energy must be added to the system to maintain the

gradient. Similarly, eddy and turbulence focusing arise from pressure gradients, similar to

thermophoresis. In addition to various transport phenomena, particles may adsorb to surfaces by

the following two processes:

Electrostriction may arise between particles with a charge or a strong permanent dipole

moment. In liquids, such charges will be solvated, but in the gas phase electrostriction can be a

significant factor. Electrostriction between two or more particles, usually of opposite sign, is a

phenomenon known as accretion.

Simple friction can play a role, such as in HEPA filters, where particles are mechanically

trapped by small gaps in a medium. Diffusion and adhesion coexist in a dynamic equilibrium,

suggesting that a sudden shift in some relevant parameter (e.g. temperature or pressure) could

shift the particle equilibrium and result in a "pulse" of free particles or a sudden shift in average

particle diameter. Also, particle concentration decreases with increasing tube length. The rate of

decrease depends on several factors, including type of tubing [Willeke and Baron, 1993].

Carranza, et al. [2001] found that particle transport efficiency was greater than 95% for particles

from 0.1 to ~1.5 Cpm in diameter for their system.










Deliquescence, a sharp rise in liquid water content at ~ 55% relative humidity, is a result

of the hygroscopicity of particles and is relevant for many salts such as calcium chloride and

magnesium chloride. Because the particles are strongly hygroscopic, under high humidity they

can absorb enough moisture to dissolve themselves. This could be a factor in situations where

particle solutions are created and then dried to form aggregates. The ability of the tower to dry

such salts may be questioned.

Clearly a host of physico-chemical processes are at work. Desirous to further develop

particle detection capabilities with the PowerChip laser, several experiments were performed.

Note here that M.D. Cheng [2003] failed to produce meaningful results from the reference

standards while testing the sub-threshold setup similar to the present study. The present work

may add further strengthen the overall effort of particle detection with lasers. Though this effort

recapitulates much of Cheng, Hahn, and Amponsah-Manager' s work, the use of the PowerChip

laser and the discussion of particle distribution (Poisson versus diffusion mediated) may lend

insight into the investigation.

LBES offers some advantages because it can both count particles and characterize them

by constituents. Several particle counting systems also provide spectral analysis (e.g. Hahn

[1998] and Cheng [2000]). Cheng proposed the use of sub-threshold breakdown for aerosol

detection (akin to sub-threshold breakdown in liquids). That is, the laser power is set just below

the breakdown threshold of air. When a particle enters the laser focus, the breakdown threshold

of air is decreased, causing plasma formation. The tacit assumption is made that the absorption

cross section of the particle is greater than that of the background carrier gas. The advantage of

using sub-threshold pulses is a reduction in background noise, since a plasma forms only in the

presence of particles of nontrivial absorption cross-section. Following this line of work, we











sought to further characterize and expand the capabilities of such a system. The next chapter

describes those efforts, including initial development, enhanced chamber setup, aerosol nebulizer

and flow gas, and long term measurements.



Fr-3rTauwr. GP~rf Wy iles slaser-lnd..rt~PMII sbeadown lm:.r19"F


loso


10-9 10-8 10-7 10-6 10-5 10-4
PULSE TIME TIME TO BREAKDOWN (SEC)


Figure 2-1: Threshold irradiance versus pulse width.





















001 0.1 10 le LOD

Suctng SkEswi~tinkpearecU.,/ Sloke ITransinc

Charyqp Dilfumn / Combined / Field




nmodesc


on,u. Ave hrolr ~ TB~ Hr*eAd Nont~iad


depostulon Datuden imp ;Youdonadatin



Sunphq D altrnkesu /ArnokMIC onesu



FNtunnon~l Dllffusn D.I LPEU~IrnahnnO necpo


runpligl




0.01 0.1 LO IQ 100
Panlcdedinner(pm)



Fnlure 2-2. Slze ra nsc ohercrool oroucntio urnilticrt imm i nli.d l982)



Figure 2-2: Size range of aerosol physics.


Aerosol Flow








Figure 2-3: Scanning Mobility Particle Sizer.

SElectronic driver/recorder


Figure 2-5: Bernoulli Effect on particle focusing.


Figure 2-4: Tapered Element Oscillating Microbalance.

Gas particles bouncing against a surface at low velocity


Gas particles bouncing against a surface at high velocity









CHAPTER 3
EXPERIMENTAL SETUP AND DEVELOPMENT

Manager [2005] worked on particle detection with the JDS MicroChip laser and nebulized

solutions of simple salts. Particle focusing occurred by nebulizing and desolvating particles,

then passing the air flow through a narrow pipette tip. The stream was interrogated by a 5 k
MicroChip laser at 50 CLJ / pulse. Results were promising but inconclusive. Inspired by the work

of Smith, Hahn, Omenetto, Amponsah-Manager, and Cheng, the present study investigated the

feasibility of the PowerChip laser for ambient aerosol monitoring at 1 k
The PowerChip laser offers the advantage of relatively high repetition rate, short pulse

width, solid state passive Q-switching, air cooling, and short cavity length, yielding essentially

single mode output. Again, all of these properties play well into a field portable design as well

as complimenting the LIBS detection. The PowerChip laser is amenable to particle detection

because its short cavity length allows longitudinal mode spacing greater than the gain bandwidth;

it enj oys virtually single mode operation with no frequency beating (Figure 16).

Bv, 1 31


While Amponsah-Manager [2005] worked mostly with solids, some particle sampling was

done. This work extends his initial investigation. The test chamber (Figure 17) was built and

used in all subsequent experiments. Several modifications were made over time to improve its

performance. Light tightness is critical since the PMT is sensitive and low background is

desirable. Tightness was improved through the addition of a laser beam tube, black electrical

tape around the housing seam, and an aluminum guard ring over the laser entry port. A mirror

placed opposite the PMT (in the bottom of the chamber) reflected light towards the PMT to

increase signal. In the future, an elliptical mirror with one focus at the laser focus and the other









focus on the PMT might prove a better choice. Lastly, the chamber was wrapped in black felt to

further reduce stray room light. Note that a beam expander would improve the focus

characteristics (sharper focus), though this might push the system into breakdown regardless of

whether a particle was present and would decrease the effective plasma volume. Depending on

the desired experimental setup, a variable diameter beam expander could serve as a simple power

control. In the limit of perfect system function, particles below a given diameter (all other

parameters, e.g. absorptivity, being equal) could be made "invisible" to the system. If the

absorption cross section is too small to create a plasma, the particle will pass undetected. Above

some minimum diameter, the cross-section will be large enough to form plasma.

The signal passed through an SR 570 amplifier, boxcar, SR 245 signal processor, and out

to a computer for data storage and display (Figure 16). Initial tests demonstrated the

effectiveness of HEPA filters at reducing the number of events detected in ambient air (Figure

14).

The system measured the emission of light from plasma formed when a particle was struck

by the laser pulse and converted to plasma. The number of photons expected for nominal 500

nm diameter, 2% (weight) silicon dioxide particles is given by:

62g 1000cm3 I
100x10b = 2mg silica in 100 CLL solution.
100g 1L cm3

particle 9 particles
0.002g -= 2.9x10
6x10-14g mL

2.9x109 particles 0.1mL I min = .x0 particles
mL min 60 sec sec

Ambient air traveled through the target chamber by laboratory exhaust flow at roughly 0.2

LPM as measured by a rotometer. If the PMT detected sufficient photons, the signal continued









through preamplifier, boxcar, and postamplifier before readout on a stripchart or storage in

computer memory. Data processing included counting the number of events greater than a

certain threshold. Initially this threshold was set at 3 standard deviations (30) from the mean, but

minor (< 0.05 V) step fluctuations in the background forced a change to the less sensitive but

more trustworthy measure of 0.2 V as threshold (Figure 15). This was well above any baseline

fluctuations. A power supply regulator (SOLA MCR 1000, Mini/Micro Computer Regulator,

catalogue # 63-13-210-05) was installed to eliminate background shifts, but was unsuccessful at

doing so. The threshold remained at 0.2 V.

Computer sampling rate:

1,000shots 3,600 sec
Expect: -24hours = 8.64x107 shots
sec hour


Found: 3,317 fls-1,0 iles = 4.98x107shots
sec

Ratio = 0.576 ~ 58% duty cycle.

This is very repeatable for all runs, leading to the conclusion that it represents the computer

writing data to memory, during which time it cannot simultaneously take data.

The amount of light scattered by the particle into the PMT we expect to be negligible

compared to the 0.2 V threshold. The following calculation bears this out. Because the particle

diameter (500 nm) is less than the laser wavelength (1,064 nm), Rayleigh scatter dominates.

Assume a 900 cone of emission circling the equator of the particle normal to the laser beam


containing all of the scattered light. Further, assume that approximately one quarter (- sr ) of


the emission is detected by the PMT:

mJ
For a 58 1064 nm laser,
pulse










3x108 na
v=C = 2.83x10'4 s1
Ai 1.064x10-6


E,, = hv = 6.6x10 3 g-n 2.83x10 's = 1.87x10-19.


6J ph ,,photons
58x10b =3.1Ix10
pulse 1.87x10-19 pulSe

ExpectedSignal =
Ph Pulses =
~ref lectivity2til attenuation, EfficiencyP~f Gain Angle, re
pulse pt sec


pulse 4 se ),,,~ sec


,,electrons Coulomb
ExpectedCurrent = 4.9x10 = 2.94x10 A
sec iselectrons
1.66x10'
sec

Expectedyoltage = 2.94x10 'A- 5002 = 14.7pVy

This is well below the 0.2 V threshold for particle hits. Using conservative values for the

reflectivity, attenuation, efficiency, and hit rate, we are confident that we do not detect

significant scattered light.

Now consider the Reynolds and Knudsen numbers regarding flow characteristics:


Reynolds Number = Re U, (3.2)


kg na 0.004n?
R, = 1.168 13.26- = 3480.4
e a kg
m S 1.78x105
na -s












The results suggest that the flow is unstable. This finding prompted a change to a longer

drying tube, permitting a lower drying gas flow rate because the particles would have more time

to dry.

Ai k,T
Knudsen Number Kn (3.3)
L (rZ~o PL)


1.38x1023 x295K
Kn= K -50x10-9n t101,300Pa =1.8x105
1/~(100x10 -9 2)

Therefore continuum mechanics are valid (for Kn < 1), as expected.

Consider now the time to to completely evaporate water:

D,
to [Hahn et al., 2001] (3.4)


(500x10-9 m2
Then to 7.14x10 's = 71pUS .
3.5x10-9


The lower flow rates should also help reduce effects such as velocity and eddy focusing,

plus increase the dwell time within the PMT viewing angle.

The particle dwell time can be calculated as follows:

Dwell time = (Flow rate) (Interaction volume)

-mi -e / 0.2cn??)- (0.2cnt)= 1.5x10- 'SOC
10,000cn? Imin

This should provide plenty of time for the detectors, which has a response time of a few

nanoseconds.

When the composition of particles is considered, the difficulty of matrix effects comes to

bear. Defined as variations in laser-target coupling secondary to variable sample composition









and surface characteristics [Harmon, et al., 2005], matrix effects can yield transients in plasma

emission that could affect the observed hit rate. Indeed, Harmon found that surface roughness is

a primary factor in experiment repeatability. The surfaces of aerosols are known to vary greatly

from spherical to needle-like [Hinds, 1999].

A final consideration is the distribution of particles, taken to follow a Poisson distribution



eZil
P(x) = (3.5)


The Poisson distribution assumes no correlations in either space or time, an assumption

which may be called into question by point source emission of particles, e.g. from a vehicle

passing close to the detector. The diffusion equation may be a more appropriate model. The

properties of the diffusion equation are complex, but a brief treatment follows. For a probability

distribution function of a single particle P, use the heat equation:

PT = DAP (3.6)

If the diffusion coefficient D is not constant but depends on P, then one gets the nonlinear

diffusion equation. The random traj ectory of a particle subj ect to the particle diffusion equation

is Brownian motion. To treat it, place a particle in R = 0 at t = 0 and find the pdf associated with

R to be:


P(R, T)= G(R, T) = (4i ~t)e e (4D) (37

For R2 = Rx2 Ry2 Rz2 (3.8)

At t = 0, P(R, T) is singular with a pdf for the particle at R = 0 given by the Dirac delta

function. The solution of the diffusion equation subject to this initial condition is the

Green Function G(R, T) given above. This treatment can be extended to a large number









of particles by a decomposition of Green functions giving the time evolution of the

particles. Such a decomposition can be generalized to any diffusive process like heat

transfer or momentum diffusion, which is the phenomenon at the origin of viscosity in

liquids. If the Poisson distribution does not hold, the diffusion equation could serve as a

new model [Willeke and Baron, 1993].

Having laid a theoretical foundation, a review of the experiments conducted is indicated.

Many experimental series were run to determine the effectiveness of particle detection with the

PowerChip laser. The first was with a HEPA filter on ambient outdoor particles. The hit rate

showed a marked decrease with installation of the filter, suggesting that the detection system was

functioning nominally (Figure 14). A drying tower was assembled and operated using

compressed air (Figure 16).

The time responses of the laser beam and the detector were measured to verify that plasma

and not scattered light was detected. Because the plasma lifetime is long compared to the laser

pulse microsecondss versus picoseconds, respectively), we conclude that plasma, not scatter, is

observed. Further, a calculation of expected scatter shows that it should produce a negligible

signal at best (Figure 18).

2ilf 2 1064x10-9 0.02nt)
Calculation of Beam waist = coo = r .x06 a=27p

58x1-6 xx0m 27x0m2 7w
58x16J 1064x10-9)
Photon flux: 'P= _, 500x10 s -1040 photon
zi3h (2.7x10 -6 n 6.6x10-34 k m 3x108
s s

The expected particle hit rate may be estimated as follows:


For 2 % by weight silicon dioxide particles of 500 nm diameter and density = 1.05
mL










2g 1000nzL g
100x10-6L -1.05 =
100g L mL

For the mass of one particle, find the voli

V=4 ar3 4 a 250x10- cm .51
3 3

Then find the mass of a single particle:


0.0021 g of silica in 100 CLL of solution.


6.55x10-14 3 -1.05 g
mL


6.9x10-14


To give the total number of particles in 10 mL:

particle
0.002 g -= 2.9x10' particles
6.9x10-14

Calculate the flow rate using the nebulization rate:

2.9x10"' particles 0.100nzL 30, particles x0 particles
10nzL min min sec

Given a counting efficiency of 1:106, this amounts to roughly 5 hits per second.

Experiments were performed using undiluted samples and approximately this rate was

observed. A more sensitive detector or higher particle concentrations must be use to see

significant hit rates.










Table 3-1: Flow regimes.
Reynolds number, Re Flow
<2300 Laminar
2300

>4000 Turbulent




















Figure 3-1: Laser and detection setup.

10 HEPA Fi
160 events/s



8-













0 50 100 150
Time. a


Figure 3-2: Effect of the HEPA filter.


200 250 300


[Reprinted with permission from Xihong Wu]

ilter




10 events/s












Nubulizer 2 LPM II
26 April 2006


1


0 10000 20000 30000 40000 50000 60000
Laser Shots


3.0
2.-

2.5
S .-

2.0 -
0.-



1.0 -


Figure 3-3: Note Baseline Shift.


1 ii 1 / I I





























Figure 3-4: Drying tower and overall experimental layout.


r.u~"'~
.I.....iiiiiirrr~-
; ; ;iiii----;;--
............ .....:: .:-::::::--;;;;;;;;;;I*-"~
.*....------------'"'


Figure 3-5: Electronics cart.


i











































Figure 3-6: Target chamber and PMT.


0.02







-0.04


Laser pulse


typical particle hit


Time, ns


Figure 3-7: Comparison of laser pulse and emission lifetime.
















0 02 -


0 00 -


a, -0 02 i




-0 06 -


-0 08 ~


-0 00000010 0000000 00000001 00000002 00000003 00000004 00000005
X Axis Title




Figure 3-8: Oscilloscope trace of laser pulse.


Pump
Energy


Free spectral range


Resonator Modes


Figure 3-9: Illustration of near single-mode operation.





















Figure 3-10: Diagram to accompany scatter calculation, using a nominal 500 nm particle close
to the PMT.


PMT


Laser O









CHAPTER 4
RESULTS

Experimental results over the past 18 months served to characterize and improve system

performance, find environmental trends, and determine limits of detection for particle size. Note

that the limit of detection for particle concentration is poorly posed because detection with

decreasing concentration simply implies a longer time interval between hits, assuming

cpntinuous flow. The question is valid and crucial for batch applications.

Styrofoam was tested, but no evidence of interaction was found, so silica powder was

used. Long time-series data collection campaigns sought long term environmental effects, as in

Tolacka et al. [2004] who found a bimodal particle distribution over 24 hours due to increased

human activity in the morning and evening. No such trends emerged in this study, excepting a

clear correlation between rainfall and decreased particle counts. Interesting peaks were found,

such as transient spikes one order of magnitude higher than surrounding peaks. No satisfactory

explanation of their origin was found.

The trials did establish the reliability of the PowerChip and associated electronics for

potential field applications to particle monitoring. A test for plasma current (MFP = 40 Cpm) was

unsuccessful. Amponsah-Manager [2005] reported a small current, but did not describe the

setup used. The effect of adhesion in tubing was measurable (Figures 19 and 20), but was

neglected in the present study.

Tests were conducted using Gelmann filters of varying pore size, though no clear

correlations were found. This could be due to dirty filters, fluctuating background signal, or

tubing effects.



































..


Number of h ts per 1 mmn as fundticn et time


150001


] Lm


5 10 15


.mme.a
25
Time, min


.........r.1III...mes ....
30 35 40) 45 50]


Figure 4-1:







14~ m


03 November Outdoor Air Background

Number of Ri1s per 1 min as function of time


I 1.


10 15 20 25 30 35 40 45 50
Time. Prin


.Figure 4-2: 03 November Outdoor Air with HEPA Filter.












rr.Jruber 3I ral, Fer I rrln ii FI.ncart) ofl lne


Figure 4-3: 50 nm Gelman Filter.

20 SeYpember 2008 20 nm Filer
Number of flits per 1 min as fuvnclion o~ftime


Figure 4-4: 20 nm Gelman Filter.

















20 September~ 20116 Shod~ Tube Backpound
Nrsaonle o iMpr 1 minas fundlon ofume


Figure 4-5: Short Tube Background.

20~ September 200 Long Tube Backgrundl
Nrsnaerof hiM pr minas fundlon ofume


Time, nan


Time, nan


Figure 4-6: Long Tube Background.













One Week, Day 2
Number of hits per 1 mmn as function of hime


4(000


35001


3000 -


P
c
1Fi
B i~4n
n
E

rjl!O


-alhj~


Time, min


Figure 4-7: 24 hour time series.





One Wieek,. Day 3
Number of hits per 1 min as function of lime
180000


200 -




Time, min



Figure 4-8: 24 hour time series.


rll)~l


I J I 1. 1 I




































1000 1200 la


0 200 400 6000 800 1000) 1200 1400 19001
Time?, min



Figure 4-10: 40 hour time series.


ILL i,.


200 4


One Week, Day 4l
Number of hits per 1 mmn as function of time


1.8








b 1
E





Z s


Figure 4-9: 24 hour time series.

One Week,. Day 5
Number of hits per 1 mmn as function of time


16000

140100

120(00

r 10000

800


9000

4000

2000


00 000 800
Time, min


,, L_.I









CHAPTER 5
CONCLUSION

The PowerChip laser was used to form plasma on ambient aerosol particles which were

detected by a photomultiplier tube and stored to a computer. A variety of subtleties arose in the

development of the system, ranging from making the detector housing light-tight to changing the

drying tower to make the flow properties more laminar. A 5-day sampling campaign produced

no clear environmental cycles and only one correlation; that between rainfall and decreased

particle counts. However, PowerChip features such as high repetition rate, short pulsewidth,

stable output, and nearly single-mode operation render it useful for many applications including

real-time particle monitoring

By taking long time-series data of ambient air and studying the size and concentration

dependence of particle counting, the PowerChip laser shows itself a steady and reliable source

for plasma excitation. The PowerChip would be a strong candidate for continuous aerosol

momitonng.

Future work may include more precise determination of minimum particle size and

improvement in efficiency through, for example, particle focusing [Wu, 2006; Erdmann, et al,

2005] and combination of the laser with an iCCD or other spectrometer to gain spectral

identification along with particle counting. Interesting experiments would include using the

spectrometer to calculate the plasma temperature to correlate the particle size versus breakdown

energy from Weyl [1989]. Lastly, particle beam focusing could dramatically improve particle

transport efficiency and, concomitantly, detection efficiency.









APPENDIX
LASER CHARACTERISTICS

Experimental Equipment

JDS Uniphase PowerChip Laser
-500 picosecond pulsewidth
-56 CLJ/pulse
1 kHz repetition rate

Hamamatsu R647 PhotoMultiplier Tube (PMT)
-Diameter = 13 mm
-Wavelength Range = 300 650 nm
-Gain 1.4 x 106

BG 3 8 filter glued to end of PMT to block laser light
~300-650 nm FWHM transmittance










LIST OF REFERENCES

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BIOGRAPHICAL SKETCH

William Paul Mason was born in Gardner, Massachusetts on 15 May 1973. The youngest

of five children, William graduated from Oakmont Regional High School in 1991 and obtained a

B.A. in outdoor leadership from Prescott College in 1995. After working as a mountain guide

and EMT, he returned to Northern Arizona University to obtain a B.S. in environmental

chemistry. Upon graduation he commissioned in the U.S. Air Force and served his first

assignment at Kirtland AFB, New Mexico as a scientist. From there, he was selected to obtain a

master' s degree and attended the University of Florida.

When not studying, William enj oys outdoor activities ranging from skydiving to cave

diving, and is a budding musician with bass guitar and violin.