Title: Optical studies of ion-molecule collisions
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Permanent Link: http://ufdc.ufl.edu/UF00098863/00001
 Material Information
Title: Optical studies of ion-molecule collisions N2+ + O2
Physical Description: xi, 167 leaves : ill. ; 28 cm.
Language: English
Creator: Murray, Lambert Edward, 1949-
Publication Date: 1977
Copyright Date: 1977
 Subjects
Subject: Collisions (Nuclear physics)   ( lcsh )
Collisional excitation   ( lcsh )
Scattering (Physics)   ( lcsh )
Physics thesis Ph. D
Dissertations, Academic -- Physics -- UF
Genre: bibliography   ( marcgt )
non-fiction   ( marcgt )
 Notes
Thesis: Thesis--University of Florida.
Bibliography: Bibliography: leaves 163-166.
General Note: Typescript.
General Note: Vita.
Statement of Responsibility: by Lambert Edward Murray.
 Record Information
Bibliographic ID: UF00098863
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: alephbibnum - 000063598
oclc - 04213816
notis - AAG8797

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OPTICAL STUDIES OF ION-MOLECULE COLLISIONS:
N2 + 0








By

Lambert Edward Murray




















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

1977















ACKNOWLEDGMENTS

The author wishes to thank the members of his super-

visory committee, especially Professors T. L. Bailey and

C. F. Hooper, for their assistance and encouragement

throughout his graduate program. He wishes to give special

credit to Dr. Ralph C. Isler, the chairman of his committee,

for his counsel, his assistance, and his patience during the

course of this research.

The author is also indebted to Helen Dickman for her

fine work in typing this paper.

Finally the author wishes to thank his wife, McKay,

for her patience, understanding, and constant encouragement

during the course of his graduate program.



























ii
















TABLE OF CONTENTS


ACKNOWLEDGMENTS . . . .

LIST OF TABLES . . . . .

LIST OF FIGURES . . . .

ABSTRACT . . . .

Chapter

I. INTRODUCTION . . . .

II. EXPERIMENTAL APPARATUS . .

Collision Apparatus .

Vacuum System . .

Ion Source . .

Beam Transport and Detect

Ion Beam Characteristics

Optical Detection System .

Spectroscopic Arrangement

Detectors . . . .

Pulse Counting System .

III. PROCEDURE . . .


Operational Definition of the Excitation and
Emission Cross Sections . . .

The Relationship of the Excitation Cross
Section to the Emission Cross Section . .

Target Excitation . . . .


i i i


Page

ii

v

vii

x



1


. . . 31









Chapter Page

Projectile Excitation . . . . . 38

Determination of Emission Cross Sections 41

The Emission Cross Section as a Function
of Energy . . . . . . . .. 42

Relative Emission Cross Sections . .. 45

Calibration of the Optical Detection
System . . . . . . . . 46

Dependence of the Emission Cross Section
on Polarization . . . . . . . 57

IV. RESULTS . . . .. .......... .. . 66

Identification of Observed Spectral Features . 66
o
Spectral Features Above 2500 A .. ... . 66

Spectral Features Below 1500 A . . . 79

Emission Cross Sections of the Molecular
Features of 112 and N2 . . . ... . 80

Emission Cross Sections of the Molecular
Features of 02 . . . . . .. . 103

Emission Cross Sections of Atomic Features . 108

Polarization Measurements . . . . . 126

V. DISCUSSION . . . . . . . . .. .. 128

Ion Beam Excited State Population . . . 128

Excitation of the N 2 B 2E State . . 132

Comparison with Charge Transfer Data . . . 144

Production of Excited Dissociation Fragments 150

VI. CONCLUSION . . . . . . . . . 160

REFERENCES . . . .... . . .. . 163

BIOGRAPHICAL SKETCH .. ..... .... . .. .167

















LIST OF TABLES


Table Page

I. Absolute emission cross sections for the
(0,0), (0,1), (1,2), and (2,3) bands of
the first negative system of N2+ arising
from col visions of N2 w ii th 02 . . . . 83

II. Absolute emission cross sections for the
(0,0), (1,0), and (2,1) bands of the second
positive system of N2 arising froni collisions
of N2 with 02 . . . . . . ... . 86

I Absolute emission cross sections for various
N2+ first negative and N2 second positive
vibrational bands arising from collisions of
N2 with 02 at a collision energy of 4.0 keV 96

IV. Absolute emission cross sections for the N +
first negative system at a beam energy of 4.0
keV . .. . . . . . . . . . 97

V. Absolute emission cross sections for the N2
second positive system at a collision energy
of 4.0 keV . .. . . . ... . 98

VI. Absolute emission cross section of the entire
N2+ first negative system as a function of
collision energy . . . .... . 101

VII. Absolute emission cross section of the entire
N2 second positive system as a function of
collision energy .. .... .. . . 102

VIII. Absolute emission cross sections for the
observed band sequences of the 02+ first
negative system at 4.0 keV . . . . . 104

IX. Absolute emission cross sections for lines
below 1750 A arising from excited dissociation
fragments resulting from collisions of N2+
with 02 at 4.0 keV . . .. . . 109









Table


Page


X. Absolute emission cross sections for lines
above 3000 A arising from excited dissocia-
fion fragments resulting from collisions of
N2+ with 02 at 4.0 keV . . . . . . 110

XI. Absolute emission cross sections for several
excited dissociation fragments as a function
of collision energy arising from collisions of
N2+ with 02 . .. . . . ... . 122

XII. Relative population of the vibrational energy
levels of the X and A states of N,' at the
collision chamber . . . . . . 131

XIII. The minimum energy defect for direct excitation
Q ,and for charge exchange excitation Qe, and
tge experimentally observed thresholds E for
the resonance lines of NI, NII, 01, and 6?I
which were observed. . . . .. . . 151

XIV. Emission cross sections of several NI and NII
lines arising from dissociative collisions of
N with 02 and with Ar at a collision energy
o 4.0 keV . . . . . . . . .. . 154


















LIST OF FIGURES


Figure Page

1. Schematic of collision apparatus . . . . 6

2. Schematic of vacuum system ... . .. . 8

3. Schematic cross section of ion source . . . 12

4. Dependence of ion beam current upon coil current
for H2+ . . . . ... . . .... 16

5. Dependence of the N2+ ion current upon electron
bombardment energy with a potential of 2.5 keV
applied to the ion chamber . . . . .. 18

6. Dependence of the N2+ ion beam current upon the
potential applied to the ion chamber with an
electron bombardment energy of 50 eV . . . 20

7. (a) Experimental configuration for determining the
mean energy of the ion beam and the energy spread.
(,) Dependence of the ion beam current upon the
retardation potential Vr .. . . . . ..23

8. Schematic of spectroscopic arrangement . . . 26

9. Schematic of pulse counting system . .. . 30

10. Dependence of the relative count rate on target
gas pressure . . . .. . 44

11. Relative detection efficiency as a function of
wavelength for the vacuum monochromator and the
EMI 9558Q phototube .. . .... . .. 48

12. Relative detection efficiency as a function of
wavelength for the Ebert mount monochromator and
and the EMI 9558Q phototube . . . ... . 50








Figure

13. Relative detection efficiency as a function of
wavelength for the vacuum monochromator and the
EMR phototube .. ......

14. Relative detection efficiency as a function of
wavelength for the vacuum monochromator and the
M EI . . . . . . . . . . .

15. Experimental arrangement for determining the
instrumental polarization

16. Instrumental polarization of the Ebert mount
monochromator as a function of wavelength


17. Emission spectra between 2500 and
from collisions of N2+ with 02 at
energy of 4.0 keV . .

18. Emission spectra between 3300 and
from coll visions of N2+ with 02 at
energy of 4.0 keV . .

19. Emission spectra between 4200 and
from collisions of N2+ with 02 at
energy of 4.0 keV .

20. Emission spectra between 4900 and
from col visions of N2+ with 02 at
energy of 4.0 keV . .

21. E mission spectra between 1525 and
from collisions of N2+ with 02 at
energy of 2.5 keV . .

22. Emission spectra between 1200 and
from col visions of N2+ with 02 at
energy of 4.0 keV . .


3400 A arising
a col vision


4300 A arising
a collision


5050 A arising
a collision


6200 A arising
a collision


1270 A arising
a collision


680 A arising
a collision


23. Absolute emission cross sections for the production
of the (0,0), (0,1), (1,2), and (2,3) bands of the
first negative system of N2+ as a function of beam
energy . . . . .

24. Absolute emission cross sections for the production
of the (0,0), (1,0), and (2,1) bands of the second
positive system of N2 as a function of beam
energy . . . . .

25. The (0,1)/(0,0) branching ratio of the N21 first
negative system for N2+ + 02 collisions as a
function of collision energy


v iii


Page








Figure Page

26. The emission cross section as a function of ion
beam energy for the entire N2+ first negative
system and the entire N2 second positive system . 100

27. The emission cross sections of the Ai = -1 band
sequence of the first negative system of 02 as
a function of collision energy . . . . . 106

28. Energy level diagram of NI . . . . . 112

29. Energy level diagram of N I . . . . .. .. 114

30. Energy level diagram of 01 . . . . . 116

31. Energy level diagram of 01i . . ...... 118

32. Absolute emission cross sections of excited
dissociation fragments arising from N2 + 02
col visions as a function of ion beam energy . . 121

33. Relative populations of the vibrational levels of
the N+ B Z1 state as a function of ion beam
ene rgy . . . . . . . . . . 135

34. Selected potential energy curves of N2 and N2 . 141

35. Total cross sections for the production of 02+ and
for the production of all slow ions in collisions
of N2+ with 02 . . .. . .. .. . 142

36. Selected potential energy curves of 02 and 02 . 148

37. Emission cross sections of the NI 1200 A line
arising from the 3s "P + 2p3 'S transition, and
the Nil 1085 A line arising from the 2s2p3 DO
2p2 3P transition compared to calculated cross
sections using the Landau-Zener formula . . . 159















Abstract of Dissertation Presented to the Graduate Council
of the University of Florida in Partial
Fulfillment of the Requirements for the
Degree of Doctor of Philosophy


OPTICAL STUDIES OF ION-MOLECULE COLLISIONS:
N2 + 02



By

Lambert Edward Murray

August, 1977


Chairman: R. C. Isler
Major Department: Physics

Inelastic collision processes of the N2 + 02 system

have been examined by studying emissions of the spectral

features in the range from 600 to 8000 A at ion impact ener-

gies from threshold to 7 keV. Emission cross sections of the

vibrational bands of the N2+ first negative (B 2E+ X 2+)

and N second positive (C n -* B 3 ) systems as well as the

emission cross section of the 02 first negative (b 4 -

a n ) system have been determined as functions of energy.

The relative population of the vibrational levels of the

N2 B state have also been determined and compared with pre-
+ +
vious measurements for the N2 + He and N2 + Ne systems. The

relative population appears to be describable in terms of








the Franck-Condon principle at the higher energies, in agree-

ment with other experiments. Thresholds and emission cross

sections have been determined as functions of impact energy

for lines originating from several excited configurations of

NI(3s 4p, 3s 2P, 3s' 2D, 3d 'P), NI (2p3 2DO), 01(3s 3So),

and 01I(2p4 4P), resulting from collision induced dissociation.

Emission cross sections for additional lines originating from

excited configurations of NI, Nil, 01, and Oil have also been

determined at 4 keV. In contrast to collisions which produce

ground state dissociation fragments, the cross section for

the production of excited dissociation fragments appears to

be relatively small.















I. INTRODUCTION

As a beam of projectiles traverses a gaseous target both

elastic and inelastic collisions can occur. When molecular

reactants are involved, the inelastic collisions may result

in excitation, ionization, or dissociation of either the pro-

jectile or the target, or both, as well as electron transfer

from one reactant to the other. In addition, reactive colli-

sions, resulting in the appearance of new molecular species,

may also occur. Ideally, experiments designed to.study these

processes should be performed by preparing both the projectile

and the target in a single state and then determining the

separate cross sections for each set of final states which

are produced. However, experimental limitations usually pre-

vent the achievement of such well-defined conditions.

The most important technique used to determine the cross

sections of the various interaction channels as well as their

angular dependence is the direct detection of the products

emerging from the collision region. However, it is often

quite difficult in this type of experiment to isolate, for

separate study, a given interaction channel from the several

possible. In such cases where the collision process leads

to an excited state which subsequently emits a photon due

to spontaneous decay, optical techniques may be utilized








to determine precisely which states have been populated dur-

ing the collision process as well as the energy thresholds

and cross sections for populating the various states, thus

providing added detail about the total cross sections for

individual interaction channels.

This dissertation reports measurements of emission cross

sections resulting from collisions of N2 with static gas 02

targets for ion beam energies from threshold to 7 keV in a

spectral range from 600 to 8000 A. Although this system is

relatively complex theoretically, the data should be of con-

siderable interest because of the importance of these mole-

cules in atmospheric processes. The only other spectroscopic

observations which appear to have been made for this system

are those reported by Liu and Broida.1 Their measurements,

however, consist only of the absolute emission cross sections

for the production of the N2 first negative system and of

two 0. lines (3947 and 4368 A) at an ion beam energy of 0.90

keV.

The measurement of emission cross sections arising from

excited dissociation fragments are of particular interest.

Few spectroscopic measurements of these features have been

made at energies less than 10 keV for any collision system.2

Most of these have involved the dissociation of H2 and H2'

although a number of measurements of the dissociation of N2

and N2 in collisions with various other species have been

made by Doering3 (N2+ + N2), Neff4 (Na+ + N2, Ne+ + N2),

Hollstein et a 5 (He+ + N2), and Holland and Maier6

(He + N2). The only spectroscopic measurements of the




3



dissociation of 02 within this energy range appear to be

those of Hughes and N 7 for H impact on 02. Of these

measurements, only those of Holland and Maier included de-

terminations of the thresholds for the production of emis-

sions from any excited dissociation fragments.















II. EXPERIMENTAL APPARATUS

Collision Apparatus

A schematic diagram of the collision apparatus used in

these experiments is shown in Fig. 1. It consists of three

principal sections: (i) a chamber for the production of

ions by electron bombardment; (ti) a differential pumping

chamber which contains a system of electrostatic lenses for

accelerating and focusing the ion beam, as well as a low

resolution Wien filter; and (iii) a collision chamber which

contains the target gas.

The system has been modularly constructed to make use

of the High Voltage Engineering Corporation's 4-inch beam

line components and coupling flanges. Each module has been

constrLcted from type 303 stainless steel, and the electri-

cal feedthroughs, ion gauges, leak valves, and observation

windows are mounted on conflat flanges and are sealed into

the various modules with oxygen-free, high-conductivity cop-

per gaskets.


Vacuum System

The source and lens chambers, which achieve ultimate

pressures of 1-2 X 10-7 torr, are coupled to a 2-inch mer-

cury pumping system by 1/4-turn butterfly valves. These

pumping systems (shown in Fig. 2) consist of a liquid nitro-

gen cooled vapor trap, a thermoelectrically cooled baffle,



















































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and a mercury vapor diffusion pump. These pumps are connected

through a liquid nitrogen trap and a molecular sieve trap to

a mechanical backing pump.

The collision chamber, which reaches an ultimate pres-

sure of 2-3 X 10-6 torr, is evacuated continuously through

the beam entrance hole (0.075 in.) in the bulkhead which sep-

arates the collision chamber from the lens chamber, and

through the slits of the vacuum monochromator, which is coup-

led to an open observation port opposite the observation win-

dow shown in Fig. 1 (see also Fig. 8). In addition a small

oil diffusion pump, employed as the low pressure reference

of a capacitance pressure sensor, is used in the initial evac-

uation of the collision chamber. This pump is not open to

the collision chamber while measurements are being made.

Airco pure grade N2 and 02 gases of quoted purity

99.9% and better than 99.5%, respectively, have been used

with no further purification. These gases are admitted to

the source and col vision chambers by means of Granville-

Phillips Variable Leak valves. Standard Bayard-Alpert type

ionization gauges and a Granville-Phillips ionization gauge

controller are used to monitor the pressure in the three

chambers of the system. In addition, a Datametrics Electron-

ic Manometer and capacitance type Barocel Sensor are used to

measure the pressure of the various gases in the collision

chamber. This is done to facilitate comparison measurements,

since the calibration accuracy of this type device is inde-

pendent of gas composition.








Ion Source

A schematic diagram of the electron bombardment ion

source is shown in Fig. 3. Except for the molybdenum grid,

all parts shown are type 304 stainless steel or boron nitride.

In order to produce a beam, the source chamber is first

evacuated, and then the butterfly valve is almost completely

closed until the pressure rises to about 2 X 10-6 torr. This

serves to minimize fluctuations in pressure in the ion cham-

ber and thus helps to maintain beam stability. The gas to

be ionized is then leaked into this section until the pres-

sure is between 10-3 and 10-4 torr. The optimum pressure for

the production of N2 was found to be about 4 X 10- torr.

The bombarding electrons are produced from a cathode

which is directly heated by current from a well-regulated

dc power supply. The cathodes are produced by painting an

RCA triple carbonate (Ba, Sr, Ca) mixture on a strip of

0.010 .n. nickel mesh 1/8 in. wide and 3/4 in. long. These

carbonates are converted to oxides by heating the cathode to

900-1000C in a vacuum. After conversion begins, electron

emission is stabilized by drawing approximately 40 mA for

about an hour.

The electrons emitted from the cathode are drawn through

the molybdenum grid by a potential difference, which can be

varied from 0 to 250 V, applied between the ion chamber and

the cathode. The electrons are then focused toward the exit

aperture of the ion chamber by an axial magnetic field which

is produced by a narrow coil wrapped around the ion chamber.

This coil consists of 275 turns of #27 copper wire with a
































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heavy formvar coating. Currents as large as 2 A are used in

the coil to produce fields of about 120 G in the region of

the extraction aperture. In order to prevent the disinte-

gration of the insulation, the coil is cooled by circulating

oil around the coil and through a simple water cooled heat

exchanger.


Beam Transport and Detection

Ions created in the ion chamber are drawn out by an

extractor potential 15-20 V lower than the potential applied

to the ion chamber itself. These ions are focused and ac-

celerated toward the collision chamber by a system of cylin-

drical electrostatic lenses. The potential applied to each

lens is adjusted empirically for maximum current in the

collision chamber. In addition, a low resolution Wien filter

is employed to eliminate N2 and N from the ion beam.

The ion beam is monitored by a General Radio type 1230-A

dc amplifier and electrometer which is connected to a Faraday

cup mounted in the collision chamber. To determine the possi-

ble effect of secondary electron emissions on the measurement

of the ion current, a potential was applied to the Faraday

cup to suppress secondary electron emission. The ion cur-

rent was measured as a function of this suppressor potential

and, within experimental error, was found to be independent

of this potential.


Ion Beam Characteristics

The dependence of the ion beam current as measured in

the collision chamber upon the magnetic field in the ion









chamber (and thus upon the coil current), the electron bom-

bardment energy, and the potential applied to the ion cham-

ber are shown in Figs. 4-6.

Although Fig. 4 is a plot for H2, ions, this plot rep-

resents the general dependence of the ion beam current.upon

the coil current which has been noted for all ions produced

in this source. While performing a measurement, the coil

current is adjusted to a region where the ion current varies

smcothly with coil current. In order to increase the sta-

bility of the ion beam, the output of the electrometer which

monitors the ion beam has been used to provide an error sig-

nal to the programming terminals of the power supply which

supplies current to the coil. Using this technique, fluc-

tuations in the ion current have been reduced to less than

0.25% while measurements are being made.8

The dependence of the N2 ion beam current on electron

bombardment energy is shown in Fig. 5 for bombardment ener-

gies between 20 and 40 eV. To obtain sufficient accuracy in

cross section measurements, the electron bombardment energy

was always maintained above 30 eV.

The ion beam energy is established by applying a posi-

tive potential of up to 7 keV to the ion chamber while the

collision chamber is maintained at ground potential. Figure

6 shows the variation of the ion current as a function of this

applied potential. To determine the actual energy of the

ions as they enter the collision chamber, a simple double

retardation screen was placed just beyond the entrance aperture

















































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Dependence of the N2 ion current upon electron
bombardment energy with a potential of 2.5 keV
applied to the ion chamber.
















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of the col vision chamber (see Fig. 7a). A plot of ion cur-

rent versus retardation voltage (solid curve) is shown in

Fig. 7b for an H + ion beam with a potential of 40 V applied

to the ion chamber and an extractor potential of 15V. The

mean energy of the ions is taken to be the potential at which

the ion current has fallen to 1/2 maximum (or where -dl/dV is

a maximum), in this case approximately 12.5 eV lower than the

applied potential. Additional tests have shown that the mean

energy of the ions is 12.3 0.5 eV lower than the potential

applied to the ion chamber, even at zero extractor voltage.

Unless otherwise noted, the beam energies quoted herein have

been corrected to give the actual energy of the ions entering

the collision chamber.

The energy spread of the ion beam, taken to be the full

width at half maximum of the change in ion beam current as

a function of retardation potential, is seen in Fig. 7b to be

approximately 3.5 eV, or about 9% of the applied potential

at 40V. Additional measurements have shown that the energy

spread is approximately 8 eV for an applied potential of

180 V for extractor potentials from 0 to 65 V. The energy

spread, therefore, is taken to be less than 5% of the beam

energy for an applied potential higher than about 200 V.

Optical Detection System

Spectroscopic Arrangement

Two types of monochromators have been utilized for the

detection of optical emissions arising within the collision

chamber: a I-meter normal-incidence vacuum instrument



















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(McPherson model No. 225) and a 1/4-meter Ebert mount instru-

ment (Jarell-Ash model No. 82-410). The vacuum instrument,

equipped with a magnesium floride coated, 600 grooves/mm
0
grating blazed for 2000 A, has a linear reciprocal dispersion

of 16.6 A/mm in the first order. This instrument is coupled

directly to the collision chamber through an open observa-

tion port (see Fig. 8). The Ebert mount instrument is mounted

on the opposite side of the collision chamber from the vacuum

instrument, separated from the collision chamber by a quartz

observation window. Light arising from the collision chamber

passes through this window and is focused upon the entrance

slit by a quartz lens. This instrument, equipped with a 1180
0
grooves/mm grating blazed for 6000 A, has a linear reciprocal

dispersion of 33 A/mm in the first order. All observations

have been made at 90" to the direction of the beam.

The entrance slits of both instruments are mounted verti-

cally, perpendicular to the beam direction, to minimize the

effect of possible variations in the beam diameter. Although

the beam is not collimated, it is estimated that distortions

of relative cross sections owing to these variations are less

than 5% over the entire range of beam energies.

The length of the beam from which light can be detected

(the observation length k) as well as the solid angle sub-

tended by the detection system is determined by the optical

limiting stops of each detection system. For the vacuum

monochromator system the limiting stops are the ruled area

of the grating (height 56 mm, width 96 mm, located 120 cm











































c

E

Q)

(U
C



U



ci



f0
L
-CO 0

U








0
u




E
SU
(U
r-










U
4-1





















rc





ft:
I
0







C)
0 <


LLI



-J i-
Or


0
or








from the beam) and the entrance slits of the monochromator.

For the Ebert mount system the limiting stops are the diam-

eter of the quartz lens and the entrance slits of the mono-

chromator. The observation length, 2, of the vacuum mono-

chromator system may be taken as 1.92 cm and will be in

error less than 3% for a slit width of 500p or less, while

the observation length of the Ebert mount system will be

approximately equal to the entrance slit width. The distance

L from the bulkhead which separates the collision and lens

chambers to the region of optical detection is approximately

1.25 cm for the vacuum monochromator and 2.21 cm for the

Ebert mount system.

Detectors

In order to observe emission spectra over a range from

600 to 8000 A, four different detectors were used: (i) an

EMI 9558Q photomultiplier tube was used from 2500 to 8000 A;
0
(ii) an EMI 6256S tube was used from 1500 to 3000 A; (iii)

an EMR 542G-09-18 tube was used from 1200 to 1700 A; and

(iv) a Bendix mouel 306 magnetic electron multiplier (MEM)

was used below 1200 A. The vacuum monochromator was employed

for all measurements below 5000 A, while the Ebert mount mon-

ochromator was employed only for measurements above 5000 A.

Although the dark count rate for the EMR tube and for

the MEM is less than 1 per second at room temperature, both

EMI tubes had to be mounted in a liquid nitrogen cooled

housing to reduce their dark count rate to about 50 per sec-

ond. To further reduce the dark count rate to a minimum of








3 per second, a permanent donut-shaped magnet was placed

about I inch from the tube face. This was done to deflect

the electrons which are thermally emitted from the unused

portion of the photocathode toward the walls of the tube,

thus preventing them from passing through the dynode chain.


Pulse Counting System

The output pulses from the detectors pass through a

preamplifier (located as closely as possible to the detector

to reduce the input capacitance) into a linear amplifier and

then a pulse-height discriminator (see Fig. 9). The ampli-

fication and the threshold of the discriminator are adjusted

to give the best signal-to-noise ratio. To obtain emission

spectra, the output of the discriminator is integrated by a

linear rate meter and then displayed by a strip chart recorder

while the monochromators are scanning the wavelength region

of interest. Relative emission rates for the various spectral

features are obtained by summing the output pulses of the dis-

criminator over a convenient time interval using a preset

timer-scaler combination.




















































2-



QJ





0,







U,


ZI u,
04

ic ci





U



In
2



U
In






30


















I a:
rn UJ



rt. u I
I-..
(n








L;Ij
~~L J




I-


cc,
C) w







_IJ
Ld z


0-_D



C rn

H -4 EL'














III. PROCEDURE
Operational Definition of the Excitation
and Emission Cross Sections

When a projectile beam of density Nb (particles/cm3)

and velocity v (cm/sec) is incident on a target gas of

density N (particles/cm3), the number of excited molecules

produced per unit time per unit.volume in a particular ex-

cited state is given by

dN.
--= N N avo ,
dt (a b ( -1)

where Ni may be the density of excited molecules in the

target Nai or in the beam bi. The constant of proportional-

ity o. is called the total cross section for exciting state i

and has the units of square centimeters. As the projectile

beam of cross sectional area A traverses a finite length Z of

the target region, the total number of excited molecules pro-

duced per unit time in state i is given by


N. = / / NaNb dA dx (111-2)
Z iA a b i

If the density of the target gas is uniform over the volume

At, then

N. = oia f [f Nbv dA] dx (1 1-3)
SA

where Ib(x) = f Nbv dA (I11-4)
A

is the flux integrated over the cross sectional area of the








projectile beam at a position x. The integrated flux of the

projectile beam as a function of position is given by


I((X) = I1(0) exp(-N a ), (111-5)

where o is the total cross section for all processes which

remove a projectile from the beam. This leads to the expres-

sion

SIb (0)
N. [.l-exp(-N a0 )]
o ( 111-6)
o


for the number of excited states i produced per unit time in

a finite distance R. If N a << the so-called thin target
ao
criterion, this last equation reduces to


Ni = iI (O)N a, (11 -7)


and the operational definition of the total cross section

for the production of state i becomes


N.
S. =a
SIb(O)Na ( 11 -8)


The assumption of single-collision conditions, i.e.,

when no projectile undergoes more than one collision as it

traverses the target chamber, has been implicit in this deri-

vation. Thus, whenever equation (1l1-7) is shown to be sat-

isfied, i.e., when N. is linearly dependent upon both N and
a
Ib(0), it is assured that the experiment is being carried out

under single-collision conditions. Under these conditions

Na is approximately uniform and IbT() = Ib(0) = NbvA so that
a D b b








Ib and NA may be monitored at any convenient point within

the target chamber.

The emission cross section is defined analogous to the

excitation cross section by the relation


G . -
1 NaI b ( I 1-9)


where J.. is the total number of photons emitted per second
s1J
in the transition i j from a length of the beam path

through the target region. In this expression the assump-

tion is also made that N and Ib are sufficiently tenuous

to assure that they may be monitored at any convenient point

within the coll vision chamber.


The Relationship of the Excitation Cross
Section to the Emission Cross Section

Although collisionally induced emission cross sections

may be of practical interest in and of themselves, they are

of most value when they can be related in an unambiguous way

to excitation cross sections. The comparison of the emission

cross section to the excitation cross section is, however,

complicated by several factors: (i) the emission cross sec-

tion is related to all processes which populate and depopulate

an excited state i, not just to the direct excitation of this

state; (ii) 'f the lifetime of an excited state is very long,

the excited particle may drift a considerable distance before

emission occurs, and the emission cross section may depend

upon the point at which observations are made; and (iii) if








the emission is anisotropic, the measured emission cross

section will depend upon the position of observation. The

remainder of this section demonstrates how the emission and

excitation cross sections can be related to each other in a

systematic way, and follows the presentation by E. W. Thomas.9

When the beam-static target configuration is employed,

the assumption generally made in relating the total emission

cross section to the total excitation cross section is that

the projectile experiences only a small change in energy and

direction as a result of a collision, while the target re-

coils only with a small velocity and at an angle of approx-

imately 90 to the beam direction. Therefore, t, a first

approximation, the direction and speed of the projectile as

well as the position of the target remains unchanged by col-

lision. Two different approaches to the comparison of the

emission cross section with the excitation cross section are,

therefore, required depending upon whether the projectile or

the target is the emitting species.


Target Excitation

There are four processes which act to populate a given

state i of the target molecule: (i) direct collisional exci-

tation by the projectile, (ii) cascades from all higher states

k which are excited during the collision process, (iii) ab-

sorption of resonant photons, and (iv) transfer of internal

energy by collisions with other target particles. Therefore

the number density of the target particles in state i increases








at a rate given by


dNa
= N vo .i + Zk N Ak + f.A oN + i o .a' (111-10)
dt as D k>I ak kI + so as x es ax
dt


The first term of this equation represents the rate of direct

coll isional excitation which has already been discussed. The

second term represents the rate of populating state i by cas-

cade from all higher states k, where Nak is the density of

target particles in state k and Aki is the radiative transi-

tion probability for the decay of state k to state i. The

third term represents the rate of repopulation of state i by

absorption of resonant photons, where A. Ni is the rate of

depopulation to the ground state by spontaneous radiative de-

cay and fi is the fraction of resonant photons which are ab-

sorbed within the target region. The last term represents

the rate of population of state i by collisional transfer of

internal energy by collisions with other target particles

where c = o N is the collisional frequency and N is
x 2. xt Xa ax
the density of target particles in all other states x.

The number density of the target particle in state i

decreases at a rate given by


dN
at
N . . + .
dt ai, j< 1, at x zx (111-11)

where the first term represents the rate of depopulation due

to spontaneous radiative decay to all lower states j, while

the second term represents the rate of depopulation due to

collisional transfer.








Once a steady state is reached, the rates of population

and depopulation must be equal, and the expression for the

equilibrium excited state density NA i is given by
aa


a b + k>i ak ki x x, ax
a E . A .. fiA + Z C (1 -12)
3] 3 < io

The number of photons of wavelength A emitted per unit time

in all directions from a length R of the beam path is given by


J.. = N -A ..A, (11 -13)
,3 ai 3

where A is the cross sectional area of the beam and A..(sec-)
'3
is the radiative transition probability for the uecay of state

i to state j. Making use of equations (111-12) and (Ill-13),

the expression for the emission cross section [see equation

(111-9)] becomes

A.A NY + T7 A / + Cn A
ij. a Nb"i k>i ak ki x xi ax
S.. = A..
N I A f A t + E c

(1 11-14)


This equation can be simplified to give
A A
oi + Zk>.i Oki + C iYx

SA .A. + c (111-15)
3j

where ki, the emission cross section for the cascade transi-

tion, has replaced the cascade term N kA ki A(N I b )-1, and
where is defined by the relation a
where y is defined by the relation y = AN (N Sa N 1b
x o: x a b








In this equation the emission cross section is related

to terms describing photoabsorption and collisional transfer,

both of which depend upon the construction and operating con-

ditions of the apparatus. As stated earlier, measurements of

emission cross sections may be of interest in and of them-

selves, but are most useful if they can be related in an un-

ambiguous way to the excitation cross section, that is, if

S.. is independent of the construction and operating condi-

tions of the apparatus. Since the photoabsorption and colli-

sional transfer terms are obviously dependent upon the tar-

get density, at sufficiently low pressures within the colli-

sion chamber, their effect should be negligible. Under these

conditions, equation (111-15) reduces to


A .
o [ +
S . A.. '7 i' ('I -16)


Here .. is no longer dependent upon target density, and J.

can be shown to be linearly dependent upon both Na and Ib [see

equation (Ii -9)]. Therefore, when J.. is shown to be linearly

dependent upon both N and I (and J . 0 as Na 0), the
a b L a
emission cross section a.. is independent of the construction

and operating conditions of the apparatus. The excitation

cross section can then be determined, if the transition prob-

abilities are known, by measuring the emission cross section

a.. and all emission cross sections due to cascade, according

to the equation,
EJ A.
S C. = 0i ki Oki
A .k (I 1 1-17)








If this equation is summed over all j
the excitation cross section becomes


S= E . (111-18)
i ji ki' (I I-18)

Thus the emission cross section can also be determined by

measuring the emission cross sections for all transitions

into and out of a given state.

Only in a relatively small number of cases is it possi-

ble to evaluate an excitation cross section explicitly in

this way, either because of the limited spectral range of a

practical detection system or because of the limited accur-

acy of calculated transition probabilities. However, in many

cases the cascade terms may be shown to be quite small, thus

simplifying the comparison of the emission and excitation

cross section.

As mentioned earlier, this treatment of target excita-

tion is based upon the assumption that the excited target

emits from the same place where it is excited. There is

presently no evidence to indicate that this assumption is

violated for projectiles colliding with atomic targets. This

may, however, not be true for excited fragments produced by

the dissociation of molecules. However, as will become clear

from the following treatment of projectile excitation, if

the lifetimes of the excited states are short (of the order

of 10-8 sec.), then this assumption may still be quite ade-

quate.








Projectile Excitation

The mechanisms leading to the population and depopula-

tion of an excited state of the projectile are essentially

the same as for the target. However, since the number den-

sity of the emitting species in a projectile beam is small,

absorption of resonant photons and collisional transfer of

excitation energy have generally been neglected. The rate

of population of the excited state i in the projectile beam,

then, is given by


dNbi ,
dt Na Nb + k>i N Ak i N. 2 Aij


(1 11-19)

where the terms on the right hand side represent the rate of

direct excitation, the rate of cascade from all higher lev-

els k, and the rate of spontaneous radiative decay into all

lower levels j, respectively.

Since the excited projectile has an appreciable velocity

and may move a considerable distance before emitting a photon,

the density of excited states in the projectile beam, and

thus the intensity of photon emissions, will depend upon the

penetration of the projectile beam through the target region.

To facilitate the calculation of ,N i as a function of pene-

tration, the time variable is converted to distance by the

relation x = vt where x is the distance penetrated by the pro-

jectile in time t. As a first approximation to the solution

of equation (111-19), the cascade term is neglected, giving









VuaNb o .
Nb =- [1-exp (-x/vri) ,
S . A (111-20)


subject to the boundary condition Nbi = 0 at x = 0. In this

equation T is equal to [z. A..]-1 and is the lifetime of

state i. The total number of photons emitted in all direc-

tions from a beam of cross sectional area A and of length Z

is given by


L+Z
J. = I I N .A dA dx. ( 11-21)
-3 L b


In this equation L is the penetration of the bean, through

the target chamber before reaching the observation region

(that region from which photons can be detected by the appa-

ratus), and 2 is the length of the beam within the observa-

tion region. Upon integration of equation (111-21), the

emission cross section becomes


oiA .A
o .. -
S Z .A. (111-22)
3<1- -3


where
VT.
A = 1 {exp (-L/wvr)cl-exp(-Z/VTi)]}


(111-23)
is a correction term due to the projectile velocity and the

lifetime of the excited state. When VT << the correction

term A = 1 and the excitation cross section would again take

the form of equation (111-16) except for the fact that the








cascade terms will also depend upon the position of excita-

tion and the lifetime of the higher levels. We will, how-

ever, define a corrected emission cross section 0.. by the

equation


i3 A (111-24)


Again the treatment of projectile excitation is based

upon the assumption that the speed and direction of the

projectile remains unchanged as it passes through the target

region.

Determination of Emission Cross Sections

To determine the emission cross section for the i j

transition it is necessary to relate the output signal S..

(counts per unit time) of the optical system to the total

number of photons J.. emitted per unit time from a length Z

of the beam path through the target region. In any practi-

cal experiment only a portion of these photons are detected.

If the emission is isotropic, the number of photons entering

the optical system per unit time S is given by
S J J


S..= J..
-0 4 1 (1 1-25)


where Q is the solid angle subtended by the optical system.

The output signal S .., therefore, is given by


S.. = S.. K(, A = J. -- K(),
-' -3 t3O 4'i ( 11-26)


where S.. is the number of photons entering the optical system
7-'J








per unit time and K(A) is the detection efficiency at the

wavelength X of the i j transition. The expression for

the emission cross section then becomes, from equation (Ill-9),

S.. 47T
7,3
J K(X) Na bt (I 1-27)


The ion beam flux Ib integrated over the cross sectional

area of the beam is related directly to the ion beam current

I measured by the Faraday cup, and at low pressure the target

gas density Na may be related to the gas pressure P in the

target chamber by the ideal gas law. If the observation length

z and the solid angle Q remain unchanged during a series of

measurements, the emission cross section may be written


a i =
K(A) (I1 -28)


where the relative emission rate a.. is defined by the relation

sj


i3 P I (111-29)


and where A is a constant as long as the observation length

., the solid angle 2, and the temperature of the target gas

remain unchanged.

To insure that the determination of emission cross sec-

tions was independent of secondary processes, i.e., that a.
z3
is independent of both Na and Ib, measurements of relative

count rates were made as a function of target gas pressure.

Figure 10 is a plot of relative count rate versus target
































T3





E


-1


C1



















O-
CZ



cJ









cn
C











C Z


- 0
o










0:
C .-





- 0

0
H 0


CU C LU




















77

















CL






(UQO
C; Wrvj




to LO I

I N UII-IWV 3L.8 L~o 3UV-3








gas pressure for the excitation of (a) the Av = 0 band se-

quence of the first negative system of 02 (b) the (0,0)

band of the first negative system of N2 and (c) the 1200 A

line of NI. The relative emission rate for these three fea-

tures is linear for pressures below 4 X 10-4 torr. Thus all

measurements of relative cross sections were made with a tar-

get gas pressure of less than 4 X 10-4 torr. In addition,

the relative emission rate was found to be linear with ion

beam current over the entire range of currents used.


The Emission Cross Section as a Function of Energy

When the emission cross section of a particular transi-

tion is measured as a function of ion beam energy, both A

and K(x) remain unchanged for all measurements and the rela-

tive emission rate o.. is directly proportional to the abso-

lute emission cross section. Relative emission rates are

measured as a function of energy by computing the ratio of

the net output signal to the product of the ion beam current

and the target gas pressure for each potential applied to the

ion chamber [equation (111-29)]. The net output signal is

taken to be the difference of the output signal obtained with

a target pressure P and the background signal obtained when

the target gas has been removed.


Relative Emission Cross Sections

The ratio of absolute emission cross sections for two

different spectral features measured at the same ion beam

energy is given by









ij ij kZ

kl kl Kij (111-30)


as long as the observation length k, the solid angle 0, and

the temperature of the target gas remain unchanged while the

two features are compared. This expression, then, defines

the relative emission cross sections for the two features,

where o.. and okZ are obtained as described above at a par-

ticular beam energy, and the ratio K(iz)/KK( j .) is obtained

by taking the ratio of the relative detection efficiency at

the two wavelengths. The determination of this relative

detection efficiency is described in the following section.


Calibration of the Optical Detection System

The relative detection efficiency is determined by the

expression

K(\I) I(Xi) Io(\2J

K(\2) I(12) I (A) (111-31)

where I(A1)/1(>2) is the relative intensity of two spectral

features measured with the optical detection system and

Io(A )/o1 0(2) is the known relative intensity of the same

two spectral features.

For wavelengths greater than 2800 A the relative detec-

tion efficiency was determined using a calibrated tungsten

ribbon lamp (Eppley model No. EPS-1055) as a light source.

Plots of the relative detection efficiency of the vacuum mono-

chromator system and of the Ebert mount system, both using










































GJ0


ca C
U'%
Cc











rru


C:4
dlo



















ru 0.
uu
U-'




















0
44g

















Q):
U Li





















u
I~n







-o
>c
(Ul





im
E




4- 0

CO

CE

44E
dI:

44 0
dim
-o

di


En

diE
Id:




48







8











,I,
/ o




o






p i p
I
t' t
r s






- I O






















C
+J

3
0
E







L
-Q








O



0




--r
1 4.








0 m







"J
C-
C 2.

















uc
u 4o
E:
441


c





0
o U











aCE
* -U
0
>- 44







"0

o C
0

E


ao




50



O
-- rF~ TI i-T---- 1 TITTTF-ni-i- i--- o



/
o0
/ o
/





0
-




/










0 0 1




0 c 0
A- ,N Ki fic;.asci 1]A,
*\i
-- -- J- J.L.i.,_L_ -.i .^UJ LL L- -J L _-



^ -: 00
































Figure 13

Relative detection efficiency as a function of
wavelength for the vacuum monochromator
and the EMR phototube.













G-







L .


El
-4



0 3;

li
LL\








L--








1200 1400 100 1800

WAVELENGTH (A
























E


2



.j
> 0





-c





c

O iL
LC





















c C .-
O *- U





m J
-x

> -c

S0)

1- E
00
>)>-










CD
U -- U
4i C





U .) >

0)
















c Z ..
>- SE







U LU
0l








o i
0) E












>C
O
WE





'U


0r

























o








-o

CO







S0 0 Uo 1O -


AOW,1OiU-3 NO!1O213G 3Al?V13a








the EMI 9558Q tube, are shown in Figs. 11 and 12, respectively,

for this spectral region. Here the error in determining the

relative detection efficiency is estimated at less than 5%.

Because there are no standard sources for the vacuum

ultraviolet region of the spectrum, the relative detection

efficiency in this region must be determined by branching

ratio techniques. In the region between 1200 and 1750 A the

relative emission rates of two pairs of NI lines (1493, 1744;

1243, 1412 A) obtained from ion beam collisions are compared

with tabulated transition probabilities10 to obtain the var-

iation in detection efficiency of the system. A plot of the

relative detection efficiency of the vacuum monocaromator

with the EMR tube is shown in Fig. 13 for this spectral reg-

ion. The results for these two sets of points are connected

by assuming that the variation in detection sensitivity is

linear between pairs of points when plotted on a semilogarith-

mic scale. There is some freedom allowed in determining the

intersection point of the two linear segments. The particular

choice made here results in a curve which is very similar to

the quantum efficiency curve for the EMR tube alone. It

should be pointed out that the quantum efficiency of the EMR

tube falls off very sharply below 1200 A, so that the detection

efficiency plotted in Fig. 13 cannot be extrapolated to lower

wavelengths. Although the absolute transition probabilities

of the NI lines may be in error by as much as 50%, the ratios

should be much less uncertain. However, the assumption that

the response curve is linear between pairs of calibration points








when plotted on a semilogarithmic scale, and the freedom

allowed in fitting the two linear segments of this curve lead

to an estimated overall uncertainty in the determination of

relative cross sections of approximately 50% for the entire

spectral range from 1200 to 1750 A. The uncertainty in ob-

taining relative cross sections for the spectral range from

1200 to 1400 A, however, is expected to be much less than 50%.

In Fig. 14 a plot of the relative detection efficiency

of the vacuum monochromator with the MEM is shown for the

spectral region between 600 and 1200 A. For the region be-

tween 1000 and 1200 A the molecular branching ratio method

was utilized. 1 Here, the relative detection efficiency has

been obtained by comparing the relative emission rates of the

1Q lines of the 2-v" progression of the Werner system of H2

produced in a microwave light source to the transition prob-

abilities given by Allison and Oalgarno.12 The light inten-

sity was sufficient to achieve 0.30 A resolution. To obtain

the relative detection efficiency below 1000 A, the relative

emission rates of two pairs of Oi lines (796,718; 673,616 A)

have been compared with tabulated transition probabilities.

This was done for Oil emissions resulting both from He ions

bombarding 02 and from a microwave light source. The detec-

tion sensitivity curve was obtained as before by assuming a

linear variation of sensitivity between pairs of points on a

semilogarithmic scale. Again, using this technique some free-

dom is allowed in determining the intersection points of the

various linear segments. Since the results differ somewhat









depending upon the source of the 011 emissions and upon the

choice of intersection points, a smooth response curve was

constructed by averaging different possible curves. This

average curve is what is presented in Fig. 14. The slope
o
of the relative response curve between 650 and 750 A is seen

to agree well with the slope of the relative quantum efficien-

cy of the MEM in this region (represented by x in Fig. 14).

The relative quantum efficiency of the MEM was obtained by

simultaneously comparing the response of the MEM at various

wavelengths to that of a photomultiplier tube coated with

sodium salicylate under the assumption that the response of

the sodium salicylate does not vary with wavelength over this

region.

The errors in calculating the absolute transition prob-

abilities of the Werner system are quoted as less than 6%.

Again, the ratio of transition probabilities is expected to

be even more accurate. Thus, within the spectral region from

1000 to 1200 A the uncertainty in determining the relative

spectral response is expected to be less than 5%. Assuming

that it does not fall off too rapidly below 1000 A, the

relative detection efficiency is expected to be in error

by less than 20% down to 900 A. The quoted error in the

transition probabilities given for the Oil lines is greater

than 50% so that below 900 A the uncertainties increase and

are probably as high as 70 100% at 700 A.








Dependence of the Emission Cross
Section on Polarization

In any practical experiment only a portion of the photons

emitted from a length of the ion beam are detected. If the

emission of these photons is anisotropic, the number of pho-

tons entering the detection system per unit time will depend

not only upon the solid angle Q subtended by the detection

system, but also upon the angle of observation, 0, measured

relative to the beam direction. It can be shown13 that the

rate of photon emission from a length I of the ion beam per

unit solid angle in a direction e is given by

ij 3-Pcos28
Iij( 4d 3-P (111-32)



where P is the polarization fraction defined by


I. J




In this last equation I.. and I.. are the number of photons

emitted per unit solid angle per unit time in the direction

0 = 90 with planes of polarization respectively parallel

and perpendicular to the beam direction.

The number of photons S.. actually entering the detec-
tJ
tion apparatus, then, is given by

S.. -= f i'( ) di ( 1 -34)


If observations are made at right angles to the beam direction,

and if 0 varies only slightly over the solid angle 0, then








S.. is given by


S.. = I..(900) = I... ( 11 -35)


In order to relate the output signal of the detection

system to the photons entering the system, the detection

sensitivity of the optical system must be known for polar-

ized light. It should be obvious that the count rate S..
"3
is given by
i N L L
S.. = [K I.. + K i .], (111-36)


where K K is the detection sensitivity for light polar-

ized respectively parallel and perpendicular to the beam

direction. Since the total number of photons entering the

detection apparatus I.. is given by


I.. = I.. + I.. = T..(C + 1), ( 1-37)
'-3 zj 1-3 '3

where C = I../ i., and the detection sensitivity for unpolar-

ized light is given by

i L L
K + K K (a+1)
K(W) = ---- ----
2 2 (111-38)

N L
where a = K /X then equation (111-36) can be written


(Ca + 1)
S..= 2K(A) I..
o3 o (a+- ) (C+I) (1 1-39)


Using equation (111-32), with 0 = 90, this equation becomes

0 3 2(C+1) K(X)
St. t 4n 3-P ( a+.(C+ ) (111-40)
sO '_ 4,1 3-P (a+1) (C+1) (1 1 1-40)








and the emission cross section becomes


o. .= - F.
"S NI N I1, (il -41)
a b a

where

(a+1)(C+1) 3-P

2(C+1) 3 (111-42)

is the correction due to the effect of polarization. This

correction term can also be written



1-P/3
r =
P(a-1)
S+
(a+1) ( 1-43)

It is clear from this last expression that if P = 0, F = 1,

and if a = 1, F = 1-P/3. Therefore, in order to actually

determine the emission cross section, the polarization frac-

tion must be determined. From equations (111-33) and (111-35)

it is clear that the polarization fraction can be written

r *-t
S.. S..
P = i AL.
S.. + si. (1 1-44)


As discussed previously the output signal S.. is determined

by the detection efficiency of the optical system. This de-

tection efficiency depends upon the polarization, so that

equation (111-44) becomes

Ir II L I tL JL
itt
= ./K (Ki /K )

S ./K + Si /K o +o (K /K )

( 1 -45)



























0


Cu
N










CL

Cu


0
2























4-
r







E
Cu
I-







2 C-
3 Cu

LCO C
-u

0L a









E
Cu

Dc
C
Cu

Cu

CU


C
Cu
0

Cu




x
w

























LM


cfli l


c- c n

o L
- -)
I c


rl
N


-I




0

.N
Z



Ij



-J
C
^ CL


cr


I <
O 0
2 [-



fd 0
, o


LJ
co












0 LJ
F- "
0 0



S -1

C C
(1_ 0
































Figure 16

Instrumental polarization of the Ebert mount
monochromator as a function of wavelength.















2.0 i-r r -E -i r i-T 1-









1.5 ,:

0$ /




- 4


S.0







0.5 -










4000 5000 5000
WAVELENGTH (A)


7000







n L
where a and a are the relative emission rates for light

polarized respectively parallel and perpendicular to the

beam direction, and where the ratio K /K is called the

instrumental polarization for the detection system, a.

The arrangement for determining the instrumental polar-

ization is shown in Fig. 15. Light from an Eppley tungsten

ribbon lamp was depolarized by a diffuser and focused by a

quartz lens through a polarization analyzer onto the entrance

slit of the monochromator. The analyzer was mounted so that

the polarization axis could be fixed either parallel or per-

pendicular to the beam direction. The output signal of the

photomultiplier was then measured at each wavelength with

the polarizer first parallel, then perpendicular. When the

incident light is unpolarized, the ratio of the output sig-

nals measured with the polarizer first parallel and then per-
L
pendicular to the beam direction, S /S is clearly equal to

the instrumentation polarization K /K A plot of the in-

strumentation polarization as a function of wavelength is

shown in Fig. 16 for wavelengths between 4000 and 7500 A.

Therefore, to determine the polarization of a given

spectral feature as a function of beam energy, the relative

emission rates for the two orientations of the analyzer are

computed for each potential applied to the ion chamber, and

equation (111-45) is evaluated using the instrumental polar-

ization plotted in Fig. 16.















IV. RESULTS

Identification of Observed Spectral Features

Spectral scans arising from N2+ + 02 collisions are

shown in Figs. 17-22 for the wavelength region between 700

and 6200 A. These scans have not been corrected for varia-

tions in the sensitivity of the optical detection system as

a function of wavelength.


Spectral Features Above 2500 A

In Figs. 17, 18, and 19 a spectral scan from 2500 to

5050 A taken with 5 A resolution is shown for a collision

energy of 4 keV. The most prominent features in this spec-

tral region are the vibrational bands of the second positive

system of N2 (C 3 B 31 ) and of the first negative sys-
S a g
tem of N + (B 2+ X 2Z+), the (0,0) band of this latter
2 u g
system being off scale in Fig. 18. The locations of many

of the band heads of these systems are indicated although not
0
all of those indicated are clearly resolved. With 5 A reso-

lution both of these systems have the appearance of single-

headed bands which are degraded toward shorter wavelengths.

Also present in this spectral region (most prominent at the

shorter wavelengths) is the second negative system of 02

(A 21 .- X 2H ). This is an extensive system of double-
u g
headed bands which are degraded to the red. Below 3000 A

other spectral features also appear to be present, but because





















0 -








NC
E >





O
u a)

0 .C



UC C4
C: _






0
U ) -u


(D- C
Oc









0 m0

- "--
4-

E



C T 4







O 0
or
o 1- .







-I-
















m 0
0 m)

























' C
"- 0 0








-E
-o












m
oJ-
U ^
CL .-







LUl f
Ea~





















0









2,


0





2 3S/SN~0t


C)
0





N,










2;if































a

N

2


u---,-




-r -.











CC











--C





















--4
2K
:4Km












O-'"d Y fl I






















C1O



r>
o -












-c- t

+ -0




0 C
O C














o"ro 0
0

C -L:0


O0 O







*- -4
C4-' 0
**O-






to-- -u
3C:



cc

0
0 o -
o L





C >

-2 C
o o
o -r

( 4- 0
0 --



3LO

C a"O




S- O

auto
0 0--4
1) u=






i -


E
UJ





70




(;
o










--- f U L






S..

-- 0- 4 -,9
-r .4 3 s IK













3- In AV't
s o o I



4,-- '5^
0 "

-/ I
-- I~




).3f- .IN L
s- ^ "
s. r~y












^ ^ 2__ ^
o IXo < -
























-c


+

C
+ fU

Z 01

Oa *



0 c
C U>


0 0
c






aE C-
O r










C
. OT
0 0







U-0





cn- .--





















U
0
ro r
C L..











E
0Cu
o o




-U --
04-






OQO
0r0



U -













-- IO
LE













A".
r

io







-- /
I')
a
N
ri-
zn t ho







0
a a
N L




















g--
Z ---




aQ
rit
^ / <









41J t 5






2

_. _
it" _.









... -
Si,-





N C
o 0- 0 ---it
Tr ~ fi fl.







'0



- -) C)-



o ot
26 -


-Q N- 0. .
a o.
/ t :- 1dO d r ^IU
t-< H 1= ^^
0u 0 0I '- ~-





<' '. -- TK- -
(i -1.^
> n ^.



a ~f0




?3S/SNOI~o:-ud do) dtij~'!li ri.Mi iir"





















Co



















-TD
0
m c
0e
+*-




















"-- CD
(o

4-f -


























L C
CA







00
-- 3










04-'
O0
0 C-u
cc



*-o >





















<- 0
L C






















mi
4- C
















CY



U-c-
fO
CO
o I- 0

C'i

-c c

C .2
r 0








a) L






0
U -



u-




(U r







LU













~p- 'ro:
0u






PH





a ~ O

0,










I~U)


a *.2





to







a -. r
c.J I







,2



too


*1-




















N
0


.:C -
c




c-j
c
+ m

C


-D
0
C 1
O

inc




0
0 >
o o<


E -


















4
OJ










































lu
c

-c~c
CL 3
0- i
C -a
0 -- ini


O Q04





L\ -


in 4 a






CL-





ri





U -


a uO





in
0E
c l-

"u
3: <






76








Ct C
H H --- .--^
,, "* J I









60 -









zI
^ H ------- ------c--~-------- *'.


F'^l.s-'-----;i -- -- --- --- --------




3/
c 7I /-^^
^-" '

H H
_^ _ _______


33S,'SNOJ.OHcd 2JO tSI'nLN 3AiIJ.U-1u


0
ro


U
-_
uJ






















0















Q)
0
+










Q))

0)
4- Z













cn c
A_-






CD,

Clo




LA 0
0u T
---0)
E )

























LiD
o -


0-o
.0 L






O 01





C 000



004



C"

>- 4
0a) 0




0-

0)0)





U-
0)0
0.

00
Oc
04-
40


Fl





78




V -
1-n ,g + '












N o
H


0 0 0

-- --- ------- 0
N H- 0















L L
Z a
-, N H -e












-- -- ^- - _
ci







0 N N c' I-



''r- ^ ------ '"----W-- _
-42















a C)



^ __


3SS/SNOI&OHd J0 aiZWru-i JAIN173U


M-.








of the relatively low signals and extensive nature of the 02

second negative emissions, these could not be unambiguously

identified.14

In Fig. 20 a scan from 5000 to 6200 A is shown taken

with a resolution of 8.5 A. This scan was also taken at a

collision energy of 4 keV. The width of the rotational struc-

ture and the consequent overlapping of the different bands of

the first negative system of 02 (b 4Z a ) together

with the relatively low signal make it impossible to identify

the individual bands. For this reason only the band sequences

(Av = 0, -1, -2) have been identified.

The atomic lines observed in the spectral region above

2500 A arise, except for four 01 lines, from transitions of

the atomic ions of oxygen and nitrogen. These lines appear

to be relatively weak compared to the molecular bands of N2

and N2 but this is due in part to the decrease in detection

sensitivity at the higher wavelengths.

Above 6200 A no additional features were identifiable.


Spectral Features Below 1500 A

Figures 21 and 22 show a spectral scan from 1500 to 700 A

taken with 5 A resolution, the collision energy being respec-

tively 2.5 and 4 keV. The most prominent features in this

spectral region are the resonance lines of both NI and 01.
0 0
Both of these features (the 1200 A line of NI and the 1303.5 A

line of 01) are off scale in Fig. 21. Other atomic lines

arising from excited dissociation fragments are also identi-

fied.15








Below 700 A a spectral feature was observed at approxi-
0
mately 673 A. This feature appeared to be broad enough to be

two unresolved lines and probably arises from the 2s3s 2pO -

2p2 3p transition of NII and the 2p23s 2p 2p3 2pO transition

of O l .

The only spectral features observed in the wavelength
0 o
region between 1500 and 2500 A (not shown) were the 1743.6 A

line of NI, which arises from the same upper state as the
0 o
1493.3 A line, and, at wavelengths greater than 2200 A, the

second negative system of 02


Emission Cross Sections of the Molecular
Features of N and N2

The emission cross sections for the (0,0), (0,1), (1,2),

and (2,3) bands of the first negative system of N2 are plot-

ted in Fig. 23 and listed in Table I as a function of ion

beam energy, while emission cross sections for the (0,0),

(1,0),. and (2,1) bands of the second positive systems of N2

are presented in Fig. 24 and Table II. The relative emission

rates for the individual features were determined at a much

larger number of impact energies than indicated in these

figures, and a smooth curve was visually fit to these origin-

al data points. Within experimental error there appears to

be no structure in the relative emission rates as a function

of energy for any of these spectral features so that the

values for the emission cross sections derived from this

smooth curve are averaged values and may well be more accu-

rate than individual data points. The error bars in Figs.







































C- a
Oa)


a-E

0*4 I.
U) a)

u*- a
a~ U-
-0 E
oala
U a
a~C 4OJ

a) 4
-~ 0




U) C

o n 4-







a) itl

a) c'



' -0


a--
a)- E





E '-U)


a) >
0 *
-~ a-
0-
U)
vl0
















O
---T Il--T-1- -ic
o x





00 <<
0 4 -
o ox < --
o x oo i,.

o o l :< --
oo Lu
0 o -



"- 0
+a -
00 0
-C;- -a] 0



- 3c
<


1 ilf I I I I I -
Q q q q q o
(uw O I)-o














Table 1. Absolute emission cross sections for
the (0,0), (0,1), (1,2), and (2,3) bands of
the first negative system of N2+ arising
from collisions of N2+ with 02.

Cross Section
Energy
(key) (10-18 cm2)
(keV)0) (0, 1 2) (2____
(0,0) (0,1) (1,2) (2,3)


0.25
0.30
0.35
0.40
0.50
0.60
0.70
0.80
0.90
1 .00
1.25
1.50
1.75
2.00
2.50
3.00
3.50
4.00
5.00
6.00
7.00


0.07
0.14
0.27
0.45
0.79
1.14
1.53
1.88
2.27
2.67
3.63
4.64
5.61
6.48
8.15
9.64
11.05
12.38
14.52
16.49
18.35


0.05
0.09
0.18
0.35
0.49
0.63
0.77
0.91
1.20
1.49
1.78
2.02
2.50
2.94
3.32
3.66
4.28
4.84
5.34


0.05
0.09
0.18
0.28
0.41
0.53
0.63
0.75
1.00
1.23
1 .44
1.65
2.05
2.38
2.72
3.08
3.63
4.22
4.75


0.05
0.09
0.17
0.24
0.33
0.41
0.47
0.53
0.67
0.78
0.88
0.96
.1 0
1 .24
1.33
1 .40
1 59
1.74
1 .85































-W
-0ol
C:





'4-
-0


aa




0o


WO
C3
C




-u u










00(
uu
75










0.. E
aj




OW




in,


UZ







0 (1)
inC





0


u
(1)









-C
C:4
0
2- u









0


Sla
0
-on





85










0



0 < -- x -
O

o < x < ~

xI
W <
-0- -1- ---




c 'x4 0
O
0C0


a C.



oJ


IJ 1 I -1 2 ___
o o q o o oq o
L jo ro o


(WO o 801).-o














Table II. Absolute emission cross sections for
the (0,0), (1,0), and (2.1) bands of the
second positive system of N2 arising
from collisions of N2+ with 02.


Cross Section
(10-18 cm2)


Energy
(keV)


(0,0)


0.30
0.35
0.40
0.50
0.60
0.70
0.80
0.90
1.00
1.25
1 .50
1 .75
2.00
2.50
3.00
3.50
4.00
5.00
6.00
7.00


0.02
0.04
0.05
0.1 1
0.18
0.23
0.32
0.39
0.50
0.78
1.10
1.38
1.69
2.30
2.77
3.19
3.57
4.26
4.80
5.30


(1,0)


0.02
0.05
0.08
0.12
0.18
0.25
0.34
0.41
0.64
0 .89
1.16
1.40
1.87
2.24
2.58
2.92
3.37
3.81
4.22


(2.1)


0.03
0.08
0 .11
0.16
0.22
0.28
0.45
0.63
0.77
0.91
1.1 1
1.42
1 .64
1.88
2.20
2.51
2.02








23 and 24 indicate the estimated error in determining the

relative cross section of a particular band as a function

of energy.

As can be seen in Figs. 23 and 24, the relative emis-

sion rates of the N2 first negative and of the N2 second

positive bands increase monotonically above an ion-beam

energy of about 0.20 keV. Below this energy the relative

emission rate levels off, and, within experimental error,

becomes constant as far down as 80 eV. The emission rate

of the (0,0) band of the N2+ first negative system remained

constant as far down as 33 eV.

It was impossible to determine whether this constant

signal resulted from emissions of the feature under observa-

tion or from the background continuum which was observed at
o
all wavelengths above 2500 A. Therefore, the energy thresh-

old for the emission cross section could not be determined.

The observed background above 2500 A may be due in part

to unresolved molecular emissions of the second negative

system of 02 In addition, no emissions from the 02 mole-

cule were identified, so that some of this apparent background

may result from unresolved 02 emissions. However, this seems

unlikely since the relative emission rate of the background,

measured at 3200, 3400, 4015, 4300, and 4900 A, was found to

be fairly constant with both wavelength and collision energy,

although there was a slight increase in the observed back-

ground at 4300 and 4900 A for energies above 4 keV. The

emission rates for all spectral features above 2500 A have

been corrected for this observed background.









In order to determine the variation of the emission

cross section for a particular molecular band as a func-

tion of energy, the relative emission rate of the band head

was measured with a 5 A band width for various ion beam ener-

gies. The relative emission rates obtained in this way

should be free of significant error unless there is consider-

able overlap of the head of one band by the rotational struc-

ture of an adjacent band and unless the rotational-energy dis-

tribution in the upper state varies significantly with pro-

jectile velocity.

To determine the amount of overlap of the head of one

band by the rotational structure of an adjacent band for the

Av = -1 band sequence of the first negative system of N2 a

high resolution scan was taken with a beam energy of 3 keV

and a target pressure of 4 X 10-4 torr. This scan indicated

that the rotational structure was relatively compact, extend-
0 0
ing only over about 40 A for the (0,1) band, 30 A for the

(1,2) band, and 20 A for the (2,3) band. The error in the

relative emission rate due to overlap was estimated at less

than 3% for this system. A high resolution scan of the

Av = +1 band sequence of the second positive system of N2

was also taken at a beam energy of 3 keV. Again the rota-

tional structure of the bands appeared relatively compact

and the error in the relative emission rates due to overlap

was estimated to be no greater than 6%.

In experiments conducted by Bregman-Reisler and Doer-

ing16 on the excitation of N2 in collisions with He and Ne,







the energy distribution among the rotational levels of the

first negative (0,0) band of N2 was found to be velocity

dependent below about 1.2 X 107 cm/sec (an ion-beam energy

of approximately 2 keV). The excitation of high rotational

states was found to increase with decreasing ion velocities

and the intensity maximum of the band contour became broader

and moved toward higher values. Such a shift in the intensity

maximum of the band contour would tend to make the relative

emission rate of a particular band measured in the manner

described above appear to decrease more rapidly than it ac-

tually does at lower energies. However, the magnitude of

the error resulting from such a shift could not b precisely

determined in all cases due to poor resolution, and no at-

tempt has been made to correct for it.

Because of a limited observation length Z, emission

cross sections arising from excited states of the projec-

tile must be corrected for both the projectile velocity and

the lifetime of the excited states according to equation

(111-23). The cross sections presented in Figs. 23 and 24

as well as in Tables I and II have been corrected using

66 nsec17 as the lifetime of all vibrational levels of the

N2+ B 2E state and 38.4 nsec18 as the lifetime of all vibra-

tional levels of the N2 C 31u state. This correction amounts

to less than 3% below 1 keV, but as much as 30% at 7 keV for

the N2 first negative system, while for the N2 second posi-

tive system it amounts to less than 3% below 3 keV and only

as much as 10% at 7 keV.




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