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Inelastic collisions of rare gas ions and excited atoms with neutral molecules

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Inelastic collisions of rare gas ions and excited atoms with neutral molecules
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Weiss, Morris Jacob, 1936-
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iv, 46 leaves : illus. ; 28 cm.

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Collisions (Nuclear physics) ( lcsh )
Chemistry thesis Ph. D
Dissertations, Academic -- Chemistry -- UF
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bibliography ( marcgt )
non-fiction ( marcgt )

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Thesis - University of Florida.
Bibliography:
Bibliography: leaves 43-45.
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Manuscript copy.
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Vita.

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Full Text
INELASTIC COLLISIONS OF RARE GAS IONS
AND EXCITED ATOMS WITH
NEUTRAL MOLECULES
By
MORRIS JACOB WEISS

A DISSERTATION PRESENTED TO THE GRADUATE COUNCIL OF
THE UNIVERSITY OF FLORIDA
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE
DEGREE OF DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
April, 1963




ACKNOWLEDGEENT

The author would like to thank Dr. E. E. Muschlitz Jr., Chairman of his Supervisory Comnittee, for assistance and advice in the carrying out of this research. He also wishes to thank the other members of his Supervisory Conmittee for their helpful suggestions in the writing of this dissertation. Finally, the author would like to express gratitude to his fellow graduate students as well as his wife, whose patience and understanding were indispensable.




TABLE OF CONTENTS
ACKNOWLEDGEMENT. . . . . ..........
LIST OF FIGURES . . . . . . . . ..
Chapter
I INTRODUCTION . . . . . . .
II HISTORY OF INELASTIC IONIC AND MOLECULAR
COLLISION PROCESSES . . . . . .
III DESCRIPTION OF APPARATUS . . . .
IV EXPERIMENTAL PROCEDURE AND RESULTS . .
V DISCUSSION . . . . . .
BIBIOGRAPHY S A RY. ... .
BIOGRAPHICAL SKETCH . . . . . . . .

Page ii iv
1
4 10 15
34
41 43 46

* ft ft
* ft ft
* ft ft
* ft ft
ft ft ft
ft ft ft
ft ft ft

t ft 0
. .




LIST OF FIGURES

Figure Page
1. Schematic Diagram of Mass Spectrometer 11
2. Diagram of Ion Source 12
3. ArH+/Ar+ Versus Backing Pressure of H2 17
4. H2 /'H2 Versus Backing Pressure of H2 18
5. D2 /D2 Versus Backing Pressure of D2 20
6. V + G2 Potential Versus Field Strength2 22
7. Repeller Potential Versus Magnetic Field Strength2 28 8. Cross Section Versus Primary Ion Energy, E 25
9. N2+ Area Versus -Jl Drawout Potential, in volts 27
10. N2 Intensity Versus Backing Pressure of N2 29
U. Mass Spectrum of Methane 30
12. Log a Versus Log E 36




CHAPTER I
INTRODUCTION
An encounter between two particles may be elastic or inelastic depending on the state of the particles surviving the collision. If no internal energy or mass is transferred between the particles., the collision is said to be elastics and inelastic if Internal energy or mass has been exchanged.
Scattering experiments reveal information about both elastic and inelastic collisions. Elastic collisions, which are not directly responsible for chemical reactions depend on the nature of the force fields surrounding the colliding particles. Much of our past knowledge of intermolecular forces has been obtained by relating certain macroscopic properties of substances such as viscosity, osmotic pressures and compressibility to the microscopic forces postulated to be responsible for them. In order for these properties to successfully explain such forces, sound and of-ten complex theories are needed to relate the observed bulk properties to the intermolecular interactions. However, elastic scattering measurements haves, in recent years, provided direct, detailed information regarding intermolecular forces.
For a chemical reaction to take place, an inelastic
collision must occur between the reacting species. If chemical




reactions are to be more completely understood, more information is needed about the nature of inelastic collisions.
Kinetic studies provide one means of obtaining information about chemical reactions. From the experimental determination of specific reaction rates and the order of a reaction, insight into a mechanism by which the reaction proceeds is possible. On the
other hand, one obtains from inelastic scattering experiments a quantity known as the inelastic collision cross section which bears a direct relation to the probability of the particular process in question. The inelastic scattering cross section is related to the specific reaction rate constant determined by kinetic studies.(') Scattering experiments have the advantage of measuring directly the concentration of reaction intermediates which may have too short a lifetime to be studied by conventional kinetic experiments. Solvent and wall effects are also eliminated, allowing the individual properties of ions and molecules to be observed,
The earliest known beam experiments were those of
Dunoyerp(2) in 1911, who used beams of sodium atoms vaporized from a reservoir into a vacuum, and showed that such beams
travelled in straight lines,
Beam techniques were significantly improved by Stern and his collaborators at Hamburg.(3)4) Recently, Amdur(5) has used high velocity atom and molecular beams to investigate intermolecular force laws. Smith and Muschlitz(6) have recently determined total scattering cross sections for collisions of




metastable helium atoms with other noble gases. More recently, Sholette and Muschlitz(7) investigated ionization cross sections
for collisions of helium metastable atoms in various gases.
The first reliable measurements on the elastic scattering of ions with neutral molecules was that of Ramsauer and Koflath.(8) More precise work on the scattering of low velocity positive ions in gases originated with Russell Fontana, and Simons in 1941.(9) This work, however, did not separate the inelastic from the elastic scattering. In later work by Simons and coworkers,(Il001 312),1814) cross sections for elastic and inelastic scattering were separately determined.
The study of ion-molecule reactions, as such, by Field and Franklin,(lS) Stevenson and Schissler,(16p17) and Giese and Maier,(18) represent more recent investigations. Stevenson and Schissler undertook a systematic investigation of the cross sections of these reactions and concluded that they require little or no activation energy and correspond to extremely rapid reaction rates. The recent work by Giese and Maier(18) represents the latest attempt to study the cross sections of such reactions as a function of ion energy.
The subject of this dissertation is the measurement of
cross sections for inelastic collisions of the rare gas ion, Ar+, and the metastable atoms, Ar* and He* in various gases. The measurements, in each case, are made utilizing a beam of the incident particles passing through a gas at low pressure. The ionic products of the collisions are then identified by mass spectroscopy.




CHAPTER II
HISTORY OF INELASTIC IONIC AND MOLECULAR COLLISION PROCESSES
Several inelastic processes are possible when a beam of positive ions, X+, is passed through a scattering gas consisting of molecules, Y2, e.g.:
X + Y2 -- Y2+ + X [11.1]
X+ + Y2 +* + X [11.2]
X+ + Y2 --- Y + Y+ + X [I.3]
X+ + Y2 Y + Y+* + X [11.4]
X+ + Y2 --- XY+ + Y [II1.5]
X+ + Y2 XY+* + Y [II.6]
Reaction [II.1] is a charge exchange process in which a fast ion, X+, is converted into a fast neutral atom, X. It is an example of a process in which little exchange of kinetic energy occurs. Reaction [II1.2] represents a special case of [II.1] in which the fast ion has sufficient kinetic energy to put the molecule ion, Y2+, into an excited state.
If the ionization potential of X is sufficiently large,Y2 may be dissociatively ionized and a process such as [I11.3] takes
4




place. Reactions of this type have been investigated by Lindholm.(19) This process may also take place if the kinetic energy of X+ is sufficiently large. In the light of the adiabatic theory(20) and experimental evidence, the kinetic energy needed to make the process significant is considerably larger than the onset energy and will not be relevant to this investigation. A special case of [11.3) is [I1.4), in which the ion, X, has sufficient energy to excite as well as ionize one atom of the diatomic molecule.
A third type of inelastic process is one involving a
transfer of an atom. Equation [11.5] represents such a process. Stevenson and Schissler have made quantitative investigations of such processes. They have found that this type of reaction occurs at incident ion energies as low as 1 ev and have cross sections many times the gas kinetic cross sections.(1617)
The existence of such reactions and the accompanying
appearance of secondary ions in mass spectrometers were definite sources of annoyance and were cited as possible sources of errors
in analytical mass spectrometry. (21,22)
The particular reactions with which this work is concerned are those of the rare gas ion, Ar+ and hydrogen and deuterium molecules, viz.:
Ar+ + H2 -- ArH+ H [ .7)
Ar+ + D2 - ArD+ + D 11.8
The positive ions are produced in an electron bombardment ion source. They are then extracted through a series of collimating




slits and finally through a gas chamber where a small fraction of the beam is scattered.
At sufficiently low scattering gas pressures the intensity of secondary ions produced by collisions of type [11.5] is given by:
11 = 1o(l e'NocdP) [11.93
where I1 = intensity of secondary ions
Io = intensity of primary ions
No = 3.54 x 1016 = the number of molecules per cm3 at
1 n pressure and 0 degrees C
2 = length of scattering region in cm
p = pressure in m corrected to 0 degrees C
a = cross section of reaction in cm2
The low pressures under which the experiment was done, enables the following approximation to be made
e-Noalp = 1 NMop [I~lO
Hence) substituting this expression in [11.9] gives, after rearrangements
Il
=1 r NaP [1.ml]
If the pressure p is known within the scattering chamber and the path length from the apparatus geometry, then the cross section can be calculated from the ratio II/I0 and No.




Metastable atoms are excited atoms whose transitions
to the ground state are forbidden by the selection rules governing electric dipole radiation, namely:
(a) I'J=0,l =O0"14- J=0
(b) 0
(c) -+ 1
where J = vector sum of L and S
L = orbital angular momentum
S = spin angular momentum
angular momentum of electron making the transition
The 21S and 23S states of helium are metastable and lie 20.61 and 19.82 ev above the ground state, respectively. The 21S state is metastable because of selection rules (a) and (c), while (b) and (c) forbid ordinary transitions of the 23S to the ground state. Similarly, argon has two metastable states, 3o and 3P2, which are above ground by U.720 and U.546 ev, respectively.
Decay to the ground state by these metastable atoms, though forbidden by electric dipole processes, do occur by double photon emission and magnetic dipole radiation, although the latter mechanisms are highly infrequent, being on the order of 10-5 as probable as electric dipole transitions. As a consequence, the lifetimes of these metastable states are on the order of tenths of seconds, whereas the lifetime of normal excited states are on the order of 10-8 seconds.




Several types of ionizing inelastic collisions between a metastable atom and a molecule are possible, e.g.:
X + Y2 -"X + Y2 + +e + k.e. [11.12]
X + Y2 X + Y2+ ++ e + k.e. [II.13]
X + Y2-- X +Y + Y + e + k.e. [II.14)
X + Y2 X + Y + Y+* + e + k.e. [11.15]
The first of these represents the conversion of electronic excitation energy to ionization and kinetic energy. This process is referred to as Penning ionization after the investigator who showed such ionization occurred in gaseous discharges.(23) Jesse and Sadauskis(24) have found the process to be of importance in the radiation chemistry of gases. Other cases of Penning processes are [11.13), in which an excited level of Y2+ can be attained, [II.14] in which Y2 is excited to a repulsive state where dissociative ionization occurs, and [II.15) where enough excitational energy is available to place the ion in an excited level. It should be mentioned that all of the processes described above are essentially ones of exact resonance, since the electron can readily take up the energy excess.
In addition to reactions [II1.12) through [11.15] are the
Hornbeck-Molnar or chemi-ionization processes which involve excited, but not metastable states and neutral molecules(25) viz.:

A* + Xy --- AXY+ + e + k.e.

[11.16]




9
A + XY AX+ + Y + e + k.e. [II.17]
Such processes have recently been investigated by Munsons Fields and Franklin. (26)




CHAPTER III
DESCRIPTION OF APPARATUS
The apparatus used in determining inelastic cross sections consists of a high-resolution, first order direction-focussing mass spectrometer, schematically illustrated in Figure 1. The basic design of the instrument is due to M. G. Inghram. A theoretical resolution of 1:600 was employed for all measurements.
The primary ions and metastable atoms used in this
experiment are generated in region 1 (Figure 1) by an electron bombardment source, shown full scale in detail in Figure 2. The electrons are produced by thermionic emission from an iridium
filament F, cataphoretically coated with thorium oxide in a manner described by Muschlitz, Randolph, and Ratti.(27) After acceleration to the anode A, the electrons pass through a
0.010 by 0.200 inch slit into the equipotential region N and are collected by trap T. Gas is leaked into the source at L, behind T.
For the ion-molecule reaction experiments, positive ions formed in the region N are pushed out of the source through a
0.010 by 0. 300 inch exit slit by the repeller R. Correction for slight beam misalignment from the apparatus geometry is accomplished by the split electrode GI, by application of potentials up to 45 volts across the plates. Suitable beam collimation and acceleration is provided by the G2 electrodes which are at the




Figure 1.--Schematic Diagram of Mass Spectrometer




--7 I (U) 0L
U1 U
A T
F z N L

Figure 2.--Diagram of Ion Source




same potential. The ions then pass through a region containing the scattering gas in a cylindrical chamber C, situated between the upper G2 electrode and J1. The chamber C makes electrical contact with Jl and is insulated from G2 by a ceramic ring, I. In this manner,, a small drawout field may be applied within the scattering region.
The mass spectrometer, collision chamber, and ion source components, with the exception of the filament, were constructed from inconel. The electrodes were fabricated directly from
0.026 inch inconel sheet. The whole assembly was supported by four precision ground glass rods and insulated by sleeves made
of glass tubing. Only two of these rods are shown in Figure 2.
Primary ions from the source, together with secondary ions formed in the chambers traverse the accelerating region 2 (Figure 1) and are given 5 kilovolts of acceleration by the time they reach the analyzer region 3. The ions then pass through region 5, containing a homogeneous magnetic field where they are mass selected, They then pass through an exit slit before impingement on the first dynode of a 16 stage Nuclide Analysis Associates electron multiplier, housed in region 4. This multiplier has a gain of 105. The electron current from the multiplier passes through an appropriate high resistance and the resulting voltage is measured by a detection system consisting of a Cary vibrating reed electrometer and a Varian Associates chart recorder.
The 5 kilovolt accelerating voltage is provided by a
variable, regulated, high-voltage supply manufactured by Northeast




Scientific Company. An electronic sweep circuit is used for sweeping the magnetic field. All mass spectra were obtained in this fashion. The field strength was precisely determined by a Harvey Wells NMR gaussmeter.
Differential pumping is performed by three three-stage mercury diffusion pumps which maintain pressures down to
1 x 10-7 mm in the instrument. An auxiliary vacuum system is employed for gas purification and handling. Two manifolds are used, one for gas leakage through a needle valve into the ion source,, the other for introducing gas into the scattering chamber through a glass capillary leak.
The helium, nitrogen, argon, deuterium, and hydrogen
used were obtained from cylinders and were purified by passage through a trap containing degassed charcoal at low temperature. Impurities in these gases were estimated to be less than one part in 103, as determined by mass spectra. The oxygen and krypton were of spectroscopic grade purity and were taken from glass flasks supplied by the Air Reduction Sales Company. Impurities amounted to approximately one part in 10'. The reagent grade methane and ethane used were supplied in cylinders by the Phillips Company and were guaranteed to contain not more than forty parts and one part of impurity per 104t respectively.




CHAPTER IV
EXPERIMENTAL PROCEDURE AND RESULTS
In order to obtain a beam of argon ions for ion-molecule cross section determinations, the following method was employed:
Argon was leaked through L (Figure 2) into the source. Ions formed in region N were pushed out by an appropriate potential on the repeller electrode. The magnetic field necessary for the focussing of argon ions was found by manual sweep of the field. The beam is further focussed by adjustment of the deflector plates G2 and the beam centering electrodes J4 and J5.
Hydrogen was then introduced into the collision chamber C where reaction with the primary beam occurred, resulting in the formation of ArH+. Since the ArH+ and Ar ions are formed in different regions of the mass spectrometer, optimum focussing
conditions for the extraction of former ions had to be determined by adjusting Jl. A potential difference of -8 volts on Jwith respect to G2 was found sufficient and was used in all the following ion-molecule cross section measurements.
To prove that the secondary ions were originating within the chamber C. a pressure dependence study was made by plotting the ratio ArH+/Ar+ against the backing pressure of hydrogen behind the chamber. The results of such a plot are illustrated




in Figure 3, in which a linear pressure relationship is obtained, indicating ArH+ formation did occur in the collision chamber.
A residual amount of mass 41 remained at zero hydrogen backing pressure. This is more than likely due to an impurity in the argon. This impurity may be ArH+ formed as a result of the ion-molecule reaction [11.7] in the source, or to another ion of mass 41. In view of the high repeller potential (100 volts) used in the operation of the ion source, the former possibility is unlikely.(I16917) Back diffusion of hydrogen into the source was shown to be negligible, even with no argon present in the
source. All these facts point to the occurrence of ion-molecule reaction [11.7] in the collision chamber.
In order to calculate the cross section for reaction [II.7] the pressure of the scattering gas must be known. To determine this pressure, a reaction of known cross section was studied. By measuring the ratio of secondary to primary ions, the pressure may be calculated from equation [Il1. The charge exchange reaction between hydrogen molecular ions and hydrogen was studied and the pressure of hydrogen in the chamber determined using the cross section measurements of Cramer.(28) Figure 4 shows the results of the H2+, H2 charge exchange experiments. The ratio of secondary to primary H2+ ions are plotted against the backing pressure of hydrogen in the collision chamber. The pressure calculated was 5.76 x 10'7 m, corresponding to a hydrogen backing pressure of 50 m.




H2 (mm

Figure 3.--ArH+/Ar+ Versus Backing Pressure of H2




2.0

LO
0
x
+
N
+
m
"r

Figure 4.--H2+ /H2+ Versus Backing Pressure of H2

10 20 30 40 50 60 70
PRESSURE H2 (mm)




The charge exchange reaction between fast ions and neutral molecules results in the production of slow ions and fast molecules. Use of this fact is made in determining the ratio of secondary to primary hydrogen ions. Since the secondaries are formed with essentially thermal energy, these ions will receive less acceleration than the fast primary ions before reaching the analyzer region. Therefore, a splitting of the H+ peak was expected to exist with the charge exchange peak appearing on the low energy side of the primary. Such a splitting was found. Furthermore, since the secondary ions are formed with little kinetic energy, their position in the mass spectrum should be essentially independent of the incident primary ion energy. Precise measurements of the field with the NMR gaussmeter showed this to be true, and indicated that the charge exchange process was being measured in the collision region. A 150 volt potential on G2 with respect to anode and a -3 volt drawout potential on J1 were used in all charge exchange measurements.
Measurements under the same conditions were made for the systems Ar+, D2 and +2 + I2. In Figure 5 are shown the results of the D2 + D2 charge exchange employed for calibration of the deuterium backing pressure. The pressure calculated, in this case, was 3.46 x 10-7 mm for a backing pressure of 50 mm of deuterium.
To obtain the cross sectional dependence of reactions [11.7) and [11.8] on energy, the incident primary ion energy




O
I)<
0
x
+ cOJ
+ CoJ
0

Figure 5.--D2+ /D2+ Versus Backing Pressure of D2




must be known. Due to the high repeller fields employed in the source, this energy is not equal to G2, the potential difference between 02 and the anode, where the ions are formed. This energy correction is determined by utilizing the mass spectrometer equation H12r2
V + G2 + R =l -- L Em.1)
where V is the main acceleration potential on the ions with respect to Jl, G2 is the potential on the G2 electrode, R is correction due to the repeller field for a particular setting of the repeller potential, and Hi, r, m, and e have their usual significance. Plotting the V + G2 potential against the square of the magnetic field strength needed to focus the argon ions results in a straight line of slope k equal to
k =mr2 .IV.2)
The results are shown in Figure 6. The repeller potential is then plotted against the square of the field strength needed to focus the argon ions. The results are shown in Figure 7. By extrapolating the results to zero repeller potential, the square of the field strength, Ho, corresponding to this potential is determined. For this case, equation [IV.I] becomes V + G2 = k *2 [IV.3J
If G2 is kept constant, subtracting equation [IV.3] from [IV.1] gives

R = k(Hl2 Ho2)

EIV.4]




O
5.12 ro
I
0
X
S508
+
504
472 474 4.76 478
H2 X 10-6 (g 2)

Figure 6.--V + G2 Potential Versus Field Strength2

480

482

484




20
I0
4728 4732 4736 4740 4744
H2 X 10-6 (g2)
Figure 7.--Repeller Potential Versus Magnetic Field Strength2




Since k can be determined from the slope of the first plot and H1, H0 are known, R can be determined. In this manner, the correction R was found to be 65 volts. Therefore, to obtain the absolute ion energy, 65 volts has to be added to the potential of G02.
The results for the cross sectional variation with energy for the ion-molecule reactions [11.7] and [11.8) are shown in Figure 8. The open circles represent the cross sections for reaction [11.7], while [11.8] is represented by black circles. The results show the cross section of the deuterium reaction to be lower than the hydrogen over the energy range studied. A slightly sharper increase for the former process in the vicinity
of 130 volts is observed.
The ion source used in the production of positive ions was also employed for the production of the helium and argon metastable atoms for studying Penning ionization processes.
The method used for production of such excited states consisted first of introducing the gas through the aperture L (Figure 2). Then the positive ions formed (Ar+ or He+) were focussed by the method outlined earlier. After this was done,
the repeller electrode was set at anode potential and the deflector plates G1(I) and G1(I1) were adjusted for maximum deflection. By this procedure, ions formed in the source are prevented from entering the collision region. That this was done is borne out by the fact that no helium ions were observed
in the mass spectra. The metastable atoms, unaffected by these




I I I I I I I

60

90

120 E (ev)

150

180

210

Figure 8.--Cross Section Versus Primary Ion Energy, E

40

30 --

201-




electric fields, diffuse upward through the collimation system where they encounter molecules in the collision chamber.
It was found necessary to operate GI and G2 at 40 volts negative with respect to anode in order to suppress electron leakage through the ion source exit slit. These conditions were maintained during all measurements.
The scattering gas first used was nitrogen with helium in the source. An ion peak appeared at mass 28 and was presumed to be N2+. The peak height was then determined as a function of the electron energy. It was found that the largest drop in intensity occurred between 32 and 24 volts electron energy# vanishing at about 24 volts, This checked well qualitatively with the appearance potential curve of He* determined by Smith and Muschlitzi(6) and afforded proof that the N2+ ions were formed as a result of inelastic collisions between helium metastable atoms and nitrogen molecules and not by electron impact, The anode was maintained at 50 volts with respect to the filament for all subsequent measurements,
To determine optimum drawout conditions for the extraction of N2+ ions, the area under the N2+ mass spectrum peak was measured as a function of the Jl drawout potential in volts, Figure 9 shows such a plot, The area increases rapidly, then levels off at about -30 volts. A drawout field of -24 volts was used in all subsequent measurements, It will be noticed that the intercept at zero area corresponds to a drawout potential of Jl of about 11 volts, indicating that the inelastic collision




30
-JI (V)

Figure 9.--N2+ Area Versus -Jl Drawout Potential, in volts

U)
O
L.
(11
z




between a meta stable helium atom and a nitrogen molecule results in the formation of very slow N'ions.
A pressure dependence plot of the N2+ intensity against the backing pressure of nitrogen was performed and is shown in Figure 10. A linear dependence of the peak intensity with backing pressure resulted. Furthermore, the N2+ peak vanished when all the helium was pumped out of the ion source chamber. This was additional proof as to the nature and origin of the ion formation.
Tests with oxygen substituted for nitrogen were performed under the same conditions. Two peaks were discovered having masses 16 and 32, respectively. The abundance ratio was on the order of 3: 1, in favor of mass 32.
identical operating conditions were used for methane and ethane. Several ion fragments of the molecules result from inelastic collisions with metastable helium atoms. The mass spectra resulting from this type of process were recorded and the relative abundance of the fragment ions were compared with conventional electron impact spectra. A typical mass spectrum of methane resulting from collisions with He* is illustrated in Figure U. The electron impact spectra were obtained in a conventional manner by introducing methane and ethane into the ion source and recording the relative abundances at various electron energies. The relative abundances of the ion fragments at 20 and 40 volts electron energy were compared to the abundances resulting from the Penning ionization process.




40 60 80 100 120
PRESSURE N2 (mm)
"0
Figure 10.--N+ Intensity Versus Backing Pressure of N2

20




CH3+

CH4

CH2+

I I I I

14.0

15.0

MASS
Figure ll.--Mass Spectrum of Methane

13.0

16.0




31
Table 1 summarizes the various systems studied. Colum I lists the metastable atom involved in the process. Column 2 lists the scattering gas. The positive ion formed is shown in column 3. The abundance of the ion fragments is listed in column 4 together with the relative abundances at the two electron energies. The probable process involved, appearance potential of the ion formed, and references are shown in the next three columns.




TABLE 1
SMHARY OF PENNING IONIZATION PROCESSES

Metastable EIn Observed Abundance
Atom Gas Formed A + XY Electron Impact Process A. P. Reference
20 ev 40 ev (ev)

100 100 10 33

N2 ( g+) 02(38 -)

02+ (xg)
+.
0 (48)
04
CH+ CH2+
[13.8 )

-- N N2+(X2 g+) + e
- 02 02+(x2<) + e
-- 02 0--0(4S) + o(P) + e
02 -+ (2D) + o100 CH4 + e CH4+ + 2e 80 --- CH3+ + H + 2e
20 --- CH2+ + H + 2e

15.577
12.2
18.69 20.53 13.12
14.39 16.5

29 29 29,30 309,31 32 32
33

He *

100
82
18

CH4

75 + 10 100
20 12

---




TABLE 1--Continued

Metastable
Atom
He
Ar*

Ion

Observed Abundance
A* + XY Electron Impact
20 ev 40 ev

Ion
Gas Formed C2H6 C2H6+
C2H5 + C2H4+
C2H3+
CU+
C2H2+
[24.6]
C3+
CH3 C2H6 C2H6 +
C2H4 +

22 26

42 1 10 41 100 60 29 15
17 90 10 100

21 100 33
20
7
26 100

Process

C2H6 + e -C2H6+ + 2e
-- C2H5+ + H + 2e ---C 2H4 +H2 + 2e
-C2H3+ + H + H2 + CH2 + 2H2 + 2e

20 100 27
12
4 22 100

Reference

A. P.
(ev)
11.5
13.05 11.44
2e 15.50
15.3
14.5 11.5
11.44

--CH3+ + CH3 + 2e C2H6 + e -*C2H6 + 2e
C2H4+ + H2 + 2e




CHAPTER V
DISCUSSION
The results for the cross sectional variation with energy for the Ion-molecule reactions 1I1.7] and [11.8] show that the Ar+, D2 system has a lower cross section than the Ar+, H2 system over the energy range studied. Unfortunately, no systematic study of these reactions have been made under similar conditions, therefore no direct comparisons can be made. Stevenson and Schissler(16'17) have made a study of these reactions in the ionization chamber of their ion source and have reported cross sections for these systems. Their cross sections, however,
have been derived from specific rate constants calculated from assumed field conditions and molecular distribution functions in the ionization region of their source,(l) hence the energy at which the hydride and deuteride ions are formed can have a wide range of values. They have reported cross sections of
5942 and 33A2 for ion-molecule reactions [11.7] and [11.8], respectively, at their highest repeller voltage of 41.1 volts/cm. The results of this work also show approximately a 2:1 ratio for the corresponding cross sections over the energy range studied. According to the theory of Gioumousis and Stevenson,(') the ratio should have been in the neighborhood of 1.38:1.




The Kr+, H2 system was tried without success in order to measure the cross section of the ion-molecule reaction Kr+ + H2 KrH+ + H [V.1]
No KrH+ was detected, although a cross section 1/10 that of [11.7] could have been measured. Stevenson(16,17) reported a much lower value for the Kr+, H2 cross section, about 1/3 that of the Ar+, H2 system.
The energy dependence of the cross sections assuming a relation, a = kEn, were determined by plotting the logarithm of the cross section a, against the logarithm of the incident ion energy, E, as shown in Figure 12. The slope n of the curve, equal to the energy dependence of a, was found to be about
-3/2 for both the Ar+, H2 and Ar+, D2 systems, remaining constant up to about 155 volts and rapidly levelling off to zero beyond this energy. For energies not much greater than thermal, the Stevenson-Gioumousis theory(l) predicts the cross section to vary inversely with the square root of the energy. For high incident ion energies the theory would not be expected to hold. It is possible, therefore, that the energy range used in this work was higher than the limit for which the theory is valid,
It should be remembered that the cross sections calculated for these systems depend directly on the correct pressure of H2 in the chamber It is not certain, in these experiments, that
all the charge exchange secondaries are being extracted from the reaction region, This would lead to low calculated pressures#




Figure 12.--Log a Versus Log E




which would result in high calculated cross sections. Since all measurements for each system were made at the same pressure, however, an error in the pressure determination would not affect the relative values of the cross sections, nor their functional dependence on energy.
The results of the inelastic collisions between metastable atoms and molecules are sumnarized in Table 1. The first mass spectrum of nitrogen with He* revealed the N2+ ion, while N was absent. The appearance potentials of N2+ and N+ are 15.557(29) and 24.301 ev,(29s30) respectively. The metastable atoms have energies greater than the former, but less than the latter, therefore, this observation is not difficult to explain.
With oxygen and He*, 02+ and 0+ were observed. Since these ions require only 12.2(29) and 18.69 ev(29s30) for ionization, respectively, there is enough energy of excitation available in He* for their formation.
The hydrocarbon gas, methane gave several ion fragments with He*, the abundances of which are summarized in column 4 of Table 1. It can be seen, in this case, that the parent ion is not the most abundant ion formed in the Penning process, whereas it is dominant in the electron impact spectra.
The peak at mass 13.3 (Figure ll) was probably due to
metastable ion decomposition of CH3+ or CH4+ just before entering the analyzer region.(35) The C+ and CH+ ions at masses 12.0 and 13.0, respectively, were not observed. The CH+ ion could have been produced by one of two processes:

CH4 + e CH+ + 3H + 2e

[V.2]




or CH4 + e --CH+ + H2 + H + 2e [V.3]
The appearance potential of the former process is reported to be 23.4 ev(37) and 18.9 ev is calculated for the latter process. It is interesting to note that [V.3) does not take place either
for electron bombardment(32) or for Penning ionization. McDowell and Warren(32) have shown [V.2] to be the process responsible for the production of CH+ by electron bombardment. Conclusions similar to those for [V.3] may be drawn for the reaction
CH4 + e C+ + 2H2 + 2e [V.4]
McDowell and Warren(32) have shown the reaction
CH4 + e C+ + 4Q + 2e, [V.5)
requiring 26.2 ev to be responsible for the production of C+ by electron impact of methane, instead of [V.4] which would require only 17.2 ev. The substitution of Ar* for He* resulted in the absence of any ions from methane. This would indicate that the fragments require more than the U.720 ev energy of excitation in Ar* available for their formation.
For reactions of He* with ethane, ions having masses 30, 29, 28, 27, 26, 24.6, and 15 were observed. No marked departures are observed when comparing the abundances resulting from the Penning and electron impact processes with the possible exception of the fragmented ions being in greater proportion, relative to the parent ion, from the Penning process than from




electron impact. The mass peak at 24.6 may be the result of a metastable ion decomposition in a manner earlier described.(35) The absence of the ions C2+ and C2H+ are supported by appearance potential measurements.(38) It is difficult to suggest an explanation for the absence of CH2
It is interesting that C2H6+and C2H4+ were the only
peaks observed for the system C2A6, Ar* The absence of other ion fragments is well accounted for by appearance potential measurements.(35) The fact that the ions CH+ and C2H4+ are formed in collisions with excited atoms having energies of 11.720 and 11.546 ev provides a means of determining the maximum appearance potentials for the processes
C26 + e -P- C2H6 + 2e [V.6]
and
C2H6 + e -- C2H4+ + H2 + 2e [V.7]
Values of iI.5,(34) ii.60,(39) and 11.78 ev(36) have been among the many reported for the former process. It seems, in the light of the present work, that the latter value of 11.78 ev is not acceptable. For the latter reactions li.44,(36) 12.090(40) and 12.1 ev(34) were reported by various investigators. The last two values are not consistent with the present investigation.
The estimation of the relative heights of the mass spectra peaks constitutes a major source of error for the Penning ionization work. Because of the high sensitivities needed to detect these ions (the largest current is 10-18 ampere),




the peaks were accompanied by a great deal of noise. The errors were estimated as 10 for the relative abundances of the ions. A statistical theory of mass spectra developed by Eyring and coworkers(4l) satisfactorily explains, in many cases, the
relative abundances of fragment ions resulting from electron impacts with molecules having many electronic and vibrational degrees of freedom. These ions result from Franck-Condon type transitions and consequently, the interaction time between an
electron and the target molecule is exceedingly short.
The Penning process differs from that of electron impact because of the relatively long duration of the collision between the target molecule and the thermal energy metastable atom. In the former process, excitation energy is transferred but in the latter, the kinetic energy of the electron is converted into excitational and vibrational energy. Furthermore, the target molecules studied in this work do not have the large number of degrees of freedom required for a statistical theory to be valid.
Ferguson(42) has calculated cross sections for Penning ionization utilizing a simple classical momentum-transfer collision model. His calculations agree, within an order of magnitude, with experimental measurements. However, no theory
has yet been developed which would predict the relative abundances of fragment ions resulting from Penning ionization.




CHAPTER VI
SUMMARY
A high-resolution mass spectrometer was used to measure the inelastic cross sections for the ion-molecule reactions Ar+ + H ArH+ + H
and
Ar+ + D2 ArD+ + D
in the energy range 60-215 ev. The rare gas ions were produced in a conventional electron bombardment source and were suitably collimated before entering a cylindrical chamber containing the scattering gas. The pressure of the scattering gas was calculated from measurements of the symmetrical charge exchange process and the known cross sections for this process.
The cross sections were found to be in the ratio of about 2:1 over the energy range studied with the first reaction exhibiting the larger cross section. The cross sections were found to vary inversely with the 3/2 power of the energy in the range 60-155 ev, rapidly levelling off to zero energy dependence beyond 155 ev.
The ion source used for the production of positive ions
was also employed for the production of helium and argon metastable




atoms for studying the Penning ionization processes of the type
A + XY u.X+ + Y+ A +e +k.e.
Here A is the metastable atom involved in the collision with a molecule,, XY. The target gases methane, ethane, oxygen, and nitrogen were used. Several ion fragments resulted from the inelastic collisions of methane and ethane with the metastable atoms. The resulting mass spectra of such fragment ions were compared to the spectra obtained from conventional electron
bombardment studies.
The measurements have demonstrated, for the first time, that fragment ions are formed in a Penning ionization process,, provided the metastable atom has sufficient energy. These ions have been identified by mass spectroscopy and significant differences in their relative abundances, as compared with those observed for electron impact, have been observed. Information relative to the upper limits for the appearance potentials of these ions and the mode of fragmentation was obtained from these studies.




BIBLIOGRAPHY
1. G. Gioumousis and D. P. Stevenson, J. Chem. Phys. 29, 294 (1958).
2. L. Dunoyer, Comptes Rendus 152, 594 (1911).
3. O. Stern, Zeits. f. Physik 39, 751 (1926). 4. 0. Stern, Zeits. f. Physik 41, 563 (1927).
5. I. Amdur, E. A. Mason, and J. E. Jordan, J. Chem. Phys. 27, 527 (1957).
6. 0. M. Smith and E. E. Muschlitz, Jr., J. Chem. Phys. 33, 1819 (1960).
7. W. P. Sholette and E. E. Muschlits, Jr., J. Chem. Phys. 36, 3368 (1962).
8. C. Ramsauer and R. Kollath, Ann. der Physik 16, 570 (1933).
9. A. S. Russell, C. M. Pontana, and J. H. Simons, J. Chem. Phys.
9, 381 (1941).
10. J. H. Simons, H. T. Francis, C. M. Fontana, and S. R. Jackson,
Rev. Sci. Instr. 13, 419 (1942).
U. J. H. Simons, C. M. Fontana, E. E. Muschlitz, Jr., and
S. R. Jackson, J. Chem. Phys. 11, 307 (1943).
12. J. H. Simons, C. M. Fontana, H. T. Francis, and i,. G. Unger,
J. Chem. Phys. 11, 312 (1943).
13. J. H. Simons, H. T. Francis, E. E. Muschlits, Jr., and
G. C. Fryburg, J. Chem. Phys. U, 316 (1943).
14. J. H. Simons, E. E. Muschlits, Jr., and L. G. Unger, J. Chem.
Phys. U, 322 (1943).
15. F. H. Field, J. L. Franklin, and F. W. Lampe, J. Amer.
Chem. Soc. 79, 2419 (1957).
16. D. P. Stevenson and D. O. Schissler, J. Chem. Phys. 23,
1353 (1955).




17. D. P. Stevenson and D. 0. Schiseler, J. Chem. Phys. 24,
926 (1956).
18. C. F. Giese and W. B. Maier, II, J. Chem. Phys. 35, 1913
(1961).
19. E. Lindholm, Arkiv. Fysik 8, 257, 433 (1954). 20. H. S. W. Massey and E. H. S. Burhop, Electronic and Ionic
act Phenomena (Oxford University Press, London, 1956),
479.
21. T. R. Hogness and R. W. Harkness, Phys. Rev. 32, 784 (1928). 22. H. D. Smyth, Revs. Mod. Phys. 8, 347 (1931). 23. A. A. Kruithof and F. M. Penning, Physica 4, 430 (1937). 24. W. P. Jesse and J. Sadauskis, Phys. Rev. 100, 1755 (1955). 25. J. Hornbeck and J. P. Molnar, Phys. Rev. 84, 621 (1951). 26. M. S. B. Munson, F. H. Field, and J. L. Franklin, J. Chem.
Phys. 37, 1790 (1962).
27. E. E. Muschlitz, Jr., H. D. Randolph, and J. N. Ratti,
Rev. Scl. Instr. 33, 446 (1962).
28. W. H. Cramer, J. Chem. Phys. 35, 836 (1961). 29. G. Hertzberg, Spectra of Diatomnic Molecules (Van Nostrand,
New York, 1959), 449, 450, 459, 558.
30. G. Hertzberg, Atomic Spectra and Atomic Structure (Dover,
New York, 1944), 00.
31. L. M. Branscomb, D. S. Burch, S. J. Smith, and S. Geltman,
Phys. Rev. 111, 504 (1958).
32. C. A. McDowell and J. W. Warren, Faraday Soc. Disc. 10, 53
(1951).
33. J. Geerk and H. Neuert, Z. Naturforseh SA, 502 (1950). 34. M. B. Koffel and R. A. Lad, J. Chem. Phys. 16, 420 (1948). 35. F. H. Field and J. L. Franklin, Electron Impact Phenomena
(Academic Press, New York, 1957), 194, 248, 249, 252
and 254.
36. J. J. Mitchell and F. F. Colemanj, J. Chem. Phys. 17, 44
(1949).




45
37, L. Smith, Phys. Rev. 51, 263 (1937). 38. J. A. Hipple, Phys. Rev. 53, 530 (1938). 39. J. L. Franklin and H. E. Lumpkin, J. Amer. Chem. Soc. 74,
1023 (1952).
40. D. P. Stevenson and J. A. Hipple, J. Amer. Chem. Soc. 64,
1588 (1942).
41. H. M. Rosenstock, A. L. Wahrhaftig, and H. Eyring, Technical
Report No. II, June 25, 1952. Univ. of Utah, Inst. for
Study of Rate Processes, Salt Lake City. 42. E. E. Ferguson, Phys. Rev. 127, 210 (1962).




BIOGRAPHICAL SKETCH

Morris Jacob Weiss was born on December 3, 1936, in
Manhattan, New York. He attended elementary school in Brooklyn, New York and graduated from Abraham Lincoln High School, in that Borough, on June, 1954. In June, 1958, he received the Bachelor of Arts from Brooklyn College, with a major in Chemistry. In September, 1958, he entered the Graduate School of the University of Florida, where he held an assistantship in the Department of Chemistry and worked toward the degree of Doctor of Philosophy.
Morris Jacob Weiss is married to the former Betty May
Marcus, a University of Florida graduate. He is a member of the American Physical Society.




This dissertation was prepared under the direction of the chairman of the candidate's supervisory committee and has been approved by all members of that committee. It was submitted to the Dean of the College of Arts and Sciences and to the Graduate Council, and was approved as partial fulfillment of the requirements for the degree of Doctor of Philosophy.
April 20, 1963

Dean, College of Arts an4 fences

Dean, Graauate School

Supervisory Committee: Chiman
- -- .




Full Text

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INELASTIC COLLISIONS OF RARE GAS IONS AND EXCITED A TOMS WITH NEUTRAL MOLECULES B y MORRIS JACOB WEISS .A DISSERTATION PRESENTED TO THE GRADUATE COUNCL OF THE UNIVERSITY OF FLORID.A IN PARTIAL F U LFILLMENT OF THE REQUIR EME NTS FOR TH E DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA April 1963

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ACKNOWLEDGEMENT The author would like to thank Dr. E E. Muschlitz, Jr., Chairman of his Supervisory Committee, for assistance and advice in the carrying out of this research. He also wishes to thank the other members of his Supervisory Committee for their helpful suggestions in the writing of this dissertation Finally, the author would like to express gratitude to his fellow graduate students as well as his wife, whose patience and understanding were indispensable ii

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TABLE OF CONTENTS A C KNOWLEDGEMENT LIST OF FIGURES . . . . . . . Chapter I II II I IV V V I INTRODUCTION HISTORY OF INEIASTIC IONIC AND MOLECULAR COLLISION PROCESSES DES C RIPTION OF APPARATUS . . EXPERIMENTAL PROCEDURE AND RESULTS . D I SCUSS I ON . . . . SUMMARY . . . . BIBLIOGRAPHY . . . . BIOGRAPHICAL SKETCH . . . . iii Page ii iv 1 4 10 15 34 41 43 46

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LIST OF FIGURES Fi g ure Page 1 Schematic Diagram of Mass Spectrometer 11 2 Diagram of Ion Source 12 3 Arll+/Ar+ Versus Backin g Pressure of 17 4 H 2 + /H 2 + Versus Backing Pressure of H 2 18 5 + + D 2 / D 2 Versus Backing Pressure of D 2 20 6 V + G 2 Potential Versus Field Strength 2 22 7 Repeller Potential Versus Magnetic Field Strength 2 23 8 C ross Section Versus Primary I on Energy E 25 9 + N 2 Area Versus -J 1 Drawout Potential, in volts 27 10 N 2 + Intensity Versus Backing Pressure of N2 29 11 Mass Spectrmn of Methane 30 12 Log a Versus Log E 36 iv

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CHAPTER I INTRODUCTION An e n coun t er be t ween two particles may be elastic or inelastic depending on the state of the particles surviving the collision If no internal energy or mass is transferred between the particles, the collision is said to be elastic, and inelastic if internal energy or mass has been exchanged Scattering experiments reveal infonnation about both elastic and inelastic collisions Elastic collisions, which are not directly responsible for chemical reactions depend on the nature of the force fields surrounding the colliding particles Much of our past knowledge of intermolecular forces has been obtained by relating certain macroscopic properties of substances such as viscosity, osmotic pressure and compress ibility to the microscopic forces postulated to be responsible for them, In order for these properties to successfully explain such forces sound and often complex theories are needed to relate the observed bulk properties to the intermolecular interactions. However, elastic scattering measurements have, in recent years, provided direct, detailed infonnation regarding intermolecular forces For a chemical reaction to take place, an inelastic collision must occur between the reacting species If chemical l

PAGE 6

reactions are to be mor e comple t ely understood, more information is needed about the nature of inelastic collision s 2 Kinetic studies provide one means of obtaining infonnation about chemical reactions From the experimental determination of specific reaction rates and the order of a reaction insight into a mechanism by which the reaction proceeds is possible On the other hand, one obtains from inelastic scattering experiments a quantity known as the in~astic collision cross section which bears a direct relation to the probability of the particular process in question The inelastic scattering c~oss section is related to the specific reaction rate constant determined by kinetic studies ,( l ) Scattering experiments have the advantage of measuring directly the concentration of reaction intermediates which may have too short a lifetime to be studied by conventional kinetic experiments S o lvent and wall effects are also eliminated, allowing the individual properties o f i o ns and molecules to be observed The earliest known beam experiments were those of Dunoyer; ( 2 ) in 19ll ; who used beams of sodium atoms vaporized from a reservoir into a vacuum and showed that such beams travelled in straight lines. Beam techniques were significantly improved by Stern and his collaborators at Hamburg .( 3 4 ) Recently Amdu i 5 ) has used high velocity atom and molecular beams to investigate inter molecular force laws Smith and Muschlitz < 6 ) have recently determined total scattering cross sections for collisions of

PAGE 7

metastable helium atoms with other noble gases. More recently, Sholette and Muschlitz{ 7 ) investigated ionization cross sections for collisions of helium metastable atoms in various gases 3 The first reliable measurements on the elastic scattering of ions with neutral molecules was that of Ramsauer and Kollath .( S) More precise work on the scattering of low velocity positive ions in gases originated with Russell,Fontana, and Simons in 1941.( 9 ) This work, however, did not separate the inelastic fran the elastic scattering In later work by Simons and coworkers,{lO,ll,l2,l 3 ,14) cross sections for elastic and inelastic scattering were separately determined The study of ion-molecule reactions, as such, by Field and Franklin, (l S ) Stevenson and Schissler, (l 6 ,l 7 ) and Giese and Maier,(lS) represent more recent investigations. Stevenson and Schissler undertook a systematic investigation of the cross sections of these reactions and concluded that they require little or no activation energy and correspond to extremely rapid reaction rates The recent work by Giese and Maier ( lS) represents the latest attempt to study the cross sections of such reactions as a function of ion energy. The subject of this dissertation is the measurement of cross sections for inelastic collisions of the rare gas ion, Ar+, and the metastable atoms, Ar* and He* in various gases. The measurements, in each case, are made utilizing a beam of the incident particles passing through a gas at low pressure The ionic products of the collisions are then identified by mass spectroscopy.

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CHAPTER II HISTORY OF INEIASTIC IONIC AND MOLECUIAR COLLISION PROCESSES Several inelastic processes are possible when a beam of positive ions, x+, is passed through a scattering gas consisting of molecules, Y 2 e g : x+ + Y Y + + X 2 2 [Il.l] [II.2] x+ + Y2 Y + y+ + X [II.3] x+ + Y2 -Y + y+* + X [II 4] [II SJ [II. 6] Reaction [I I .l] is a charge exchange process in which a fast ion, x+, is converted into a fast neutral atom, X It is an example of a process in which little exchange of kinetic energy occurs Reaction [II 2] represents a special case of [II.l] in which the fast ion has sufficient kinetic energy to put the molecule ion Y2+, into an excited state If the ionization potential of Xis sufficiently i.arge, may be dissociatively ionized and a process such as [II 3] takes 4

PAGE 9

5 place Reactions of this type have been investigated by Lindholm ( l 9 ) This process may also take place if the kinetic energy of x+ is sufficiently large In the light of the adiabatic theory ( 2 o ) and experimental evidence, the kinetic energy needed to make the process significant is considerably lar ger than the onset energy and will not be relevant to this investigation A special case of [II 3] is [II.4], in which the ion, x+, has sufficient energy to excite as well as ionize one atom of the diatomic molecule A third type of inelastic process is one involving a transfer of an atom Equation [II,5) represents such a process Stevenson and Schissler have made quantitative investigations of such processes They have found that this type of reaction occurs at incident ion energies as low as 1 ev and have cross sections many times the gas kinetic cross sections .< 16 17 ) The existence of such reactions and the accompanying appearance of secondary ions in mass spectrometers were definite sources of annoyance and were cited as possible sources o f errors ( 21 22 ) in analytical mass spectranetcy The particular reactions with which this work is concerned are those of the rare gas ion, Ar+ and hydrogen and deuterium molecules, viz : [II 7] [II.8] The positive ions are produced in an electron bombardment ion source They are then extracted through a series of collimating

PAGE 10

slits and f i n ally through a g as chamber where a small fr action of the beam is scattered 6 At sufficiently low scattering gas pressures the intensity o f secondary ions produced by collisions of t ype [ II 5] is given by : where 1 1 = intensity of secondary ions I 0 = intensity o f primary ions [II 9] N 0 = 3 54 x 10 16 = the number o f molecules per cm 3 at 1 ll!ll pressure and O degrees C i = length of scattering region in cm p = pressure in mm corrected to O degrees C cr = cross section of reaction in cm 2 The low pressu~es under which the eMperiment was done enables the following approximation to be made: [l I. 10] Hence substituting this expressi o n in [ II. 9] gives af--ter rearrang.ement i [ II.1 1] If the pressure p is known within the scattering chamber and the path length .R. from the apparatus geometry ; then the cross section can be calculated from the ratio 11 / 1 0 and N 0

PAGE 11

7 Meta stabl e atom s ar e exc i ted atoms whose transi t ions t o t h e g roun d st a te are f orbi d den by t h e selec t ion rules g overnin g e lec t ric dipole ra d ia t ion, namel y : where ( a ) 61 = o, 1 (b) 6S = O (c) b i = l J=O ~ J=O J = vector StDil of Land S L = orbital angular momentum S = spin angular momentum --' = angular momentum of electron makin g the transition The 2 1 s and 2 3 s states of helium are metastable and lie 20 61 and 19 82 ev above the ground state, respectively The 2 1 s state is metastable because of selection rules (a ) and (c), while ( b) and ( c) forbid ordinary transitions of the 2 3 s to the ground state Similarly, c:rgon has two metastable states, 8F 0 and 3p2, which are above ground by ll 720 and ll 546 ev, respectively Decay to the ground state by these metastable atoms though forbidden by electric dipole processes; do occur by double photon emission and magnetic dipole radiation, although the latter mechanisms are highly infrequent being on t he order o f 105 as probable as electric dipole transitions As a consequence the lifetimes of these metastable states are on the order of tenths of seconds, whereas the lifetime o f normal excited states are on the order o f 10 8 seconds

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Several types of ionizing inelastic collisions between a metastable atom and a molecule are possible, e g : 8 x* + Y 2 X + Y 2 + + e + k e [II .12.] +* X + Y2 X + Y2 + e + k.e [11.13] x* + Y 2 -X + Y + y+ + e + k .e. [II 14) x* + Y 2 -X + Y + y+* + e + k e [II 15) The first of these represents the conversion of electronic excitation energy to ionizati,on and kinetic energy. This process is referred to as Penning ionization after the investigator who showed such ionization occurred in gaseous discharges .< 23 ) Jesse and SadauskiaC 24 ) have found the process to be of importance in the radiation chemistry of gases Other cases of Penning processes are [11 13], in which an excited level of Y 2 + can be at t ained [I I.14] in which Y 2 is excited to a repul~ive atate where I dissociative ionization occurs and [11 15] where enough excitational energy is available to place the ion in an excited level It should be mentioned that all of the processes described above are essentially ones of exact resonance since the electron can readily take up the energy excess In addition to reactions [Il 12) through [II 15] are the Hornbeck -Molnar or chemi-ionization processes which involve excited, but not metastable states and neutral molecules< 25 ) viz : A*+ XY -AY:/+ + e + k e [II 16]

PAGE 13

9 A + XY --AX+ + Y + e + k e [ II. 17) Such processes have recently been investigated by Munson Field and Franklin .< 26 )

PAGE 14

CHAPTER III DESCRIPTION OF APPARATUS The apparatus used in determining inelas t ic cross sections consis ts o f a high-resolution, fi rst order direction-focussing mass spectrometer, schematically illustrated in Figure 1. The basic design o f the instrument is due to M G. Inghram. A theoretical resolution of 1:600 was employed for all measurements The primary ions and metastable atoms used in this experiment are generated in region 1 ( Figure 1 ) by an electron bombardmen t source, shown full scale in detail in Figure 2. The electrons are produced by th ermionic emission from an iridium filament F, cataphoretically coated with thorium oxide in a manner described by Muschlitz, Randolph, and Ratti. ( 27 ) After acceleration t o the anode A, the electrons pass thr ough a 0 010 by 0.200 inch slit into the equipotential region N and are collected by trap T Gas is leaked into the source at L, behind T For the ion-molecule reac:tion experiments, positive ions formed in the region N are pushed out of the source through a 0 010 by 0 300 inch exit slit by the repeller R C orrection for slight beam misalignment from the apparatus geometry is accomplished by the split electrode<_ by application of potentials up to 45 volts across the plates Suitable beam collimation and acceleration is provided by the G 2 ele c trodes which are at the 10

PAGE 15

11 5 Figure 1.--Schematic Diagram of Mass Spectrometer

PAGE 16

I G2 GI (I) "'r\... il II I I I cI n n ~t ,JI' rf77 I I QL .L _J. .-4I I v "' Figure 2.--Diagram of Ion Source 12 I I G I (ll)

PAGE 17

same potential. The ions then pass through a region containing the scattering gas in a cylindrical chamber C, situated between the upper G2 electrode and J1 The chamber C makes electrical contact with J1 and is insulated from< by a ceramic ring, I. In this manner, a small drawout field may be applied within the scattering region The mass spectrometer, collision chamber, and ion source components, with the exception of the filament, were constructed from inconel The electrodes were fabricated directly from 0 026 inch inconel sheet The whole assembly was supported by four precision ground glass rods and insulated by sleeves made of glass tubing. Only two of these rods are shown in Figure 2 Primary ions from the source, together with secondary ions formed in the chamber, traverse the accelerating region 2 ( Figure 1 ) and are given 5 kilovolts of acceleration by the time they reach the analyzer region 3 The ions then pass through region 5 containin g a homogeneous magnetic field where they are mass selected They then pass through an exit slit before impingement on the first dynode of a 16 stage Nuclide Analysis Associates electron multiplier housed in region 4 This multiplier has a gain of 10 5 The electron current from the multiplier passes through an appropriate high resistance and 13 the resulting voltage is measured by a detection system consisting of a Cary vibrating reed electrometer and a Varian Associates chart recorder. The 5 kilovolt accelerating voltage is provided by a variable, regulated, high-voltage supply manufactured by Northeast

PAGE 18

\ Scientific Company An electronic sweep circuit is used for sweeping the magnetic field All mass spectra were obtained in this fashion. The field strength was precisely determined by a Harvey Wells NMR gaussmeter Differential pumping is performed by three three-stage mercury diffusion pumps which maintain pressures down to 1 x 107 mm in the instrument An auxiliary vacuum system is employed for gas purification and handling Two manifolds are used, one for gas leakage through a needle valve into the ion source the other for introducing gas into the scattering chamber through a glass capillary leak The helium nitrogen argon, deuterium, and hydrogen used were obtained fran cylinders and were purified by passage through a trap containing degassed charcoal at low temperature Impurities in these gases were estimated to be less than one part in 10 3 as detennined by mass spectra The oxygen and krypton were of spectroscopic grade purity and were taken from glass flasks supplied by the Air Reduction Sales Compa ny Impurities amounted to approximately one part in 10 5 The reagent grade methane and ethane used were supplied in cylinders by the Phillips Company and were guaranteed to contain not more than forty parts and o ne part of impurity per 10 4 respectively I 1 4

PAGE 19

CHAPTER IV EXPERIMENTAL PROCEDURE AND RESULTS In order to obtain a beam of argon ions for ion-molecule cross section detenninations, the following method was employed: Argon was leaked through L (Figure 2 ) into the source Ions fonned in region N were pushed out by an appropriate potential on the repeller electrode The magnetic field necessary for the focussing of argon ions was found by manual sweep of the field The beam is further focussed by adjustment of the deflector plates G 2 and the beam centering electrodes J 4 and J 5 Hydrogen was then introduced into the collision chamber C where reaction with the primary beam occurred, resulting in the formation of ArH+ Since the Arlt+ and Ar+ ions are formed in different regions of the mass spectrometer, optimum focussing conditions for the extraction of fonner ions had to be detennined by adjusting J 1 A potential difference of -3 volts on J 1 with respect to G 2 was found sufficient and was used in all the following ion-molecule cross section measurements To prove that the secondary ions were originating within the chamber C, a pressure dependence study was made by plotting the ratio ArH+ / Ar+ against the backing pressure of hydrogen behind the chamber The results of such a plot a re illustrated 1 5

PAGE 20

16 in Figure 3, in which a linear pressure relationship is obtained; indicating ArH+ formation did occur in the collision chamber. A residual amount of mass 41 remained at zero hydrogen backing pressure. This is more than likely due to an impurity in the argon. This impurity may be ArH+ formed as a result of the ion-molecule reaction (11.7] in the source, or to another ion of mass 41 In view of the high repeller potential {100 volts) used in the operation of the ion source, the former possibility is un1ikely.(l,l 6 17 ) Back diffusion of hydrogen into the source was shown to be negligible, even with no argon present in the source. All these facts point to the occurrence of ion-molecule reaction [Il.7] in the collision chamber. In order to calculate the cross section for reaction (11.7), the pressure of the scattering gas must be known. To determine this pressure, a reaction of known cross section was studied By measuring the r a tio of secondary to primary ions, the pres sure may be calculated from equation [II. 11]. The charge exchange reaction between hydrogen molecular ions and hydrogen was studied and the pressure of hydrogen in the chamber determined using the cross section measurements o f Cramer.( 2 8) Figure 4 shows the results of the H 2 +, } charge exchange experiments. The ratio of secondary to primary H 2 + ions are plotted against the backing pressure crf hydrogen in the collision chamber. The pressure calculated was 5.76 x 107 mm, corresponding to a hydrogen backing pressure of SO mm.

PAGE 21

6 5 4 0 X +3
PAGE 22

LO 0 X + N I .................... + N I 10 20 30 40 50 60 70 PRESSURE Hz (mm) Figure 4.--H 2 +'/H 2 + Versus Backing Pressure of H 2

PAGE 23

The charge exchange reaction between fast ions and neutral molecules results in the production of slow ions and fast molecules Use of this fact is made in determining the 19 ratio of secondary to primary hydr o gen i o ns Since the secondaries are formed with essentially thermal energy, these ions will receive less a c celeration than the fast primary i o ns before reaching the analyzer region Therefore a splitting of the + peak was expected to exist with the charge exchange peak appearing on the low energy side of the primary Such a splitting was f o und Furthermore, since the secondary i o ns are formed with little kinetic energy their position in the mass spectrum should be essentially independent of the incident primary ion energy Precise measurements of the field with the NMR gauss meter showed this to be true and indicated that the charge exchange process was being measured in the collision region A 150 volt potential on( with respect to anode and a 3 volt drawout potential on J 1 were used in all charge exchange measurements Measurements under the same conditions were made for the systems Ar+ D._z and D.i + D.i I n Figure 5 are shown the results of the D 2 + D 2 charge exchange employed for calibration of the deuterium backing pressure The pressure calculated in this case was 3.46 x 10 7 mm for a backing pressure of 50 mm of deuterium To obtain the cross sectional dependence of reactions [It 7] and [ II, 8] on energy the incident primary ion energy

PAGE 24

2.0 LO 1.5 0 X + N 0 '+' 1.0 N 0 10 20 30 40 50 60 70 PRESSURE D 2 {mm) Figure S.--D2+ 1 /D2+ Versus Backing Pressure of D2

PAGE 25

21 must be known. Due to t he high repeller fields employed in the source, this energy is no t equal to G 2 the potential di f ference between G 2 and the anode, where the ions are fonned This energy correction is determined by utilizing the mass spectrometer equation [IV l] where Vis the main acceleration potential on the ions with respect to J 1 G 2 is the potential on the G 2 electrode ~ R is correction due to the repeller field for a particular setting of the repeller potential and H 1 r, m; and e have their usual significance Plotting the V + c 2 potential against the square of the magnetic field strength needed to focus the argon ions results in a straigh t line of slope k equal to r2 k = 2mTe [ I V .2 ] The results are shown in Figure 6 The repeller potential is t hen plotted against t he square of the field strength needed to focus the argon ions The results are shown in Figure 7 By extrapolating the results to zero repeller potential the square of the field strength H 0 corresponding to this potential is detennined For this case, equation [IV l] beco mes [ I V 3] I f G 2 is kept constant subtracting equation [IV 3) from [ I V l) gives [IV 4]

PAGE 26

> r<) I 0 X 5 16 5.12 5.08 + > 5.04 4.72 4.74 476 478 480 4.82 4.84 H 2 X 10-G (g 2 ) Figure 6. --V + G 2 Potential Versus Field Strength 2

PAGE 27

40 30 ,,..... > ---a:: 20 10 4.728 4.732 4.736 4.740 4744 H 2 X IO 6 (g 2 ) Figure 7.--Repeller Potential Versus Magnetic Field Strength 2

PAGE 28

Since k can be determined from the slope of the first plot and H 1 H 0 are known, R can be determined. In this manner, the correction R was found to be 65 volts. Therefore, to obtain 24 the absolute ion energy, 65 volts has to be added to the potential of G 2 The results for the cross sectional variation with energy for the ion-molecule reactions [II.7] and [II.8] are shown in Figure 8. The open circles represent the cross sections for reaction [II.7], while [II SJ is represented by black circles. The results show the cross section of the deuterium reaction to be lower than the hydrogen over the energy range studied A slightly sharper increase for the former process in the vicinity of 130 volts is observed. The ion source used in the production of positive ions was also employed for the production of the helium and argon metastable atoms for studying Penning ionization processes. T he method used for production of such excited states consisted first of introducing the gas through the aperture L (Figure 2 ). Then the positive ions formed (Ar + or He+) were focussed by the method outlined earlier. After this was done, the repeller electrode was set at anode potential and the deflector plates G 1 (I) and G 1 (II) were adjusted for maximum deflection By this procedure, ions formed in the source are prevented from entering the collision region. That this was done is borne out by the fact that no helium ions were observed in the mass spectra. The metastable atoms, unaffected by these

PAGE 29

(\J O
PAGE 30

electric fields diffuse upward through the collimation system where they encounter molecules in the collision chamber It was found necessary to operate G1 and G2 at 40 volts negative with respect to anode in order to suppress electron leakage through the ion source exit slit These conditions were maintained during all measurements The scattering gas first used was nitrogen with helium in the source An ion peak appeared at mass 28 and was presumed to be N 2 + The peak height was then determined as a function of the electron energy. It was found that the largest drop in intensity occurred between 32 and 24 volts electron energy j vanishing at about 24 volts This checked well qualitatively with the appearance potential curve of He* detennined by Smith and Muschlitz, ( 6 ) and afforded proof that the N 2 + ions were formed as a result of inelastic collisions between helium metastable atoms and nitrogen molecules and not by electron impact. The anode was maintained at 50 volts with respect to the filament for all subsequent measurements, 26 To detennine optimum drawout conditions for the extraction of N 2 + ions t the area under the N2+ mass spectrum peak was measured as a function of the J 1 drawout potential in volts Figure 9 shows such a plot. The area increases rapidly then levels off at about -30 volts A drawout field of -24 volts was used in all subsequent measurements, It will be noticed that the intercept at zero area corresponds to a drawout potential of J 1 of about volts indicating th at the inelastic collision

PAGE 31

6 5 (/) -C ::) 4 >.... 0 .... -3 ..0 ....
PAGE 32

between a metastable helium atom and a nitrogen molecule results in the fonnation of very slow N 2 + ions. 28 A pressure dependenee plot of the N 2 + intensity against the backing pressure of nitrogen was performed and is shown in Figure 10 A linear dependenoe of the peak intensity with backing pressure resulted Furthermore the N 2 + peak vanished when all the helium was pumped out of the ion source chamber This was additional proof as to the nature and origin of the ion formation Tests with oxygen substituted for nitrogen were performed Wlder the same conditions Two peaks were discovered having masses 16 and 32 t respectively The abundance ratio was on the order of 3:1, in favor of mass 32 Identical operating conditions were used for methane and ethane Several ion fragments of the molecules result from inelastic collisions with metastable heliwn atoms The mass spectra resulting from this type of process were recorded and the relative abundance of the fragment ions were compared with conventional electron impact spectra A typical mass spectrum of methane resulting from collisions with He* is illustrated in Figure 11 The electron impact spectra were obtained in a conventional manner by introducing methane and ethane into the ion source and recording the relative abundances at various electron energies The relative abundances of the ion fragments at 20 and 40 volts electron energy were compared to the abundances resulting from the Penning ionization process

PAGE 33

8 6 > E + ~4 2 20 40 60 80 100 120 PRESSURE N 2 {mm) Fi g ur e 10.--~+ Int ensity Versus Backing Pressure of N 2

PAGE 34

? 13 0 CH + 2 14.0 15 0 MASS Figure 11 ,-Mass Spectrum of Methane 16.0

PAGE 35

31 Table 1 summarizes the various systems studied. Column l lists the metastable atom involved in the process6 Column 2 lists the scattering gas The positive ion formed is shown in column 3 The abundance of the ion fragments is listed in column 4 together with the relative abundances at the two electron energies The probable process involved appearance potential of the ion formed and references are shown in the next three columns

PAGE 36

TABLE 1 SUMMARY OF PENNING IONIZATION PROCESSES Metastable ton Observed Abundance Atom Gas Formed A + XY. Electron Impact Process A._ P Reference 20 ev 40 ev ~ ev ) He* 1 N2 ( ~11 Nz + ( x2;/ ) 100 ? N 2 + ( X 2 Eg+ ) + e 15 577 29 0 2( 32:g -) 02+ ( X2,rg ) 100 10 o 2 o 2 + ( x21tg ) + e 12 2 29 o+ ( 4 s ) 33 o 2 o+ ( 4 s ) + o ( 3i> ) + e 18 69 29 30 o 2 o+ ( 2 D) + o20 53 30 31 CH 4 CH + 4 75 10 100 100 C H 4 + e CH 4 + + 2e 13 12 32 C H+ 3 100 82 80 CH:/+ H + 2e 14 .. 39 32 CH2 + 20 18 20 C H2 + + R.z + 2 e 16 5 33 to) t.) (lS S] 12

PAGE 37

TABLE 1 -Continued Metastable Ion Observed Abundance Atom Gas Formed A*+ "XY Electron Impact Process A P Reference 20 ev 40 ev ( ev) C 2H6 C 2H6+ 42 10 22 26 C 2 H 6 + e --c 2H6+ + 2e 11.5 34 He C2H5+ 41 20 21 --C 2H5+ + H + 2e 13 05 35 C 2H4 + 100 100 100 + -C 2H4 + H2 + 2e ll 44 36 C2H3 + 60 27 33 c H + 2 3 + H + H 2 + 2e 15 50 35 C 2H2 + 29 12 20 C 2 J+ + 2"2 + 2e 1 5 3 35 [24 6] 15 CH+ 3 17 4 7 --c H 3 + + C H 3 + 2e 14 ~ 35 Ar* C 2H6 C2H6 + 90 10 22 26 C 2H6 + e -c 2 u6+ + 2e 11.5 34 I:>' C'3 C 2H4+ 100 100 100 C2H4+ + H2 + 2e ll 44 36

PAGE 38

CHAPTER V DISCUSSION The results for the cross s ectional variation with energy f or the ion-molecule reaction s [ II.7] and [II 8] show that the A r+ D 2 system has a lower cross section than the Ar+, H 2 system over the energy range studied Unfortunately, no systematic study of these reactions have been made under similar conditions, therefore no direct comparisons can be made Stevenson and Schissler< 16 17 ) have made a study of these reactions in the ionization chamber of their ion source and have reported cross sec t ions for these systems Their cross sections however, have been derived from specific rate constants calculated from assumed field conditions and molecular distribution functions in the ionization region of their source,(l) hence the energy a t which the hydride and deuteride ions are formed can have a wide range of values They have reported cross sections of 59i 2 and 33A 2 for ion-molecule reactions [11.7) and [ II. SJ, respectively, at their highest repeller voltage of 41.l volts/cm The resul t s of this work also show approximately a 2:1 ratio for the corresponding cross sections over the energy range studied According to the theory of Gioumousis and Stevenson, ( l ) the ratio should have been in the neighborhood of 1 38~1. 34

PAGE 39

The Kr+, H 2 system was tried without success in order to measure the cross section o f the ion-molecule reaction 35 [V l] No Krll+ was detected, al~hough a cross section 1 / 10 that of [II. 7] could have been measured Stevenson < 16 ,l 7 ) reported a much lower value for the Kr+, H 2 cross section, about 1 / 3 that of the Ar+, H 2 system The energy_dependenee of the cross sections assuming a relation, a = kEn, were determined by plotting the logarithm of the cross section a, against the logarithm of the incident ion energy, E, as shown in Figure 12 The slope n of the curve, equal to the energy dependence of o, was found to be about 3 /2 for both the Ar+, H 2 and Ar+, systems, remaining constant up to about 155 volts and rapidly levelling off to zero beyond this energy For energies not much greater than thermal the Stevenson-Gioumousis theory ( l ) predicts the cross section to vary inversely with the square root of the energy. For high incident ion energies the theory would not be expected to hold It is possible, therefore, that the energy range used in this work was higher than the limit for which the theory is valid. It should be remembered that the cross sections calculated for these systems depend directly on the correct pressure of H2 in the chamber, It is not certain, in these experiments, that all the charge exchange secondaries are being extracted from the reaction region This would lead to low calculated pressures ;

PAGE 40

1.6 1.5 1.4 b <.9 1.3 0 _j 1.2 1.1 1.0 19 2.0 2 1 LOGE 2 .2 F i g u re l2 .--L og cr Vers u s L og E 36 2 3 24

PAGE 41

which would result in high calculated cross sections Since all measurements for each system were made at the same pressure ; however an error in the pressure determination would not affect the relative values of the cross sections nor their functional dependence on energy 37 The results of the inelastic collisions between metastable atoms and molt?cules are summarized in Table 1. The first mass spectrum of nitrogen with He* revealed the } + ion, while was absent The appearance potentials of N 2 + and N+ are 1s ss1 < 29 ) and 24 30 1 ev ,< 29 3 o ) respectively The metastable atoms have energies greater than the former but less than the latter therefore this observation is not difficult to explain With oxygen and He* o 2 + and o+ were observed Since these ions require only 12 2 < 29 ) and 18 69 ev < 29 3 o ) for ionization respectively there is enough energy of excitation available in He* for their formation The hydrocarbon gas methane, gave several ion fragments with He*, the abundances of which are summarized in co lumn 4 o f Table 1 I t can be seen in this cas e, that the parent ion is not the most abundant ion f o rmed in the Penning process, whereas it is dominant in the electron impact spectra The peak at mass 13 3 ( Figure 11 ) was pr o bably due to metastable ion decomposition o f C H 3 + or C H 4 + just bef o re entering the analyzer regi o n .( 35 ) The c + and C H+ ions at masses 12 0 and 13 0 respectively were not o bserved The CH+ ion could have been produced by one of two processes : C H 4 + e --C H+ + SH + 2e [ V. 2]

PAGE 42

38 or [V.3] The appearance p o tential of the former process is rep o rted to be 23.4 ev} 37 ) and 18,9 ev is calculated for the latter process It is interesting to note that [V. 3] does not t ake place either for electron bombardment< 32 ) or for Penning ioniza t ion McDowell and Warren < 32 ) have sh o wn [V.2] to be the process responsible for the production of C H+ by elec t ron bombardmen t Conclusions similar to those for [V 3] may be drawn for the reaction McDowell and Warren ( 32 ) have shown the reaction C H4 + e c + + 4H + 2e, requiring 26.2 ev to be responsible for the production of C 4 by electron impact of methane, instead of [V.4] which would [V 4] [V. 5] require only 17 2 ev The substitution of Ar for He resulted in the absence of any ions from methane. This would indicate that the fragments require more t han the ll.720 ev energy of excita t ion in Ar* available for their formation For reactions of He with ethane, ions having masses 30, 29, 28, 27, 26, 24.6, and 15 were observed. No marked departures are observed when comparing the abundances resulting from the Penning and electron impact processes with the possible exception of the fragmen t ed ions being in greater proportion, relative to t he parent ion, from the Penning process than from

PAGE 43

electron impact The mass peak at 24 6 may be the result of a metastable ion decomposition in a manner earlier described .( 35 ) The absence of the ions C 2 + and C 2H+ are supported by appearance potential measurements .( 3 s ) It is difficult to suggest an explanation for the abs enc e of C H2 + It is i nteresting that C 2 H 6 +and C 2 H 4 + were the only peaks observed for the system C 2H6, Ar* The absence of other ion fragments is well accounted for by appearance potential measurements .( 35 ) The fact that the ions C 2 H 6 + and C 2 H4+ are fonned in collisions with excited atoms having energies of 11 720 and ll 546 ev provides a means of detennining the maximum appearance potentials for the processes 39 [V 6] and [V 7] Values of u s ,( 34 ) 11.60, ( 39 ) and 11.78 ev < 36 ) have been among the many reported for the fonner process It seems in the light of the present work that the latter value of 11 78 ev is not acceptable For the latter reaction 11 44 ( 36 ) 12 09 ,( 4 o ) and 12 1 ev ( 34 ) were reported by various investigators. The last two values are not consistent with the present investigation The estimation of the relative heights of the mass spectra peaks constitutes a major source of error for the Penning ionization work Because of the high sensitivities needed to detect these ions ( the largest current is 1018 ampere ),

PAGE 44

the peaks were accompanied by a great deal of noise. The errors were estimated as for the relative abundances of the ions. A statistical theory of mass spectra developed by Eyring and coworkers( 41 ) satisfactorily explains, in many cases, the relative abundances of fragment ions resulting from electron impacts with molecules having many electronic and vibrational degrees of freedom. These ions result from Franck-Condon type transitions and consequently, the interaction time between an electron and the target molecule is exceedingly short. The Penning process differs from that of electron impact because of the relatively long duration of the collision between the target molecule and the thermal energy metastable atom. In the former process, excitation energy is transferred but in the latter, the kinetic energy of the electron is converted into excitational and vibrational energy. Furthermore, the target molecules studied in this work do not have the large number of degrees of freedom required for a statistical theory to be valid. 40 Ferguson< 42 ) has calculated cross sections for Penning ionization utilizing a simple classical momentum-transfer collision model. His calculations agree, within an order of magnitude, with experimental measurements. However, no theory has yet been developed which would predict the relative abundances of fragment ions resulting from Penning ionization.

PAGE 45

CHAPTER V I SUMMARY A high-resolution mass spectrometer was used to measure the ine las t ic cross sec ti ons for the ion-molecule reactions and in the energy range 60-215 ev The rare gas ions were produced in a conventional electron bombardment source and were suitably collimated before entering a cylindrical chamber containing the scattering gas The pressure 0 the scattering gas was calculated from measurements of the symmetrical charge exchange process and the known cross sections for this process The cross sections were fom1d to be in the ratio of about 2:1 over the energy range studied with the first reaction exhibiting the larger cross section The cross sections were fom1d to vary inversely with the 3 / 2 power of the energy in the range 60-155 ev; rapidly levelling off to zero energy dependence beyond 155 ev The ion source used for the production of positive i o ns was also employed for the production of helium and arg o n metastable 41

PAGE 46

atoms for studying the Penning ionization processes of the type + A + XY X + Y + A + e + k e Here A is the metastable atom involved in the collision with a molecule XY The target gases methane, ethane oxygen and nitrogen were used Several ion fragments resulted from the inelastic collisions of methane and ethane with the metastable atoms The resulting mass spectra of such fragment ions were compared to the spectra obtained from conventional electron bombardment studies The measurements have demonstrated for the first time that fragment ions are formed in a Penning ionization process provided the metastable atom has sufficient energy These ions have been identified by mass spectroscopy and significant differences in their relative abundances as compared with those observed for electron impact have been observed Information relative to the upper limits for the appearance potentials of these ions and the mode of fragmentation was obtained from these studies 42

PAGE 47

BIBLIOGRAPHY 1. G Gioumousis and D P Stevenson t J Chem Phys ~ 294 ( 1958 ). 2 L. Dunoyer Comptes Rendus 152 594 ( 19ll ). 3 o Stern Zeits f Physik !?_ 751 ( 1926 ). 4 o Stern Zeits f Physik _g, 563 ( 1927 ). 5 I. Amdur E A Mason and J 527 ( 1957 ). E Jordan J C hem Phys !Z_, 6 G M Smith and E E Muschlitz Jr., J C hem Phys ~ 1819 ( 1960 ). 7 w P Sholette and E E Muschlitz Jr ., J. C hem. Phys ~, 3368 ( 1962 ). 8 C. Ramsauer and R Kol.lath Ann der Physik 16, 570 ( 1933 ). 9 A S Russell C. M Fontana and J H Simons J Chem Phys 2., 381 ( 1941 ). 10 J. H Simons H T Francis C M Fontana, an d S R Jackson Rev Sci Ins tr ll 419 (1 942 ). 11 J H Simons C M Fontana E E Muschlitz, Jr ., and S R Jackson J. C hem Phys ll 307 ( 1943 ). 12 J H Simons C. M Fon tana H T Francis and t G Unger J C hem Phys _g,, 312 ( 1943 ). 13 J. H Simons H T Francis E E Muschlitz Jr ., and G C. Fryburg J C hem Phys .!,! 316 ( 1943 ). 14 J H Simons E E Muschlitz J r ., and t G. Unger J C hem Phys ,,!!, 3 22 ( 1943 ). 15 F H Fi.eld J. L Franklin and F W Lampe J. Am
PAGE 48

44 17 D P Stevenson and D O Schissler J Chem Phys 24, 926 ( 1956 ). 18 C. F Giese and W B Maier II J Chern Phys 35 1913 ( 1961 ). 19 E Lindholm Arkiv Fysik _! 257 433 ( 1954 ). 20 H S w Massey and E H S 13urhop Electronic and Ionic ~act Phenomena ( Oxford University Press London 1956 ), 47 21. T R Hogness and R w Harkness Phys Rev !! 784 ( 1928 ). 22 H D Smyth Revs Mod Phys 2, 347 ( 1931 ). 23 A A Kruithof and F M Penning Physics i, 430 ( 1937 ). 24 W P Jesse and J Sadauskis Phys Rev .!22_ 1755 ( 1955 ). 25 J Hornbeck and J P Molnar Phys Rev !_! 621 ( 1951 ). 26 M 27 E 28 w 29 G 30 G 31 t 32 c. 33 J 34 M 35 F S B Munson, F H Field and J L Franklin J C hem Phys .,, 1790 ( 1962 ). E Muschlitz Jr ., H D Randolph and J N Ratti Rev Sci Instr ~, 446 f l962 ). H Cramer J Chem Phys ~ 836 ( 1961 ). Hertzberg S~ectra of Diatood c Molecules ( Van Nostrand ; New York, 159 ), 449 450,459,558 Hertzberg Atomic s 5 ectra and Atomic Stru c ture ( Dover New York l944 ) 2 o M Branscomb D S Burch S J. Smith; and S Geltman Phys Rev 111 504 ( 1958 ). A McDowell and J. W Warren Faraday Soc Disc 10 53 ( 1951 ). Geerk and H Neuert z Naturforsch 2,!, 502 ( 1950 ). B Koffel and R A Lad J C hem Phys 1! 420 ( 1948 ). H Field and J L Franklin Electron !~act Phenomena (Academic Press, New York 1957 ) 194 48 249, 252 and 254 36 J J Mitchell and F F Coleman J C hem Phys 44 ( 1949 ).

PAGE 49

45 37 ~ L Smith Phys Rev g 263 ( 1937 ). 38 J A Hipple Phys ~ Rev ~ 530 ( 1938 )~ 39~ J L, Franklin and H E Lumpkin J Amer. Chem Soc ]i 1023 ( 1952) 40 D P Stevenson and J A Hipple J 1588 ( 1942 ). Amer ~ C hem. Soc .2,! 41 H M R o sens t ock, A L Wahrhaf1:ig, and H Eyring, Technical Report No II, June 25 1952, Univ of U tah, Ins t f or Study of Rate Processes Salt Lake C ity 4 2 E E Ferguson Phys Rev .!!?_, 210 ( 1962 ).

PAGE 50

BIOGRAPHICAL SKETCH Morris Jacob Weiss was born on December 3, 1936, in Manhattan New York He attended elementary school in Brooklyn New York and graduated from Abraham Lincoln High School, in that Borough, on June 1954 In June 1958 he received the Bachelor of Arts from Brooklyn College, with a major in Chemistry In September, 1958 he entered the Graduate School of the University of Florida where he held an assistantship in the Department of Chemistry and worked tow~rd the degree of Doctor of Philosophy Morris Jacob Weiss is married to the farmer Betty May Marcus a University of Florida graduate He is a member of the American Physical Society 46

PAGE 51

This dissertation was prepared 11J1der the direction of the chairman of the candidate s supervisory committee and has been approved by all members of that committee It was submitted to the Dean of the C ollege of Arts and Sciences and to the Graduate C ouncil and was approved as partial fulfillment of the requirements for the degree of Doctor of Philosophy. April 20 1963 Dean, Gradua te School Supervisory Commi tte e:


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