• TABLE OF CONTENTS
HIDE
 Front Cover
 Acknowledgement
 Table of Contents
 List of Tables
 List of Figures
 Introduction
 Survey of metastable atoms and...
 Description of apparatus
 Experimental procedure
 Results and discussion
 Appendices
 References
 Biographical sketch
 Back Cover














Title: Associative and dissociative ionization of gases on impact of metastable atoms
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Title: Associative and dissociative ionization of gases on impact of metastable atoms
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Creator: Herce, John Austin, 1940-
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Copyright Date: 1967
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Table of Contents
    Front Cover
        Page i
        Page ii
    Acknowledgement
        Page iii
    Table of Contents
        Page iv
    List of Tables
        Page v
    List of Figures
        Page vi
    Introduction
        Page 1
        Page 2
        Page 3
    Survey of metastable atoms and inelastic processes
        Page 4
        Page 5
        Page 6
        Page 7
        Page 8
        Page 9
    Description of apparatus
        Page 10
        Page 11
        Page 12
        Page 13
        Page 14
        Page 15
    Experimental procedure
        Page 16
        Page 17
        Page 18
        Page 19
        Page 20
        Page 21
        Page 22
        Page 23
        Page 24
        Page 25
        Page 26
        Page 27
        Page 28
        Page 29
        Page 30
        Page 31
        Page 32
    Results and discussion
        Page 33
        Page 34
        Page 35
        Page 36
        Page 37
        Page 38
        Page 39
        Page 40
        Page 41
        Page 42
        Page 43
        Page 44
    Appendices
        Page 45
        Page 46
        Page 47
        Page 48
        Page 49
        Page 50
    References
        Page 51
        Page 52
    Biographical sketch
        Page 53
        Page 54
    Back Cover
        Page 55
        Page 56
Full Text













ASSOCIATIVE AND DISSOCIATIVE
IONIZATION OF GASES ON IMPACT
OF METASTABLE ATOMS











By
JOHN AUSTIN HERCE


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
December, 1967

































To

Sam and Archie











ACKNOWLEDGMENT


The author wishes to express his sincere appreciation to

Professor E. E. Muschlitz, Chairman of his Supervisory Committee,

for his assistance and guidance, to Dr. R. J. Cross, who was re-

sponsible for the design and construction of the molecular beam

source, to Dr. K. D. Foster, for his helpful suggestions and aid

in performing the experiments, and to the.other members of his

Supervisory Committee for their helpful suggestions in the writing

of this dissertation.

The author is also grateful to the American Chemical

Society Petroleum Research Fund for the financial assistance that

made this work possible.


iii









TABLE OF CONTENTS


ACKNOWLEDGMENT ........................

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

LIST OF FIGURES .......................

Chapter


I. INTRODUCTION ................................................

II. SURVEY OF METASTABLE ATOMS AND
INELASTIC PROCESSES .........................................

III. DESCRIPTION OF APPARATUS ....................................

IV. EXPERIMENTAL PROCEDURE ......................................

V. RESULTS AND DISCUSSION .......................... .........

Rare Gas Systems ............................................

He 02 System .............................................

He CH and He CD4 Systems .............................

Future Direction of Research ................................

APPENDICES .........................................................

REFERENCES .........................................................

BIOGRAPHICAL SKETCH ................................................


Page
iii

v

vi


11 111 1


11: :111












LIST OF TABLES


Table Page

1. Determination of Absence of Highly Excited
States in Beam ............................................... 25

2. Individual Ion Abundances and Ionization Cross
Sections for the Rare Gas Systems ............................ 36

3. Individual Ion Abundances and Ionization Cross
Sections for the Metastable Helium Oxygen
System ....................................................... 40

4. Individual Ion Abundances for the Metastable
Helium Methane and Methane d4 Systems .................... 42

5. Calculated Values of R/(l + R) as a Function
of the Energy of Exciting Electrons .......................... 50











LIST OF FIGURES

Figure

1. Cross Section of Apparatus ..........................

2. Isometric View of Collision Chamber .................

3. Primary Beam Intensity Versus Electron Energy .......

4. Ion Intensity Versus Drawout Potential ..............

5. Ion Intensity Versus Argon Backing Pressure .........

6. Ion Intensity Versus Argon Backing Pressure .........

7. Ion Intensity Versus Helium Backing Pressure ........
8. Per Cent Ion Abundance Versus R/(1 + R) for He + Ar

9. Per Cent Ion Abundance Versus R/(l + R) for He + Kr

10. Per Cent Ion Abundance Versus R/(1 + R) for He + 02
11. Per Cent on Abundance Versus R + R) for He + CH
12. Per Cent Ion Abundance Versus R/(1 + R) for He + CD
12. Per Cent Ion Abundance Versus R/(l + R) for He + CD4


13. Schematic Diagram of Potential Energy Curves Describing
Energy Transfer Processes ....................................


Page

11


11111:_11















CHAPTER I


INTRODUCTION


In the past much of our knowledge of intermolecular forces

has been obtained by relating certain macroscopic properties to the

microscopic forces responsible for them. The theory of gaseous trans-

port processes such as diffusion, heat conductivity and viscosity -

was developed on the basic assumption that the molecules behave like

point-centers of force. In general, these calculations depended upon

detailed information concerning the variation of the molecular veloc-

ity distribution of the gases, and unless the gases were in thermal

equilibrium, these variations could not be calculated directly. Fre-

quently, very complicated theories are required to circumvent a lack

of detailed knowledge of the interactions taking place. With the

advent of molecular beam techniques, however, direct and often de-

tailed information may be obtained regarding the intermolecular forces

acting between atoms and molecules.1'2

A molecular beam may be defined as a unidirectional beam

of neutral atoms or molecules at such low pressures that the effect

of molecular collisions is essentially negligible. Ordinarily a molec-

ular beam is formed by effusive flow of atoms or molecules through

an orifice,with subsequent beam definition by one or more apertures

aligned with the first. The earliest molecular beam experiments were

those of Dunoyer in 1911. His apparatus consisted of an evacuated

1










glass tube divided into three separately evacuated chambers. Sodium

metal introduced into the first chamber was vaporized, whereupon a

deposit of sodium was noted in the third chamber. He concluded from

these experiments that the sodium atoms traveled in straight lines.

The work of Stern and Knauer in 19264,5 provided the basis for many

of the molecular beam techniques which have followed. One of the

most important applications of molecular beams has been the measure-

ment of spins and magnetic moments of both atoms and molecules by

deflection in an inhomogeneous magnetic field.67

Collisions between particles may be classified as either

elastic or inelastic processes. This classification is solely de-

pendent on whether the particles undergoing collisions suffer a

change in either mass or internal energy. Elastic collisions may

be viewed in terms of the interaction potential existing between the

colliding species; on the other hand, inelastic collisions involve

a change in the internal energy of the reactants or an atom-inter-

change between them.

A chemical reaction implies an inelastic collision between

the reacting species. Excess energy may appear either as a change

in the internal energy or kinetic energy of translation of the inter-

acting particles. Scattering experiments have furnished a means for

investigating the kinematics of these reactions and enable the de-
8
tection of short-lived reaction intermediates. It is obvious that

these short-lived species could never be studied by a measurement of

some bulk property.

Collisions of excited neutrals with thermal energy atoms










or molecules resulting in ionization were discovered by Penning in

9
his studies of gaseous discharge processes. On introducing an argon

impurity into a pure helium discharge, a larger current was attained

than that measured for the pure helium discharge. Penning reasoned

that this current enhancement resulted from the ionization of argon

by metastable helium atoms. Hornbeck and Molnar0 investigated the

formation of rare gas molecular ions and concluded that these ions

result from collisions between excited and ground state rare gas

11
atoms. Sholette and Muschlitz obtained the first cross sections

for ionizing collisions of metastable helium atoms in various gases.

Cermik and Herman12 made the first comprehensive study of the ion-

ization and fragmentation of the lower hydrocarbons by metastable

rare gas atoms.

It is the purpose of this work to investigate associative

and dissociative ionization resulting from impact of thermal energy

helium and neon metastable atoms on various gases.












CHAPTER II


SURVEY OF METASTABLE ATOMS AND INELASTIC PROCESSES


Excited atoms or molecules with lifetimes greater than

a microsecond are generally described as metastable, the transi-

tion to the ground state being forbidden by the selection rules

governing electric dipole radiation. These selection rules are:

(a) AJ = 0, +1 J = 0 + J = 0

(b) AS = 0

(c) Al =+ 1

where

1 = orbital angular momentum of an electron

J = vector sum of L and S

L = orbital angular momentum = li
i
S = spin angular momentum.

The 2 S0 and 23 S metastable states of helium lie 20.61 and 19.82 eV

above the ground state, respectively. Decay of the 21 S states to

the ground state is forbidden by selection rules (a) and (c), while

(b) and (c) forbid transitions of the 2 3S state to ground. Simi-
3 3
larly, neon has two metastable states, P2 and P0, which lie 16.62

and 16.71 eV above the ground state.

Although forbidden by electric dipole processes, decay of

metastable atoms to the ground state can occur by double photon










emission and magnetic dipole radiation. These latter mechanisms are
-5
about 10- as probable as ordinary electric dipole radiation, and,

consequently, the lifetimes of these metastable states are on the

order of a second. For comparison, lifetimes of excited states for

which electric dipole radiation is allowed are approximately 10-9

second.

No experimental determination of the metastable helium life-

times is available, but calculated lifetimes are on the order of one
13 1
second or greater.13 The 2 S0 metastable state can be quenched in

14
the presence of a strong electric field. Holt and Krotkov14 attained

(90 + 2) per cent quenching of the 21 S state of helium at a field

strength of 226 kv./cm.

Beams of excited particles have been produced by gaseous

discharge and electron impact. The former method usually results in

a higher intensity of the excited species, but is not very selective

and produces several atomic or molecular excited states. Selective

excitation of a beam of neutrals can be attained by electron impact,

but care must be taken to ensure that beam collimination is not de-

stroyed by recoil of the excited particles. Normally, the beam of

controlled energy electrons is located either directly in front or

in back of the first colliminating aperture of the beam source.

Electron ejection from metal surfaces by incident meta-

stable atoms is known to occur, and this property has often been

used for metastable atom detection. Oliphantl5 first observed elec-

tron ejection from a gas-covered molybdenum surface by metastable








16
helium atoms. Greene extended this work to include argon and neon

metastable atoms. Stebbings7 measured the electron yield from a

gas-covered gold surface. He concluded that his value of 0.29 for

the electron yield represented that for the triplet state of meta-

stable helium. More recently, MacLennanl8 reported the absolute

electron yield of helium and neon metastable atoms incident on an

atomically clean polycrystalline tungsten surface. The value for
1 3
helium is 0.306 + 0.025, where the 2 S0 and 2 S1 yields are equal

within his experimental error; the neon value is 0.215 + 0.020. This

latter determination did not include resolution of the contributions

of the two metastable states.

Excitation of helium by electron impact has been studied

by various methods. The most recent determinations of excitation

efficiency curves of the singlet and triplet states of helium are

those of dermakl9 and Dugan, et al.20 Cermak obtained separate ex-

citation functions in the energy range 20-60 eV for 2 S0 and 2 S1

helium from studies of the kinetic energy of the electrons released

in the Penning ionization of argon by metastable helium atoms. Dugan,

et al. obtained these excitation functions in the range 25-135 eV by

observing the deflection of the beam of metastable atoms in an inhomo-

geneous magnetic field. These authors report the ratio of the singlet

to triplet functions which vary from 0.26 to 2.37 in the energy range

studied.

Several types of ionizing collisions between a metastable

atom and an atom or molecule are possible, e.g.:








+ -
X + Z X + Z + e (II-1)

X + Z XZ + e (11-2)
+
X + Y2 X + Y2 + e (11-3)
+
X + Y2 X + Y + Y + e (11-4)
+ -
X + Y2 XY2 + e (11-5)
+
X + Y2 XY+ Y +Y e (11-6)

The first of these, Penning ionization, represents the conversion of

electronic excitation energy to ionization and kinetic energy. This

process occurs in gaseous discharge and in the radiation chemistry of
9,21
gases. Other cases of Penning ionization are (11-3) and (11-4),

where the molecule-ion may be formed in an excited repulsive state

resulting in dissociative ionization. It is possible in all cases

that these ions may be formed in excited states, but usually the re-

action is nearly resonant, for the electron can carry off the excess
22
energy. Herman and Cermak have studied reactions of types (II-1)

and (11-2) and obtained the ratio of cross sections for the formation

of Hg and ArHg for collision of metastable argon atoms with mercury
23
atoms. Muschlitz and Weiss cite ionization cross sections for the
+ + *
production of 02 and 0 in the He 02 system. Individual total

ionization cross sections for the 2 S0 and 2 S1 metastable states of

helium on various gases have been reported by Sholette and Muschlitz.11

Reaction (11-5) was first investigated by Hornbeck and Molnar.10

For this process to be energetically possible at thermal energies, it

is necessary that the ionization potential of X be less than the

energy required for the process XY2+ + X + Y2. These same authors









also investigated the formation of the homonuclear rare gas molec-

ular ions (11-2) and concluded that they result from the reaction

of an excited, not necessarily metastable, rare gas atom with its
24
ground state counterpart. Kaul, et al.24 have also investigated

these same processes and have shown that there are at least two ex-
+ 25
cited states of helium contributing to the formation of He2 Kaul

has extended these studies to other rare gas systems and reported the

lifetimes of those excited states responsible for the production of
26
the rare gas homonuclear ions. Munson, et al. have measured the

appearance potentials for all homonuclear and heteronuclear rare gas

ions with the exceptions of HeXe and the radon compounds.

Since the beam of metastable helium atoms produced by elec-

tron impact is composed of atoms in more than one metastable state,

the following treatment has been developed to isolate the contribu-

tion of each excited state to the total ionization. This has been

applied to the systems in which the composition of the metastable

beam is known (metastable helium) and more than one ion was produced

in the collision.

Let

AO = per cent ion abundance produced by the mixed beam
A1
A1 = per cent ion abundance due to (2 SO)He
3 *
A3 = per cent ion abundance due to (2 S1)He
1 3 *
R = (2 S0)He /(2 S )He = ratio of metastable states
in the mixed beam.

If the observed ion abundance is considered as the sum of contributions









from each metastable state, then

A0 = [R/(l + R)] Al + {1 [R/(l + R)]} A3

= (A1 A3)[R/(1 + R)] + A3, (11-7)

where R/(1 + R) = fraction of (21S0)He in the mixed beam.

The value of R/(1 + R) has been calculated from the re-

sults of Dugan, et al.20 who measured the ratio (2 S )He/(2 S )He

as a function of the energy of the electron beam used to produce

these excited states. The above treatment suggests that a plot of

the experimentally measured ion abundances expressed as a per cent

of the total ionization versus R/(1 + R) should result in a straight

line of slope (A1 A3) and intercept A These data may then be

extrapolated to yield the per cent ionization due entirely to a single

metastable species. From knowledge of the total ionization cross

section for a particular system, that is a measure of the total ion-

ization occurring irrespective of the identity of the product ions,

individual ionization cross sections for each metastable species can

be calculated.














CHAPTER III


DESCRIPTION OF APPARATUS


The experimental arrangement used for this work consists

of a 12 inch radius-of-curvature, 600 sector, magnetic deflection,

first order direction-focusing mass spectrometer modified for molec-

ular beam experiments. A detailed drawing of the beam source, elec-

tron gun, collision chamber and ion accelerating-ion focusing lens

is shown in Figure 1.

The atomic beam was produced by introducing the beam gas

through an array of seven-micron-diameter glass capillaries affixed

to the leading edge of the bellows. This arrangement provides an

adjustable beam source having three degrees of freedom for the pur-

pose of alignment. The source produces a 1/2 inch long by 0.030 inch

wide beam. A portion of the atomic beam is excited in the source re-

gion by an electron beam traversing region N. The resultant beam

composed of metastable atoms and ions then passes through a 1/2 inch

long and 0.040 inch wide slit, D, which serves only to define the

two separately pumped regions. The beam enters the collision chamber

through a 1/2 inch long slot of adjustable width which was set at

0.030 inches.

Excitation of the beam gas was accomplished by electron

bombardment. A cross-sectional view of the electron gun is given in

Figure 1. This electron gun is of cylindrical symmetry and consisted






1t
Mass
Analyzer


J7
\\ \\\ \.-i 6
j9 ______ _____ ;_/
PJ 8 J 4 Pump
C J 5- /J 3
-J 2

S.P '

R S Gas





Pumnp


Figure 1. Cross Section of Apparatus.
I1









of two concentric cylinders C and A with the filament F positioned

in the annular region formed by them. Electrons were produced by

thermionic emission from a directly heated iridium ribbon filament

which had been thoriated by cataphoretic deposition as described by
27
Muschlitz. Electrons were accelerated and reflected across the

equipotential region N by maintaining the outer cylinder C at a nega-

tive potential and the anode A at a positive potential with respect

to the filament. The entire electron gun was fabricated from stain-

less steel. The anode consisted of 0.007 inch tungsten wire wound

about six stainless steel rods. All spacers and insulators were

ceramic. The entire electron gun assembly was insulated from ground

and could be operated at a fixed potential with respect to ground by

means of an auxiliary power supply.

A field of 8.3 kv./cm. was maintained across plates S.P.

to deflect all ions and to quench any highly excited long-lived states

present in the beam. A manually operated shutter S was placed direct-

ly behind these plates, so that the beam of metastables could be in-

tercepted to obtain a zero reading.

The rectangular collision chamber was constructed from brass

and heavily gold plated to obtain surfaces with equal electron-ejection

efficiency. An additional isometric view of the collision chamber is

given in Figure 2. Pressures in the collision region can be measured

with the thermocouple gauge. The electrometer electrode E is employed

for measurement of either total ionization cross sections or metastable

beam intensity. Gaskets fabricated from teflon sheet provide electrical

insulation and were found to be reasonably leak tight.










Teflon Gasket


Electror

Repeller


Box


Thermocouple


Teflon Gasket


Figure 2. Isometric View of Collision Chamber.









In normal operation ions produced in the collision chamber are

extracted at right angles to the incident metastable beam by a drawout

field between the repeller R and J,. Ions emerging from the slit in

Jl are gradually accelerated through a potential difference of five

kilovolts provided by a Northeast Scientific Company, high voltage

power supply. In the course of this acceleration the ion beam is

focused by a series of electrostatic lenses, Jl through J9. The ion

beam is then mass analyzed by a homogeneous magnetic field and finally

impinges upon the first dynode of a secondary electron multiplier.

The field current of the analyzer is electronically regulated

and swept by a power supply and control circuit constructed in this

laboratory. The magnetic field is determined within 0.01 per cent

by a Harvey Wells N.M.R. Gaussmeter. The electron multiplier is a

twenty-stage device having copper-beryllium dynodes and manufactured

by Nuclide Analysis Associates as their model EM-1. The multiplier
5
has a measured gain of 9 x 10 for the mass 39 isotope of K' produced

by surface ionization.

Ion intensity measurements were made by utilizing a pulse-

counting method. The signal from the electron multiplier is ampli-

fied by a Tektronix, Inc., type 1121 amplifier followed by a linear

pulse amplifier constructed in this laboratory. This pulse amplifier

also performs as a pulse shaper and adjusts the baseline over-and

under-shoot. A combination Beckman Counter, model 6020A and Beckman

Printer, model 1453 provide both visual display and printed tape read-

out of the ion intensity. Typical counting intervals of one to ten









seconds were employed.

An auxiliary vacuum system was used for gas manipulation

and purification. The beam and scattering gases were introduced

into their respective chambers through two Vactronic variable-leak

valves. The helium, neon, argon and oxygen were obtained from cyl-

inders and were purified by passage through an adsorption trap filled

with degassed charcoal at low temperatures. These gases are estimated

to have less than one part in 103 impurity as indicated by mass anal-

ysis. The methane used was Phillips Company reagent grade and guar-

anteed not to contain more than 40 parts impurity per 10 The

methane-d4, a Nichem Incorporated product, was observed to contain

approximately one part impurity in 103 and was taken from a glass

flask sealed directly into the auxiliary vacuum manifold. The kryp-

ton, an Air Reduction product, was obtained directly from cylinders

and was guaranteed to contain no more than 36 ppm impurities.













CHAPTER IV


EXPERIMENTAL PROCEDURE


In order to produce and align the beam of metastable atoms,

the following procedure was employed. Helium gas was leaked into the

source region, and a pressure of 100 microns was maintained behind

the capillary array (Figure 1). Excitation of the atomic beam was

accomplished in region N by electron impact, and the resultant beam

of metastable atoms was directed on the entrance slot of the colli-

sion chamber. A 90-volt potential difference across the deflecting

plates S.P. was found to be sufficient to deflect any positive ions

present in the beam. The entire collision chamber was maintained at

-20 volts with respect to the electrode E, which was connected to a

Cary, Model 31, vibrating-reed electrometer. An input resistance of

10 ohms was used. In this mode of operation, a negative current

due to electrons ejected from the walls of the collision chamber by

the metastable atoms was measured at E; this current is proportional

to the metastable beam intensity, since

I_ = y0 (IV-l)

where:

I = measured electron current,

I0 = metastable beam intensity, and

y = metastable helium electron-
ejection efficiency of gold.









By manual adjustment of the movable bellows arrangement, the signal

I was maximized for optimum beam alignment.

The metastable beam current was then measured as a function

of the energy of the electron beam (at constant electron beam current).

The results arc shown in Figure 3. It is seen that the mctastable

beam current exhibits a maximum at 30 electron volts and drops off

at higher electron energies. Qualitatively, this agrees quite well

with the appearance potential curve of He determined by Smith and

Muschlitz.2

In the absence of a metastable beam but with a target gas

in the collision chamber, electrons were purposely accelerated into

the collision chamber by operating the electron gun at 100 volts

negative with respect to ground and with zero deflection field across

plates S.P. The mass spectrometer was then focused for optimum sen-

sitivity on the parent ion resulting from electron impact. This pro-

cedure not only afforded a simple means of focusing the spectrometer

but also provided a convenient means for calibration of the gaussmeter

with an ion of known charge to mass ratio. This procedure was re-

peated for all scattering gases used in this study.

The scattering gas first used was argon with helium in the

beam source. An ion peak appeared at mass 40, coinciding with the Ar+

obtained by electron impact. To determine optimum extraction condi-

tions for the Ar the ion intensity was measured as a function of

the voltage applied to the repeller R. Figure 4 shows a plot of ion

intensity against this drawout potential. The ion intensity increased








27

U)O
S24 O
UO
0 "
0 21

>.-
I- 18

F--

12

LOU
M 9

6-
.< 6

0- ,

/_ 1 i i_ I I I
10 20 30 40 50 GO 70 80

ELECTRON ENERGY (uncorrected) electron volts
Figure 3. Primary Beam Intensity Versus Electron Energy.










800


0
600-

6 )
C)
0 /
400-


-
d. 200





-2 0 2 4 6 8 10 12 14 16

DRAWOUT POTENTIAL (volts)


Figure 4. Ion Intensity Versus Drawout Potential.









quite rapidly at low drawout potential and saturated at +4 volts.

Every scattering gas used in this study showed this same dependence,

and all subsequent measurements were made at a 4 volt extraction

potential.

A plot showing the pressure dependence of the Ar and HeAr

ion intensities on argon backing pressure was made and is shown in

Figure 5. A linear dependence of ion intensity with backing pressure

was observed. Both ions vanished when the argon was pumped out of
*
the collision chamber. Figure 6 shows a similar study for the Ne Ar

system. Using nitrogen as the scattering gas, only N2 was observed.

The N2 intensity was measured as a function of the helium backing

pressure. A linear dependence was found as shown in Figure 7. These

observed linear dependence support the assumption that the ions were

formed in a single collision process between the metastable atoms and

the scattering gas. When the shutter S was positioned to intercept

the beam of metastable atoms, no ions were observed. This served as

additional proof of the nature and origin of the ion formation.

Excitation of the atom beam by electron impact, although

reasonably selective, does not preclude the possibility that excited
1 3
states other than the 2 S0 and 2 S1 metastable states were present

in the beam. With this in mind, certain experiments were performed

to substantiate the assumption that the beam of excited atoms con-

tained only these metastable states. Table 1 summarizes these experi-

ments. The first column lists the metastable atom involved in the

collision process and the energy of its metastable states. Column































60
BACKING


400







200







0


Ion Intensity Versus Argon Backing Pressure.


600


80 100 120
PRESSURE (TORR)


40
ARGON


14.0


130


Figure 5.


















S 750

0


>- 500
I-




ON
LIF

250 NAr




o1 I I I I I
20 40 GO 80 100 120 14-0 160
ARGON DACK(ING PRESSURE (TO, R)
Figure 6. Ion Intensity Versus Argon Backing Pressure.






























100 200
HELIUM BACKING


300 400 500
PRESSURE (TORR)


Figure 7. Ion Intensity Versus Helium Backing Pressure.


1000


800


600


400



200



0


600









two lists the scattering gas. The positive ion or ions capable of

being produced if highly excited states were present in the beam are

listed in the third column. Appearance potentials for these ions

and references are given in the last column. In all of the systems

studied no ions were observed, indicating the absence of highly ex-

cited states. It has been recently observed that Ne+ and He+ ions

are produced in collisions of highly excited neon and helium atoms

in various gases.29'30 The cross sections for such reactions are

about four orders of magnitude larger than Penning ionization cross

sections. No He or Ne ions were observed in this study.

The intensity of ions resulting from energetically allowed

reactions was found to be independent of the quenching field maintained

across S.P. (Figure 1) in the range 8.3 11 kv./cm. A simple cal-
14
culation based upon the results of Holt and Krotkov14 showed that

3 per cent of the 21 S metastable helium atoms were quenched in

the 8.3 kv./cm. electric field used in all the experiments. This is

a further indication that the beam of metastable atoms was free of

highly excited states. The small change in the 21 S population due

to the quenching field is not sufficient to affect the results within

the experimental error of the measurements.

Of primary interest in this study was the variation in the

amount and type of ionization process with respect to changes in the

composition of the beam of metastable atoms. This was accomplished

by measuring the ion intensities as a function of the energy of the

electron beam. These ion intensities are listed in Appendix A as










Table 1. Determination of Absence of Highly Excited States in Beam.


Metastable Atom
*
He
(19.81 eV)
(20.61 eV)


*
Ne
(16.62 eV)
(16.71 eV)
*
Ar
(11.55 eV)
(11.72 eV)


Target Gas


Ion Appearance Potentiala (eV)


(He )

(He )

(Ne+)

(HeNe+)


(Ne )


(Ne2+)

(He+)

(HeAr )


24.58

23.3

21.56

23.4, 22.6

21.56

20.9


24.58

17.9


See Ref. 26.










per cent ion abundances of the ions formed from collisions of meta-

stable helium atoms on argon, krypton, oxygen, methane and methane-d4.

In the case of the He Kr system, all isotopes of the parent ion

were detected, but only the ion intensities involving the most abun-

dant isotopes were used in the calculations. The Ne Ar and Ne Kr

systems were also studied, and the experimental determinations are

also given in the Appendix A.

Figures 8, 9, 10, 11 and 12 show the variation in the per

cent ion abundances of the product ions with changing composition of

the beam of metastable atoms. These plots have been constructed ac-

cording to eq. (11-7). The fraction of singlet metastable helium

atoms R/(l + R), which increases with increasing electron energy,
20
was calculated from the results of Dugan, et al. and is listed

in Appendix B. Each experimental point represents an average value de-

termined from three to four experimental measurements which agree

within 3 per cent in all cases. Since the experimental data do fit

a straight line, the assumption that the total amount of ionization

may be treated as the sum of the individual contributions of each

metastable state appears to be valid. This behavior also supports

the initial premise that interactions between particles in the beam

are negligible.

Since we are primarily concerned with the individual con-

tributions of each metastable state, these values at R/(l + R) = 1

and 0 and their most probable errors were determined by a least
31
squares treatment. For simplicity, the errors for each system will






100

..... .... -- .. .......A r+

80
w
0
z

z
2
D 60

z
0
H 40-



20

HeAr+
o---o--o--o--o------


0.2 0.4 0.6 0.8 LO
R/(I+R)
Figure 8. Per Cent Ion Abundance Versus R'/(1 + R) for He + Ar.








100


Kr+
80-
w
O
0
z
Q
D 60


O
i:
z
0
40 -
LJ
zL
w
0


20

HeKr+
0~~ ------o------- -o---o- I ---


0.2 0.4 0.6 0.8 1.0
R/(I+R)
Figure 9. Per Cent Ion Abundance Versus R/(l + R) for He + Kr.






100 I



0+
80-
L)



0
<
2
2


20 -

40










0.2 0.4 0.6 0.8 1.0
R /(+ ( R)
Figure 10. Per Cent Ion Abundance Versus R/(1 + R) for He + 02'
0








0.2 0.4 0.6 0.8 1.0


Figure 10. Per Cent Ion Abundance Versus R/(1 + R) for He + 02. o







100



80
Ld

0
z
D 60 CH--




20

40
20


I---- O-0---0-0I ,------o


0.2 0.4 0.6 0.8 1.0
R/(I+R)
Figure 11. Per Cent Ion Abundance Versus R/(1 + R) for He + CH .






100




80 --
80
w








tii
z










(2 0
a +
z 60CD




60






-----r---0--- <>0--- ---
20




0.2 0.4 0.6 0.8 1.0
R/(I+ R)
Figure 12. Per Cent Ion Abundance Versus R/(1 + R) for He + CD4.





32


be tabulated in the discussion. Both the experimental procedure

and the treatment of these data are the same for all systems. The

total ionization cross sections used to obtain the individual cross
11
sections were taken from the work of Sholette and Muschlitz. The

estimated error in these cross sections is + 15 per cent.










CHAPTER V


RESULTS AND DISCUSSION


Various mechanisms can be proposed to explain the manner

in which electronic excitation energy of an atom may be transferred

to an atom or molecule. Two direct mechanisms are schematically

represented in Figure 13. Consider now the upper and lower poten-

tial energy curves. During collision the internuclear distance be-

tween A and B diminishes until point X or Y is reached. In this

region a spontaneous release of an electron can occur and the system

passes by Franck-Condon transition to the lower potential curve. If

this transition occurs at Y, then the molecular ion AB+ may be formed

(associative ionization). A transition occurring at X will result

in the system passing to the lower curve at X' where the dissociation

energy of AB is exceeded. This will result in the formation of three

particles: an electron, an ion, and a neutral.

A second direct mechanism is that of curve crossing. Dur-

ing collision the system (A + B) follows the upper potential curve

as before, but there is a finite probability that at the internuclear

separation r this system can cross over and follow the lower poten-

tial curve (A + B ). The excess energy AE appears as kinetic energy

of the products A and B This will leave atom A in the ground state

and molecule B in a pre-ionizing state such that subsequent ioniza-

tion or dissociation may occur. Although this mechanism offers an





























I








I I
















X~ 1-


-- -4--


INTERNUCLEAR


Figure 13.


SEPARATION


Schematic Diagram of Potential Energy Curves
Describing Energy Transfer Processes.


I









adequate explanation for Penning ionization, it would be difficult

to apply this explanation to systems in which associative ionization

occurs.

Consider now another mechanism in which the excited A and

B form a collision complex (AB) which then can decay through vari-

ous product channels. For this mechanism to be applicable, the life-

time of the complex must exceed the time for a few rotations of the

complex. This mechanism has the distinct advantage over the previous

one cited because it may be applied to all systems studied. Penton
32
and co-workers32 have employed a similar mechanism in explaining the

isotope effect found in the He H2, He D2 systems. Because of

its wide applicability, the collision complex mechanism will be used

in analyzing the present results.

For simplicity, the results will be discussed in three
*
sections: the rare gas systems, the He 02 system and the He -

methane and methane d4 systems.


Rare Gas Systems

There is a significant change in the relative amounts of

ionization of argon and krypton with changing metastable beam compo-

sition (Figures 8 and 9). The ion abundances and cross sections for

each metastable state of helium are given in Table 2 with the re-

sults for the Ne rare gas systems also.

In both metastable helium systems, the Penning ionization

cross sections for the 2 1S state are larger than those for the 23S1










Individual Ion Abudances and Ionization Cross Sections
for the Rare Gas Systems.


Product
System Ion


Per Cent Ion Abundance
(2 S0)He (23S1)He


2
Cross Section A
(1 (23 *
(25S0)He (2 S )He


91.5 + 0.3

8.5 + 0.3

92.6 + 0.4

7.4 + 0.4


87.1 + 0.2

12.9 + 0.2

88.6 + 0.2

11.4 + 0.2


Product
System Ion


Per Cent Ion Abundance
(average)


o2
Cross Section A
(average)


*
Ne Ar



Ne Kr


81.6 + 0.7

18.4 + 0.7

78.4 + 0.5

21.6 + 0.5


Table 2.


*
He Ar



*
He Kr


Ar+


HeAr+

Kr+

HeKr+


7.0

0.6

8.3

0.7


6.6

1.0

8.0

1.0


Ar+


NeAr+

Kr+

NeKr+








state. The simple calculations of Ferguson33 predicts a ratio of the

singlet to triplet cross sections of about 1.1 to 1.2, which is due
1 3
to the difference in van der Waals coefficients for the 2 SO and 2 S1

states. The ratio of the cross sections determined in this study are

1.06 and 1.04 for the He Ar and He Kr systems, respectively.

The individual contributions for each of the metastable

states of neon, 3P0 and 3P2, could not be determined for lack of suf-

ficient data. Since the energy difference between these two states

(16.62 and 16.71 eV) is small, a large difference in their behavior

would not be expected. However, the relative population of these states

does change according to the energy of the electrons used to produce

them. Values for the ratio 3P2 P0 have been measured in the energy
34
range 25 35 eV and vary from 0.51 to 1.39.3 Within the experi-

mental error of the measurements, the data shown for metastable neon

systems are independent of electron energy and hence independent of

the beam composition. Unresolved cross sections for both Penning and

associative ionization have been calculated for the Ne Ar system

(Table 2), using the previously measured total ionization cross section

of MacLennan.18 The relative amount of associative ionization in the

Ne systems is approximately twice as large as that occurring in the

analogous reactions involving He

Because of the existence of these associative ionization

processes, it is tempting to envision these reactions as proceeding

via a collision complex. Consider a mechanism whereby a collision

complex is formed from an excited atom and an atom or molecule. This










complex may either eject an electron, forming the associative ion,

or break up, leaving the excited helium atom in its ground state

(11S0) and the atom or molecule in a pre-ionizing state which may

subsequently ionize. We will now restrict the discussion to an ex-

cited atom atom system and look upon the mechanism for ionization

in the following way:
3 + -
(2 S )He + Z (HeZ) HeZ + e


+ -
He + Z Z + e

In these experiments the target gas Z is in its ground state and

has a singlet configuration. Conservation of spin demands that

the pre-ionizing state have a triplet configuration. In order to
*
form the atom in this pre-ionizing state Z without a reversal of

electron spin, an electron exchange between the two atoms must occur

to produce a helium atom in the singlet ground state; therefore, we

would expect the probability for formation of Z from the singlet

He (which does not require an electron exchange) to be greater than

for the triplet. Considering the production of either ion as pro-

ceeding through the two competing reaction channels, the ratio (HeZ /Z )
1 *
should be less for reactions involving the (2 S0)He than for the
3 *
(2 S1)He The experimental results show that this ratio is smaller

for 2 S0 metastable atoms. This effect is not observed in the meta-

stable neon systems as would be expected from the above argument since

both metastable states of neon have a triplet configuration.









He 02 System


As can be seen from Figure 10 and the values listed in

Table 3, there is a definite difference (far outside the limits of

experimental error) in the amount of 0+ and 0+ produced by each

metastable state of helium. The data reported here agree within
23
20 per cent with previous results,23 but is considered more precise

since greater metastable beam intensity and increased detection

sensitivity were obtained by modifications in the apparatus.

Adopting the same model as that for the rare gas systems,

we may envision the ionization process proceeding in the following

manner:
+ He0+ + (0 + e)
He + 02 (+HeO2 )+-
2 (H -* HeO + e



He + 0 0 + e
2 2
+
0 + (0 + e)

The ions He+ and HeO+ were not observed in the mass spectra. Evi-

dently the collision complex cannot be sufficiently stablized by the

loss of an electron to produce these ions.

The production of 0 ions is much more probable in collisions

of singlet metastable atoms. The ratio of cross sections for the

formation of this ion from the triplet versus singlet states is 0.42.

A closer look into the various processes resulting in production of 0+

is in order.










Table 3. Individual Ion Abundances and Ionization Cross Sections
for the Metastable Helium Oxygen System.


Per Cent Ion Abundance
1 3 *
(2 S0)He (2 S1)He

85.4 + 0.8 93.9 + 0.3


02
Cross Section A
(2 S0)He (2 S1)He


12.0


13.1


14.6 + 0.8


6.1 + 0.3


System

*
He 02


Product
Ion


02


2.0









The pertinent electron impact appearance potentials are:35

e + 02 0( S) + 0 (17.2 + 0.2)eV V-l
+4 1
e + 02 0 ( S) + 0( D) + e (19.2 + 0.2)eV V-2

e + 02 0( 2D) + 0 (20.4 + 0.2)eV V-3

If the 0+ production is considered to proceed through three possible

reaction channels with the products shown above, then the third

channel would be energetically impossible for 2 3S helium. This offers

a possible explanation for the observed decrease in the 0+ cross sec-

tion,with decreasing fraction of singlet helium atoms in the beam.


He CH4 and He CD4 Systems


The experimental results for the ionization of methane and

methane d4 on impact with thermal energy metastable helium atoms

are shown in Table 4. Using the known appearance potential data for
36
methane and methane d4, it may be shown in each case that all

ions are observed that correspond to energetically possible frag-

mentation processes.

There is but a slight difference in the amount and type

of fragmentation occurring in the two systems for a particular meta-

stable state, and these differences may well be within experimental

error; however, the amount of fragmentation for the two metastable

states is definitely different, and this difference should be con-
+ + +
sidered real. It should be noted that only the CH CD CH2
+ 1 *
and CD2 ion abundances are greater for (2 SO)He. No adequate

explanation of these facts can be offered at the present time.










Table 4. Individual Ion Abundances for the Metastable Helium Methane
and Methane d4 Systems.


Per Cent Ion Abundance
1 3 *
(21 S)He (2 S1)He

37.9 + 1.3 46.7 + 0.6

34.4 + 0.9 46.3 + 0.4

56.7 + 1.4 49.1 + 0.6

59.0 + 0.9 51.0 + 0.4

5.4 + 1.4 4.2 + 0.6

6.6 + 0.9 2.7 + 0.4


20 eV electrons


50.0



41.0



9.0


21.3 eV photons


55.5



35.4



9.1


See Ref. 23.

CSee Ref. 37.


Product
Ion

+
CH4
+
CD4
+
CH3
+
CD3
3
CH2 +
+
CD2
2









In Table 4 the mass spectra for impact of 20 eV electrons23
37
and 21.3 eV photons are also shown. There are a number'of signi-

ficant differences in comparison with the spectra corresponding to

impact of metastable helium atoms. The most abundant ion is CH3

for the He CH4 systems, but when the incident particles are

electrons or photons, the parent ion CH4 is the more abundant.

Evidently, the interaction potential of the colliding species is a

major factor in determine the fragmentation patterns.


Future Direction of Research

Future experiments with this apparatus should be extended

to the study of the fragmentation patterns of other simple molecules:

e.g., C2H6, C2H, C2H2 and N20. Also, measurements of the negative

ion spectra of He + 02 would be invaluable in determining the re-

action paths pertinent to the corresponding 0 production.

Three mechanisms have been considered for reactions in-

volving metastable atoms. Determinations of the velocity dependence

of the cross sections could differentiate between a Franck-Condon

mechanism and a curve crossing mechanism. Landau-Zener theory pre-

dicts a maximum in the velocity dependent cross section if curve

crossing is involved. Measurements of the angular distribution of

the product ions should be also undertaken to establish the kinematics

of these reactions. Determination of the kinetic energy of Penning

ions may yield useful information regarding the shape of the potential

energy curves of these systems and the kinetic energy of the pre-

ionizing atoms which may be produced in such reactions.








The process, X + M X + (M + e), has recently been ob-

served to occur in collisions of highly excited rare gas atoms with
30
various gases. The cross sections for the processes are four orders

of magnitude larger than Penning ionization cross sections. In the

absence of the quenching field, such processes may occur and be in-

vestigated in the present apparatus. By studying the total amount and

type of ionization with varying quenching field strength, these pro-

cesses may be differentiated from Penning ionization. In the same

manner, the field strength at which these highly excited states field

ionize may be measured. The reaction, Ar + He HeAr + e, may

also be studied. There is some evidence for the existence of states
** 26
of Ar of energy as high as 18 eV. If this pre-ionizing state is

sufficiently long-lived and in the absence of a quenching field, HeAr+

production is energetically possible for impact of excited argon atoms

on helium.





































APPENDICES











APPENDIX A


ORIGINAL DATA

Metastable Helium in Argon


Electron Energy (eV)
26
30
40
50
60
80


Per Cent Ion Abundance
Ar+ HeAr+
88.1 11.9
88.6 11.4
89.2 10.8
89.4 10.6
89.5 10.5
90.3 9.7


Metastable Helium in Krypton


Electron Energy (eV)
26
30
40
50
60
80


Per Cent Ion Abundance
Kr+ HeKr+
89.9 10.2
89.6 10.4
90.5 9.5
90.8 9.2
91.1 8.9
91.4 8.6








Metastable Neon in Argon


Electron Energy (eV)

30

40

50

60

80


Per Cent Ion Abundance
Ar NeAr

80.1 19.9

81.6 18.4

81.3 18.7

82.5 17.5

82.6 17.4


Metastable Neon in Krypton


Electron Energy (eV)

30

40

50

60

80


Per Cent Ion Abundance
Kr+ NeKr+

77.4 22.6

77.9 22.1

78.3 21.7

78.9 21.1

79.7 20.3









Metastable Helium in Methane


Electron Energy (eV)


Per Cent
+
CH4

44.8

42.6

42.6

41.5

40.0


Ion Abundance
+ +
CH CH2

50.2 5.0

52.4 5.0

52.7 4.7

53.5 5.0

53.8 6.2


Metastable Helium in Methane d4


Electron Energy (eV)


Per Cent
+
CD4


43.3

42.7

41.1

39.6

38.6

38.8


Ion Abundance
+ +
CD CD2


53.0 3.7

53.4 3.9

54.8 4.1

55.6 4.8

55.8 5.6

56.1 5.1








Metastable Helium in Oxygen


Electron Energy (eV)


Per Cent Ion Abundance
02 0+

91.9 8.1

91.4 8.6

89.4 10.6

89.2 10.8

88.5 11.5











APPENDIX B

METASTABLE BEAM COMPOSITION

Table 5. Calculated Values of R/(l + R) as a Function of the Energy


Electron Energy (eV) R = 21S0/23S1 R/(1 + R) = 21S/(21S + 23 S1)

26 0.30 0.23
30 0.50 0.33
40 0.90 0.47
50 1.2 0.55
60 1.5 0.60
80 1.9 0.66












REFERENCES


1. N. F. Ramsey, Molecular Beams (Oxford University Press, London,
1956).

2. J. Ross, ed., Molecular Beams (Interscience Publishers, New
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3. L. Dunoyer, Compt. Rend. 178, 1475 (1911).

4. 0. Stern, Z. Physik 39, 751 (1926).

5. F. Knauer and 0. Stern, Z. Physik 39, 764 (1926).

6. W. Gerlach and 0. Stern, Ann. Physik 74, 673 (1924).

7. W. Gerlach and 0. Stern, Ann. Physik 76, 163 (1925).

8. D. R. Herschbach, Molecular Beams (Interscience Publishers,
New York, 1966), Chap. 9.

9. A. A. Kruithof and F. M. Penning, Physica 4, 430 (1937).

10. J. A. Hornbeck and J. P. Molnar, Phys. Rev. 84, 621 (1951).

11. W. P. Sholette and E. E. Muschlitz, Jr., J. Chem. Phys. 36,
3368 (1962).

12. V. Cermak and Z. Herman, Collection Czechoslov. Chem. Commun.
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13. G. Breit and E. Teller, Astrophys. J. 91, 215 (1940).

14. H. K. Holt and R. Krotkov, Phys. Rev. 144, 82 (1966).

15. M. L. E. Oliphant, Proc. Roy. Soc. (London) A124, 228 (1929).

16. D. G. Greene, Proc. Roy. Soc. (London) 63, 876 (1950).

17. R. J. Stebbings, Proc. Roy. Soc. (London) A241, 270 (1957).

18. D. A. MacLennan, Phys. Rev. 148, 218 (1966).

19. V. Cermik, J. Chem. Phys. 44, 3374 (1966).









20. J. L. G. Dugan, H. L. Richards and E. E. Muschlitz, Jr., J.
Chem. Phys. 46, 346 (1967).

21. W. P. Jesse and J. Sadauskis, Phys. Rev. 100, 1755 (1955).

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31, 649 (1966).

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(North-Holland Publishing Company, Amsterdam, 1964), p. 1073.

24. W. Kaul, P. Seyfried and R. Taubert, Z. Naturforsch. 18a, 432
(1963).
e
25. W. Kaul, Compt. Rend. Conf. Intern. Phenomnes lonisation Gas 6
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of Papers Published at 13th Mass Spectrometry Conference, A.S.T.M.
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34. R. J. Hammond, unpublished data.

35. H. D. Hagstrum, J. Chem. Phys. 23, 1178 (1955).

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BIOGRAPHICAL SKETCH


John Austin Herce was born in Newark, New Jersey, on

November 10, 1940. After graduating from Valley High School in

Orange, New Jersey, he entered Villanova University, Villanova,

Pennsylvania. He received the Bachelor of Science Degree with

major in Chemistry in June, 1962 and the Bachelor of Science

Degree with major in Physics in June of the following year.

In September, 1963, he entered the Graduate School of

the University of Florida. During that time he held the positions

of graduate assistant and Petroleum Research Fellow in the Depart-

ment of Chemistry.









This dissertation was prepared under the direction of the

chairman of the candidate's supervisory committee and has been ap-

proved by all members of that committee. It was submitted to the

Dean of the College of Arts and Sciences and to the Graduate Coun-

cil, and was approved as partial fulfillment of the requirements

for the degree of Doctor of Philosophy.


December, 1967




Dean, Colleg/ of Afts and Sciences




Dean, Graduate School






Supervisory Committee:


Chairman a




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