Spatial and energetic aspects of electron energy deposition


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Spatial and energetic aspects of electron energy deposition
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vi, 227 leaves : ill. ; 28 cm.
Jackman, Charles Herbert, 1950-
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Collisions (Physics)   ( lcsh )
Nitrogen   ( lcsh )
bibliography   ( marcgt )
theses   ( marcgt )
non-fiction   ( marcgt )


Thesis--University of Florida.
Includes bibliographical references (leaves 220-226).
Statement of Responsibility:
by Charles Herbert Jackman.
General Note:
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University of Florida
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Dr. A.E.S. Green has helped the author a great deal in his efforts

to complete this work. The author sincerely appreciates this guidance.

He also wishes to thank Dr. R.H. Garvey and Dr. R.A. Hedinger for their

helpful discussion about the dissertation. David Doda, David Killian,

E. Whit Ludington, George Sherouse, and Ken Cross were instrumental in

providing assistance with computer problems and other dissertation-related


Woody Richardson, Marjorie Niblack, and Wesley Bolch were extremely

helpful in drafting the figures. The final manuscript was then typed and

refined by Adele Koehler. The author is grateful to Adele for her prompt

and professional assistance.

The author wishes to thank Joseph Pollack for aiding in editorial

matters concerning the dissertation. A thorough reading and criticism

of the dissertation by the author's committee (including Dr. A.E.S. Green,

Dr. L.R. Peterson, Dr. T.L. Bailey, Dr. S.T. Gottesman, and Dr. G.R.

Lebo), Dr. UJ.L. Chameides, and Dr. A.G. Smith was extremely helpful.

The author is especially grateful to his parents, Rev. and Mrs. H.W.

Jackman, and to his sister, Kathi Jouvenat, for their encouragement and

support throughout graduate school.

The author gratefully acknowledges financial support from the De-

partment of Physics and Astronomy and the Graduate School of the Univer-

sity of Florida and from NASA grant number NGL-10-005-008.



ABSTRACT. . . ... ..




A. Energy Deposition Techniques . .

B. Monte Carlo Energy Deposition Techniques .

SECTIONS FOR N2. .................. ..

A. Elastic Differential and Total Cross Sections for N2 .

B. Inelastic Differential and Total Cross Sections
for N2 '................... .....

C. Total Cross Section (Elastic Plus Inelastic) .


A. Brief Discussion of the Monte Carlo Calculation. .

B. Computer Programs and Machinery Used in the Monte
Carlo Calculation. . . .

C. Detailed Discussion of the Monte Carlo Electron
Energy Degradation Technique . .

1. First Random Number, R. .............

2. Second and Third Random Numbers, R2 and R3 ...

3. Fourth Random Number, R .. .....

4. Fifth Random Number, R5. ..............




















5. Sixth Random Number, Rg. ............. 67

6. Multiple Elastic Scattering Distribution Used
Below 30 eV. . .. ..... .. 67

7. Value of the Cutoff Energy, 2 eV ... 74

D. Statistical Error in the Monte Carlo Calculation 75


A. Excitation of the N B2 + State. .... .. 77
2 u
B. Range of Electrons . .... .. 80

C. Previous Experimental and Theoretical Work on the
3914 A Emission of N ........ 81

D. Range Results and Longitudinal Intensity Plots from
the Monte Carlo Calculation. . ... 84

E. Intensity Plots in the Radial Direction. .. 87


A. Effects of Ionization Differential Cross Section on
the Intensity Distributions. . .... 99

B. Influence of Inelastic Differential Cross Sections
on the Intensity Distributions . .. 104

C. Comparison of Different Elastic Phase Functions on
the Electron-N2 Collision Profile. . .. 104
D. Influence of Different Elastic Phase Functions on
the Intensity Profiles . .. 112
E. Effects of the Total Elastic Cross Section on the
Electron Energy Degradation. . ... 121


A. Energy Loss of Electrons in N2 ........... 125
B. Spatial Yield Spectra for Electrons Impinging on N2. 130

1. Three Variable Spatial Yield Spectra ...... 132

2. Four Variable Spatial Yield Spectra. .. 143

VIII CONCLUSIONS. . ... ..... 152



A MONTE CARLO PROGRAM ................... 155

B GETDAT PROGRAM .............. ....... .... 200

REFERENCES . . . .. 220

BIOGRAPHICAL SKETCH ....................... 227


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


Charles Herbert Jackman

August, 1978

Chairman: A.E.S. Green
Major Department: Physics and Astronomy
The spatial and energetic aspects of the electron energy degradation

into molecular nitrogen gas have been studied by a Monte Carlo method.

Perpendicularly monoenergetic incident electrons with energies from 0.1

through 5.0 KeV were injected into the N2 gas. This Monte Carlo de-

gradation scheme employed previously developed N2 cross sections with new

phenomenological differential elastic and doubly differential ionization

cross sections. All these agree quite well with experimental work and

are consistent with the higher energy theoretical total cross section

fall-off with energy.

Information has been generated concerning the following topics:

1) range values and 3914 A intensity profiles for the longitudinal and

radial directions which can be easily compared with experimental work;

2) a sensitivity study characterizing the influence of the input cross

sections on the spatial energy deposition of the electrons; 3) the rate

of energy loss by the electrons as they interact with the N2 gas; and

4) spatial yield spectra for incident electron energies in the range
from 0.1 to 5.0 KeV (evaluated between 2 eV and the incident energy)

which are analytically characterized for future work on atmospheric prob-

lems dealing with incident energetic electrons.



Calculating the spatial and energetic aspects of the energy deposi-

tion of intermediate energy electrons (with incident energies from 100

to 5000 eV) in the earth's atmosphere is a difficult, yet intriguing,

problem. These intermediate energy electrons (hereafter called IEEs)

include the highest energy photoelectrons, a large bulk of the auroral

electrons, and many secondary electrons produced by solar protons and

cosmic rays.

These electrons lose most of their energy through ionization events,

electronic excitations, vibrational excitations, and rotational excita-

tions. Elastic collisions reduce the electron energy slightly, but

mainly these interactions influence the direction of motion of the


The atmosphere is dominated by the presence of molecular nitrogen

up to a height of about 150 kilometers. Even above this altitude (at

least up to 300 km), M2 continues to play a substantial role in the

atmospheric processes. For this reason the study of the influence of

impinging electrons on molecular nitrogen is the major thrust of this


One aspect of this study is the formulation of a complete cross

section (differential and total) set for IEEs impacting on N2. The very

difficult problem of modeling the interactions of the impinging IEEs in

the upper atmosphere is then reduced in complexity. Since N2 interacts

with electrons similar to the way that other atmospheric gases interact

with electrons, it follows that differential and total cross section sets

for these gases could be assembled in a like manner.

Another aspect of this work is a sensitivity comparison among several

of the influences on the electron energy deposition. The spatial energy

degradation is vitally linked to the elastic phase function used. Since

there are data available on the elastic differential cross sections of

N2 as well as the energy degradation resulting from electron impact on

N2, a comparison illustrating the effects of a variation of the elastic

phase function is quite useful. Other influences on the spatial energy

deposition, including ionization and excitation differential cross

sections and the total elastic cross sections, are also considered in

this work.

In order to deal with these spatial and energetic aspects of elec-

tron energy degradation, a Monte Carlo (which may be abbreviated MC)

calculation is used. The MC technique, depending on how it is used, can

be the most accurate energy deposition approach. Use of this MC method

at various incident energies helps in the assemblage of the best cross

section set for N2 and provides the easiest way of comparing some of

the influences on the spatial energy deposition.

The details of this undertaking are discussed in Chapters II through

VIII. A paragraph summary of each chapter is given below.

The second chapter presents a brief review of some of the standard

electron energy deposition methods. The continuous slowing down approxi-

mation, discrete energy bin, Fokker-Planck equation, two-stream equation

of transfer, and the multi-stream equation of transfer are all included

in section II.A. The MC method which was used in this study along with

three other MC approaches are briefly described in section II.B.

This MC approach requires knowledge of differential and total cross

sections. The third chapter discusses the cross sections that were used

for N2. Section III.A includes the elastic differential and total cross

sections. The inelastic differential and total cross sections are next

discussed in section III.B. Section III.C, then, considers the total

(inelastic plus elastic) cross section of N2.

In Chapter IV, the MC calculational procedure is considered. A

brief discussion of the approach is given in section IV.A. The computer

programs and machinery used in this work are discussed in section IV.B

with the programs listed in appendices A and B. A detailed discussion

of the MC electron energy degradation technique is presented in section

IV.C. Finally, the statistical error resulting from the Monte Carlo

calculation is given in section IV.D.

The MC three-dimensional intensity plots with comparison to experi-

ment are given in Chapter V. The excitation of the N2 BE state is

discussed in section V.A with the concept of range being defined in
section V.B. Previous experimental and theoretical work on the 3914 A

emission from N2 is considered in section V.C and section V.D presents

some range results and intensity plots in the longitudinal direction

from this MC calculation. Section V.E, then, concludes the chapter with

a comparison between the MC intensity plots in the radial direction and

the experimental data.

The main concern of Chapter VI is a sensitivity study. The effects

of the ionization differential cross section on the intensity distribu-

tions reconsidered in section VI.A. Section VI.B, then, discusses the

influence of inelastic differential cross sections on the intensity

distribution. A comparison of different elastic phase functions on the

electron-N2 collision profile (no energy loss) is given in section VI.C.

Next, the influence of different elastic phase functions on the electron

energy deposition is presented in section VI.D. Finally, section VI.E

considers the influence of the total elastic cross section on the

electron energy deposition.

The energy loss plots and yield spectra from the MC calculations

are given in Chapter VII. Section VII.A presents the energy loss plots

and section VII.B includes a discussion of the yield spectra.
Chapter VIII concludes this paper with a summary of this work and

its impact on the spatial and energetic aspects of the electron energy

deposition problem.



Several standard energy deposition techniques will be discussed in

this chapter. In the first section, II.A, several general ways for

treating the degradation of the energy of charged particles will be re-

viewed briefly. The second section, II.B, includes a brief sketch of

four Monte Carlo energy deposition schemes: The MC approach applied in

this work and three other MC techniques.

A. Energy Deposition Techniques

Since the turn of the century, researchers have been studying the

energy degradation of rapidly moving particles in a medium. Initial work

in this area was carried out by Roentgen, Becquerel, Thompson, Bragg,

Rutherford, Bohr, and other founders of modern physics.

These pioneers in the energy degradation process were mainly con-

cerned with the medium affecting the particle. In order to solve this

complex energy degradation problem, most of the early workers used a

local energy deposition approximation. Even today many degradation

problems can be solved fairly accurately with this local approximation.

One of the earliest local energy deposition methods is that of the

continuous slowing down approximation (hereafter called CSDA) first

used by Niels Bohr (1913, 1915). Bethe (1930) expanded on this work and

used an approximate theoretical treatment (valid at high energies) to

describe the slowing down of particles in a medium.

This work of Bethe (1930) required knowledge of the stopping power,
(the rate at which energy E is lost from the impinging particles
incident along the x axis). This stopping power is given by
dE _+
d = n S Wi. (E) (2.1)
dx i 1
(see Dalgarno, 1962, p. 624) where the summation S includes integration
over the continuum (thus allowing for energy loss through ionization),
Wi is the energy loss for the ith state, and ai(E) is the cross section
for excitation to the ith state at energy E. Knowledge of the stopping
power then leads to information about the mean distance traveled by the
particle (referred to as the range). This range R(E) of a particle of
energy E is simply given by
E dE
R(E) = (2.2)

Atmospheric researchers are more interested in the effects that the
particles have on the medium rather than the medium affecting the par-
ticles. These effects could include both spectral emissions by the con-
stituents and heating of the atmosphere.
Green and Barth (1965) were the first workers to adapt a variation
of the CSDA to the problems in aeronomy. In this approach the total
energy loss function L(E) = -(!) ) is determined by
(E- I)/2 daI,(ET)

L(E) = I Wk k(E) + + Ijal (E) + I ) T da(ET) dT (2.3)
k J j j 0 dT

where Wk is the threshold for excitation to the kth state, ak(E) is the
cross section for excitation to the kth state at energy E, I. is the
J daj.(E,T)
threshold for ionization and excitation to the jth state, and dT

is the secondary differential ionization cross section for the creation

of a secondary electron of energy, T, from a primary electron of energy
E. The loss function with detailed atomic cross sections (hereafter

called DACSs) was used to make reasonable estimates of the ultraviolet
emissions resulting from an aurora event. In this approach, the excita-

tions Jk(E) of the kth state resulting from an electron of energy E were
simply represented as

E ak(E')
Jk(E) = I _- dE' (2.4)

Green and Dutta (1967) built on this work and used the Born-Bethe

approximations, the Massey-Mohr-Bethe surface, the Bethe sum rule, and
a "distorted" oscillator strength to lay the groundwork for extension
of the DACS approach to other gases. Jusick, Watson, Peterson, and
Green (1967), Stolarski, Dulock, Watson, and Green (1967), and Watson,
Dulock, Stolarski, and Green (1967) applied this approach to helium,

molecular nitrogen, and molecular oxygen, respectively.
Stolarski and Green (1967) used this CSDA to calculate auroral

intensities with these DACSs and Green and Barth (1967) applied this
method to the problem of photoelectrons exciting the dayglow. Other

atmospheric physicists (namely, Kamiyami, 1967; and Rees, Stewart, and
Walker, 1969) started around this same time and also employed a CSDA type

approach to that problem of energetic electrons depositing their energy
in the atmosphere.
The oldest discrete energy apportionment method is that of Fowler
(1922-23) which is directly related to the Spencer and Fano (1954)

approach (see Inokuti, Douthat, and Rau, 1975). The Fowler equation is

solved by building on the lower-energy solutions to obtain the higher
energy solutions. The Spencer-Fano method introduces the electron at
the highest energy and solves the equation at successively lower ener-
gies. An approach similar to the Spencer-Fano method was developed by
Peterson (1969) and is called the discrete energy bin (hereafter called
the DEB) method.
Peterson (1969) pointed out certain differences between the CSDA and
the DEB results. In particular, he noted that the DEB method tends to
predict higher populations of some excited states than does the CSDA.
In the modification of the DEB method by Jura (1971), Dalgarno and
Lejeune (1971), and Cravens, Victor, and Dalgarno (1975), the equilibrium
flux or degradation spectrum f(E,E ) (for incident energy Eo and electron
energy E and in units of # cm-2 sec1 eV-1) of Spencer and Fano (1954)
is obtained directly. Douthat (1975), in an effort to make the degrada-
tion spectra suitable for applications, proposed an approximate method
of "scaling." Unfortunately, this method is quite cumbersome and not
very accurate. This impelled Garvey, Porter, and Green (1977) to seek
an analytic representation of the degradation spectra and, despite its
complex nature, they found an analytic expression to represent this
spectra for H2.
The concept of the "yield spectra" U(E,E ) was first initiated
through a modified DEB approach given in Green, Jackman, and Garvey
(1977) in an effort to find a quantity with simpler properties than the
degradation spectra. By utilizing the product

U(E,Eo) = aT(E) f(E,Eo)

where OT(E) is the total inelastic cross section for an electron of
energy E, one defines a quantity U(E,E ) which not only has a simpler

shape than f(E,Eo) but also has approximately the same magnitude for

all substances. This yield spectrumcan also be represented analytically.

It effectively embodies the non-spatial information of the degradation

Jackman, Garvey, and Green (1977a), using this modified DEB,
elaborated on the differences between the DEB method and the CSDA. The

more accurate modified DEB method was found to produce consistently more

ions per energy loss while at the same time producing less excitations
of some of the low lying states when compared with the CSDA. The CSDA,
although inexpensive to use, appears to be ill-suited for calculations

requiring an absolute accuracy better than about 20%. Since auroral and

dayglow intensities are rarely measured to better than this accuracy, the

CSDA has been adequate for most purposes of concern in aeronomy. How-

ever, with future improved measurements it should be purposeful to
utilize more accurate deposition techniques.

Several recent spatial electron energy deposition studies have been
concerned with the spatial aspects of auroral electron energy deposition.

Walt, MacDonald, and Francis (1967) employed the Fokker-Planck diffusion

equation to give a detailed description of kilovolt auroral electrons.
The Fokker-Planck equation, as given in the Strickland, Book, Coffey,

and Fedder (1976) paper, is written

Snz)a(Eaz E [ 2) a.(zE,u)]
"n(z)a(E)az 2a{ET aF aE

+ aE- [L(E)4(z,E,E)] (2.5)

where 4(z,E,p) is the flux (in units of cm-2 sec-1 eV1 sr-1), z is the
distance into the medium along the z axis, E is the electron energy,


and p is the cosine of the pitch angle associated with the direction of

motion of the electron. The symbols n, a, Q, and L are the number den-

sity of the scatterers, the total cross section (both elastic and in-

elastic), the momentum transfer cross section, and the loss function,


The momentum transfer cross section, given in terms of the differ-

ential elastic cross section, d(E)is written as
2n 7r
Q(E) = i dE (1 cose)sineded4 (2.6)
0 0d

This Fokker-Planck equation may be thought of as a CSDA approach to

the spatial energy degradation problem. Its solution, therefore, is only

accurate in the higher energy regime.

Banks, Chappell, and Nagy (1974) were able to calculate the emission

as a function of altitude for a model aurora using the Fokker-Planck

equation for electrons with energy E > 500 eV along with a two-stream

equation of transfer for electrons with energy E 5 500 eV. The two-

stream equation of transfer solves the transport of electrons in terms

of the hemispherical fluxes of two electron streams 0 (E,z), the electron

flux upward along z, and 4-(E,z), the electron flux downward along z.

The steady state continuity equations for 0 and 0 can then be written


+L -1 n k k k +
dz Pk a ee

+ 1 kk- + q +q (2.7)
k k ee 2



o- = 1 Yn k k k -
dz k k + ea

1 kk+
+ 1 n kk + + q- (2.8)
k e e 2

ak k (2.9)
a ai

q (E,z) = ~ nk(z) {P (E')a (E'-E)-(E,z)

+ [1 pj(E')]c a(E'-E)+(E ,z)} (2.10)

k k (E,_ E)+(Ez)
q-(E,z) = C nk(z) I {pk E')aj( E)+(E'z)
E >E

+ [1 Pa(E')]a (E'-E)a-(E',z)} (2.11)
ai a3

and z is the distance along a magnetic field line (positive outward);
nk(z) is the kth neutral species number density; p (E) is the electron
backscatter probability for elastic collisions with the kth neutral
species; ak(E) is the electron total scattering cross section for the
kth neutral species; q(E,z) is the electron production rate in the
range E to E+dE due to ionization processes (both electron ionization
and photoionization); q~ is the electron production in the range E to
E+dE due to cascading from higher-energy electrons undergoing inelastic
collisions; p k is the electron backscatter probability for collisions
with the kth neutral species resulting in the jth inelastic process; and
k. is the inelastic cross section for the jth excitation of the kth neutral


This approach combined these two methods of electron energy deposi-
tion in order to find a reasonable solution to the very difficult auroral
energy deposition problem. The Fokker-Planck method is accurate only

at large incident energies; therefore, it should only be used at ener-
gies above 500 eV. The two-stream equation of transfer approach, on the
other hand, is more accurate at energies below 500 eV. This combination
then provided a very reasonable solution to the auroral electron spatial
deposition problem for a reasonable amount of calculation.

The Fokker-Planck equation and the two-stream equation of transfer
may both be derived from the Boltzmann equation or the general equation

of transfer. This general equation of transfer, in one of its simpler
forms, is written as (from Strickland et al., 1976)

Sd(z E,)= -n(z)o(E)O(z,E,p)

+ n(z)o(E) f R(v',',v,E'E)(z,E',p')dE'dP' (2.12)

(assuming a steady state condition and no external fields) where

R(p',p,E',E) a G(E) (2.13)

with the sum over all processes. The symbols p and y' are the cosines
of the pitch angles e and e' which are associated with the directions n
and i' given in Figure 2.1.
The first term on the right hand side of Eq. (2.12) represents the
scattering out of p. The R(p',p,E',E) in the second term is the proba-
bility (eV- (2wsr)- ) that a collision of an electron of energy E' and
direction p' with some particle will result in an electron of energy E


0 8' /





- ~1

Figure 2.1 The directions denoted by n' and n are the incident and
scattered directions of the electron, respectively.



and direction P. The integral in Eq. (2.12) is over all possible ener-

gies E' and directions of motion p'.

Strickland et al. (1976) studied the auroral electron scattering

and energy loss with a multiangle equation of transfer. Their approach

is one of the most accurate yet applied to auroral electrons. This multi-

angle method of solution is more realistic than the two-stream approach

and it is computationally more difficult as well.

The methods discussed above are the "state of the art" approaches

(excluding the Monte Carlo methods which are discussed in section II.B)

to the IEEs degrading in the atmosphere. Other approaches used by

Jasperse (1976, 1977) and Mantas (1975) are mainly concerned with photo-

electrons and will not be discussed here.

The Monte Carlo approach can rival any of these electron energy

deposition methods in accuracy when used in the proper manner. This

stochastic technique for solving the deposition problem will be con-

sidered next in section II.B.

B. Monte Carlo Energy Deposition Techniques

Another method of solving the spatial energy deposition problem is

the use of the Monte Carlo approach. The MC technique, which is used

in this paper, is a stochastic method of degrading an energetic electron.

The approach can be one of the most exact methods of electron energy

deposition. Briefly, one electron is taken at a time and allowed to

degrade in energy collision by collision. This deposition attempts to

mimic the randomness of the actual degradation process that occurs in



Many MC schemes have been applied in all areas of physics. Some

are more exact and more detailed than others. Since virtually all the

MC methods are run on the computer, the most exact approaches cost the

most computer time and money. The precision of the technique must be

balanced against a finite computer budget.

Three approaches using the MC deposition scheme, that have been

applied to electrons impinging on the atmosphere, are discussed below.

Brinkmann and Trajmar (1970) applied experimental differential electron

impact energy loss data in a MC computation for electrons of 100 eV

energy. Because of the large amount of input cross sections in numerical

form, only electrons of 100 eV incident energy were degraded with this


In the lower electron energy regime (below 25 eV), Cicerone and

Bowhill (1970, 1971) used a MC technique to simulate photoelectron dif-

fusion through the atmosphere. This method, which included both elastic

and inelastic processes, predicted escape fluxes from the atmosphere.

Berger, Seltzer, and Maeda (1970, 1974) (hereafter called BSM)

worked with high energy electrons (with energies from 2 KeV to 2 MeV).

They employed a MC approach that has two variations which are pointed out

below. They treat inelastic collisions in a continuous slowing down

manner. The energy deposited by the electrons along their path is

assumed to be equal to the mean loss given by the loss function, L(E),

from Rohrlich and Carlson (1954).

The angular deflection resulting from elastic collisions has been

evaluated by two separate methods in BSM. One approach employed the

multiple scattering distribution of Goudsmit and Saunderson (1940)

applied to the screened Rutherford cross section given in BSM. The


other approach involved a sampling of each elastic collision. Appli-
cation of the BSM technique to a constant density medium and no
magnetic field gave good agreement with laboratory experiments (Grun,

1957; and Cohn and Caledonia, 1970).

In this study, a MC method was needed that could be applied to IEEs.
The basic equation of transfer is solved with the use of the MC approach.
This equation can be rewritten as

dU(P,z,E,E d
cYTTE dz

= -n(z)U(u,z,E,Eo)

E+AEEl as
+ n(z)

f pe(u',p,E',E)U(u',z,E',E )du'dE'

o +1
+ n(z) J f PION1i(u',vE',E)U(i',zE',E )dp'dE'
i 2E+Ii -1

+ n(z)

paj (p',v,E',E)U(v',z,E',E )dp'dE'

No external fields

U(v,z,E,E ) is the
that there is only

aT(E) is the total

are included here and a steady state is assumed. The
"yield spectra" (in eVl sec-1 sr-1) and it is assumed

one neutral scattering species. In this equation
cross section (elastic + inelastic)for the species,

AEElas = 2E(1 cose) electron


is the energy loss during an elastic collision, p (v',p, E',E) is the
probability during an elastic collision with a neutral specie that an
electron with energy E' and direction p' will result in an electron of



energy E and direction i, PIONi(p',,, E',E) is the probability during an
ionization collision with a neutral species that an incident electron

with energy E' and direction p' will result in a secondary electron of

energy E and direction p, and p .a(',p,E',E) is the probability during

an inelastic collision (excitation or ionization) with a neutral specie

that an incident electron with energy E' and direction p' will result in

the incident electron being scattered into direction u with energy E.

Some techniques from each of the other three MC methods were in-

cluded in this work. Some new approximations and assumptions were made,

however, to enhance the accuracy of the computations as well as simplify

some of the calculations. These assumptions are discussed in detail in

Chapter IV.

In this MC work the information is stored in a collision by collision

manner on a magnetic tape. Once all the case histories are generated,

then, the tape is scanned and any correlations of interest may be deter-

mined. The choice of altitude and energy intervals is specified only

during the scanning of the tape. This method allows for greater flexi-

bility in minimizing the statistical uncertainties of the results,

while, at the same time retaining good spatial or energy resolution

(Porter and Green, 1975).

All the degradation methods mentioned in this chapter require cross

sections as input. The cross sections used in this MC work are, there-

fore, discussed in the next chapter.



In this chapter differential and total cross sections for electron

impact on N2 will be discussed. Section III.A reviews the elastic cross

sections of N2 and discusses three models for representation of these

properties. In section III.B the inelastic cross sections of N2 are

presented with their relationship to theory and experiment. Section
III.C, then, concludes this chapter with a discussion of the total

(elastic plus inelastic) cross section for N2. Any energy degradation

technique requires knowledge of these cross sections for a complete

evaluation of the energy loss by electrons in a gas.

A. Elastic Differential and Total Cross Sections for N2

One of the most common differential elastic cross section forms is
the screened Rutherford cross section which can be expressed in the form

do 224
do 22 Z2 e 2] Krel(e) (3.1)
p v (1 cose + 2n)
where Krel(e) is the spin-relativistic correction factor.

The familiar Rutherford cross section unscreenedd case) which can be

derived from classical scattering theory is given by

2 4
do Z e4
ce2 (3.2)
d 2 v2(1 cose)2




sin2 1 cose

Here, an electron is elastically scattered by a nucleus of charge Z

using the Coulomb potential

V(r) = Ze2 (3.3)

with r being the distance between the two particles.

Treating scattering in a quantum mechanical approach with the use

of the Born approximation and a potential of the form

V(r) Ze -r (3.4)
V(r) r

where y is a positive but small quantity approaching 0, Eq. (3.2) can

again be derived. The Born approximation, using the potential in Eq.

(3.4), is only valid in certain angle and energy regimes (Mott and

Massey, 1965, pp. 24 and 466). At sufficiently high angles and low

energies, the Born approximation fails. The range of validity varies

for different substances and for N2 the Born approximation is probably

not accurate at all angles for energies less than 100 eV and at large

angles for energies less than 500 eV.

Equation (3.2) does, however, go to infinity when e approaches 0.

This differential cross section also leads to an infinite value in the

total elastic cross section. Both of these results are unreasonable for

elastic scattering of electrons by atoms and molecules. The most popu-

lar way of correcting this unreal behavior is to add a screening param-

eter n. Equation (3.1) portrays this resulting form.


Equation (3.1) has a maximum at e = 0 and a minimum at a = 1800.

At energies below 200 eV, experimental results indicate a minimum in the

elastic differential cross sections at about 900 with a strong forward

scattering peak at e = 0 and a secondary backward scattering peak at

e = 1800.

In Figure 3.1 experimental data for energies at E = 30 and 70 eV are

presented. These data are taken from Shyn, Stolarski, and Carignan (1972)

with the small circles denoting 30 eV points and the crosses denoting

the 70 eV data.

Later on in this section the screened Rutherford cross section and

another analytic model of the differential elastic cross section are

compared with experimental data. Before discussing the differential

cross section in more detail, first, consider the total elastic cross


Several experiments have been performed deriving the total elastic

cross sections for N2. There have also been several theoretical studies

on the N2 elastic total cross sections. Two recent reviews of the data

available on this subject are Kieffer (1971) and Wedde (1976).

A plot of all this data would obscure the analytic total cross

sections specifically considered in this work. Consequently, only data

from Sawada, Ganas, and Green (1974) (theoretical), Shyn, Stolarski, and

Carignan (1972) (experimental), and Herrmann,Jost, and Kessler (1976)

(experimental) are plotted in Figure 3.2. The sets of data overlap to

a degree such that the disagreement in absolute magnitude of the total

cross sections is readily apparent.

In view of this disagreement, no experimental or theoretical data

areassumed to be absolutely correct and some average of this data is








a I


o X
oxx -


0 30 60 90 120 150
8 (Degrees)


Figure 3.1 N2 experimental electron impact elastic cross section data
from Shyn, Stolarski, and Carignan (1972). o's denote data
from E = 30 eV and the x's denote data from E = 70 eV.


o< 0
- 10


- -

I I I "I

10 r- r- 0 *.-

d Q
EC0 .oU

r- *r- r- 4

1 0 Ln (") C
4- S. A

4t) -Mc

C0 CD ),-

U O In (I1
Ur-- -
1-- 03
cU 4.o n

.) V) U)- LL
to 0 Q. C

*r- (fl (n

0r r0 'a
- *-' O u

0- **- *cI-

4 ON ca 4.
r- 0o

U 4) .0
u0 *c?

(0 C r

*r- .- *

ro V C c
5- C *0
U r- n-W
a, -s-s .-
-- M < au
CM 10 ** 10 U*- *r-
Z CD O Z- i-



iZ n (r








(V !) (3)
0 '0



desirable. An analytic function representing the total elastic cross
section is most easily used in a computer program. Consider now an
analytic form derived from the differential screened Rutherford cross
Knowledge of the differential cross section implies knowledge of the

total elastic cross section as they are simply related by
2w r
o(E) = f sineded (3.5)
0 0

where 0 is the azimuthal angle. Substituting Eq. (3.1) into Eq. (3.5),
the total elastic cross section, OR(E), resulting from the screened
Rutherford cross section is very simply given as

Z2 51.8
R(E) E2 n(1 + n) (3.6)

-16 2
If E is given in eV then OR(E) is in units of 101 cm The screening

S 1.70 x 105 Z2/3 (37)
n c T(r + 2)

according to the Moliere (1947, 1948) theory. Berger, Seltzer, and
Maeda (1970) assumed that n was a constant value and decided on nc = 1
as its best value. The T in Eq. (3.7) is the electron energy in units
of the electron rest energy so that T = E/mc2. In the energy regime of
interest (E s 5 KeV), T << 2, and Z = 7. Noting these observations,
Eq. (3.7) can be rewritten as n % 1
The total cross section from Eq. (3.6) is plotted in Figure 3.2
as the dash-dot line. A noticeable difference is evident between this
model and the experimental values at practically all energies.


Using the form

q F[1 (W)O8
o(E) = F[- ( C (3.8)
Ec W

implemented first by Green and Barth (1965), where q = 651.3 A2 eV2,
the total elastic cross section for N2 was characterized fairly well in
the range from 30 to 1000 eV using the parameters a = 1, 8 = 0.6,
c = 0.64, F = 0.43, and W = 2.66. The E"-064 dependence of Eq. (3.8)

at the larger energies is similar to that seen by Wedde and Strand (1974)
for N2. This form does not represent the data as well below 30 eV and,
in fact, is not defined below an energy of 2.66 eV.
Porter and Jump (1978) have used a more complex total elastic cross
section form which is written as

{ ^E
c(E) = T
n(n + 1)[V+ + E2+ ]

F2G2 F G2
+ 1G12 2+ 22 (3.9)
(E- E1) + G (E E + G

Here, n = U

and for N2: T = 2.5 x 10-6 cm2 F1 = 7.33

U = 1.95 x 10-3 eV El = 2.47 eV
V = 150 eV G2 = 24.3 eV
X = -0.770 F2 = 2.71

G1 = 0.544 eV E2 = 15.5 eV

In the large energy limit, the total cross section falls off as 1/E,
similar to the screened Rutherford cross section. This form does con-
tain two other terms (the second and third terms) which were introduced


phenomenalogically to describe the low energy shape and Feshbach reso-


If either Eq. (3.8) or (3.9) is used as the total elastic cross

section, the differential elastic cross section must be normalized such

2ir i
f f P(e,E) sinededo = 1 (3.10)
0 0

where P(e,E) is the phase function and the differential cross section can
be written as

= o(E) P(e,E) (3.11)

With this in mind consider now three separate phase function forms.

The first phase function form is very similar to the screened Rutherford
cross section and it is written here as

PMl(e,E) = -1 1 2(3.12)
2n[(2 + a(E)) a(E) l][ cose + a(E)]

This is known as model 1. The parameter "a" acts in a similar manner to

the "2n" term in the denominator of the screened Rutherford cross sec-

tion form and is written

E a2
a(E) = al 1 eV

The second phase function form (model 2) includes the forward
scattering term of Eq. (3.12) along with a backscattering term and is
given as


PM2(,E) = -f(E)
S2r[(2 + a(E))-1 a(E)-l][ chose + a(E)]2

(1 f(E)) (3.13)
27[(2 + c(E))-1 c(E)-1][1 + cose + c(E)]2
(E/fl 2
f(E) = (E/fl)
(E/f) 2+ f3

a(E) = a l(-iv)


c(E) = cl[1 --) ]

Irvine (1965) was one of the first researchers in scattering prob-
lems to use a phase function containing forward and backward scattering
terms. He applied a sum of two Henyey-Greenstein functions to the prob-
lem of photon scattering by large particles. Porter and Jump (1978)
also have used a sum of two terms (one for forward scatter and one for
backward scatter). They fitted experimental data at several separate
energies with their form. Use of their differential cross section form
in a deposition calculation probably would require the use of spline
functions or other interpolative techniques.
The third phase function (model 3) is now considered. At small
angles the differential cross section shows a near exponential-like fall
off. This behavior has been pointed out by several experimenters (see,
for example, Shyn, Carignan, and Stolarski, 1972; and Herrmann, Jost,
and Kessler, 1976). It was this experimental observation that led to


model 3 which is written as

fl(E)[l b2(E)]e-B/b(E)
M3(,E) = 2w b2(E)[ + e-"/b(E)

2w[(2 + a)- a-1][1 cose + a]2

[1 fl(E) -
S2[(2 + c(E))-1 c(E)

(E/f l)12
fl(E) =f 21p
[(E/fl1) + f13]

(E/f21) 22
f2(E) = f2
[(E/f21) 22+ f23]

f2(E) = 1 fl(E)

f(E) = f2(E)[1 fl(E)]

-1 ][l + cose + c(E)]2

for E > 200 eV

for E < 200 eV

b(E) = b (E )b2

Cc(E) = (
c(E) = c [1 (C2

The parameters used for the rest of this study in Eq. (3.14) are

fl = 100 eV

fl2 = 0.84
fl3 = 1.92

a = 0.11

bI = 0.43
b2 = -0.29




f21 = 10 eV cI = 1.27

f22 = 0.51 c2 = 12 eV

f23 = 0.87 c3 = 0.27

This form is more complex than the other phase function models but

it does describe the experimental differential cross section data the

most realistically. It includes an exponential term for the near ex-

ponential-like forward scattering as well as a backward scattering term

for electron energies below 200 eV.

Comparisons of the screened Rutherford and model 3 cross sections

are given in Figures 3.3 and 3.4 at the two energies of 30 eV and 1000 eV.

Both forms are normalized to the total elastic cross section form of

Eq. (3.9). This modified screened Rutherford cross section vastly under-

estimates the forward scattering from e = 00 to 30, overestimates the

scattering in the range from 6 = 300 to 120, and underestimates the

scattering at the larger angles with e = 1200 to 1800. Model 3 does a

fairly reasonable job of representing the differential cross section data

at both of these representative energies and the other energies as well.

Although there is not a large amount of energy loss during an

elastic collision, there is some. Using classical considerations (see

Green and Wyatt, 1965), the energy loss is approximately given by Eq.

(2.15). For molecular nitrogen and 6 = 900, the energy loss is about

8 x 10-5 E.

The MC approach, being a stochastic process, uses the concept of

probability for scattering within a given angle interval. In order to

compare phase functions, the probability for backscatter may be compared.

Figure 3.3
(a and b)

N2 electron impact elastic differential cross sections.
The screened Rutherford (dashed line) and the model 3
(solid line) are compared with the experimental data of
Shyn et al. (1972), x, and Herrman et al. (1976), o, at
the energies of 30 eV (Figure 3.3a) and 1000 eV (Figure



%.- I

i1 1 1 I I I I 1 I a I I I
0 30 60 90 120 150 180
e (Degrees)

Figure 3.3a




b|C \\


10 4 I I I I I I I 1 1 1 1 1 I
0 30 60 90 120 150 180
9 (Degrees)

Figure 3.3b

Figure 3.4
(a and b)

N2 electron impact elastic differential cross sections
between 0 and 200. The screened Rutherford (dashed
line) and the model 3 (solid line) are compared with the
experimental data of Shyn et al. (1972), x (the a's
denote extrapolated points), and Herrmann et al. (1976),
o, at the energies of 30 eV (Figure 3.3a) and 1000 eV
(Figure 3.3b).




0 4 8 12 16 20
8 (Degrees)

Figure 3.4a





8 12
8 (Degrees)

Figure 3.4b


This probability, PB(E), is simply calculated with

PB(E) = 2n ~ (3.15)
f d sinededo

In Figure 3.5, PB(E) from the screened Rutherford and model 3 are

compared with other theoretical (Wedde and Strand, 1974) and experi-

mental (Shyn et al., 1972) values. Model 3 does have a tendency to

estimate less backscatter than the screened Rutherford at the larger

energies. (The PB(E) curves for model 3 and the screened Rutherford do

tend to converge at 5 KeV however.) The dominant exponential-like for-

ward scattering is the reason behind this behavior. The discontinuity

observed at 200 eV in model 3 values results from the lack of the back-

scatter characteristic above this energy.
The elastic scattering collisions influence mostly the direction of
travel of the electrons. There is some energy loss during an elastic

collision (as pointed out above), but this loss is not important for
electrons with energies above 2 eV colliding only with N2 particles.
Inelastic collisions, on the other hand, result in a fairly sub-

stantial energy loss with some scattering. Consider now the differential
and total cross sections for these inelastic events.

B. Inelastic Differential and Total Cross Sections for N2

Inelastic collisions are divided into two types: 1) electron ex-
citation and 2) electron ionization. In the excitation process the

electron is excited to a higher state which may either be an optically


10 o x

0 \

2 3
g10 100

m -

102 103
E (eV)

Figure 3.5 Backscatter probabilities for the screened Rutherford
(dashed line) and the model 3 (solid line) are compared
with Wedde and-Strand (1974), x; and Shyn et al. (1972), o.


allowed or an optically forbidden transition. This transition for many

molecules leads to a repulsive state which can dissociate the molecule.

In N2, dissociation of the molecule in this manner is virtually non-

existant because N2 is a very stable homonuclear molecule in which both

the singlet and triplet states are found to be strongly bound. As a

consequence of this, the main process for dissociation is through pre-

dissociation of stable electronic terms by repulsive states that are

themselves strongly optically forbidden in direct excitation.

Porter, Jackman, and Green (1976) (hereafter called PJG) compiled

branching ratios for dissociation of N2 using several experimental and

theoretical papers (see, for example, Winters, 1966; Rapp, Englander-

Golden, and Briglia, 1965; Polak, Slovetskii, and Sokolov, 1972; and

Mumma and Zipf, 1971). In PJG the efficiencies for the production of

atomic nitrogen from proton impact were determined.

This study does not include a calculation of the atomic nitrogen
production; however, the PJG branching ratios with the yield spectra,

discussed in section VII.B, can be applied to calculate this atomic

yield. The excitation events, not resulting in the dissociation of the

N2 molecule, are either electronic or vibrational transitions. Cross

sections for these transitions are taken from both PJG and Jackman,

Garvey, and Green (hereafter called JGG) (1977b).

In the ionization event an electron is stripped from the molecule

and given some kinetic energy. The ionization cross section is a sub-

stantial portion of the total inelastic cross section above 50 eV

(compare Figures 3.6 and 5.1) and the total amount of energy loss is

always > the lowest ionization threshold (which is 15.58 eV for N2).

Subsequently, most of the energy loss of the electrons (for energies







Figure 3.6

Figure 3.6


N2 electron impact cross sections. The total inelastic,
Eq. (3.16) (solid line), total elastic, Eq. (3.9) (dashed
line), total inelastic plus elastic, Eq. (3.16) plus
Eq. (3.9) (dash-dot line), and the experimental inelastic
plus elastic values (Blaauw et al., 1977), x, are pre-
sented here.


> 50 eV) is from the ionization collisions. These ionization cross

sections were also taken from PJG and JGG. The total inelastic cross

section found by summing these inelastic cross sections was fit with

the function
TI) qF[1 W (aB a ,4EC
q0F[l (T) in(t + e)
TI(E) = WEw (3.16)

This form has the characteristic Born-Bethe In E/E fall off behavior at

the large energies. The parameters a = 1, 8 = 4.81, C = 0.36, F = 4.52,

and W = 11 were found with the use of a nonlinear least square fitting

program which fit Eq. (3.16) to the sum of all the inelastic cross

sections. From 30 eV up to 5 KeV this form was used for the total

inelastic cross section.

Below 30 eV much structure in the total inelastic cross section is

evident. At these low energies, the total inelastic cross section can

be read numerically into the MC program. This total inelastic cross

section is illustrated by the solid line in Figure 3.6.

Consider now the scattering of the two electrons involved in an

electron impact ionization collision. In reality, only the incident

electron is scattered. The other electron is simply stripped from the

molecule and given kinetic energy in a certain direction of travel.

Experiments are unable to distinguish between the incident electron and

the electron stripped from the molecule. In this paper, the ionization

event is assumed to cause scattering of both electrons. The scattering

angle of either is then measured with respect to the incident electron's


After the collision event the electron with the higher energy is

designated the primary electron and the electron with the lower energy


is called the secondary electron. There should be a correlation between

the primary and secondary scattering, but this mutual influence is dif-

ficult to quantify. The impinging electron ionizes a many body par-

ticle, a molecule of nitrogen, thus momentum and energy can be conserved

without all the momentum and energy shared by the two resulting elec-

trons. Only recently has work been done on triply differential cross

sections for N2 and this interaction was measured only at one energy

E = 100 eV (see Jung, Schubert, Paul, and Ehrhardt, 1975). More ex-

perimental and theoretical work needs to be done in this area before

any generalization can be made concerning the correlation between the

primary and secondary scattering.

The primary and secondary scattering will thus be treated sepa-

rately in this work. In dealing with the scattering of the primary

electron after an ionization collision, a differential ionization cross

section form based on the Massey-Mohr-Bethe surface of hydrogen, is

used. The form (with a the Bohr radius, and Re, the Rydberg energy) is
do oe )/2 F(x) (3.17)
dTdn Wx E (

where x = (Ka )2 is the momentum transfer, W is the energy loss in the

collision process which is equal to the ionization potential, I, plus

the kinetic energy of the secondary, T, and F(x) is a complex function

given in PJG.

Equation (3.17) is very highly forward peaked for small energy

losses but becomes less forward peaked as the energy loss becomes sig-

nificant when compared with the incident energy, E.

The secondary electron is also scattered in the ionization event.

Probably the most comprehensive work that exists on secondary doubly


differential cross sections is that of Opal, Beaty, and Peterson (1972).
(More recent data by DuBois and Rudd (1978) agrees with their work.)
This data indicates a preferred angle range in the scattering process
(usually between 450 and 900) at all primary and secondary energies.
A Breit-Wigner form has been chosen to represent the data. Here,

do S(E,T)C2 (3.18)
dTdn [C2 + B(cose cose )2]Nf

B(E) = 0.0448 + (72o e)

C() = 36.6 eV
C(T) = (T + 183 eV )

o6(E) = 0.873 + ,(E)
0 (T + eB(E))

OA(E) = 20 eV + 0.042 E

OB(E) = 28 eV + 0.066 E

-2-C -1 (1 + cose ) -1 (1 cose )
Nf = -_ _tan ------ ] tan-l [
cC 0

S(E,T) =d = A(E)r2(E)/[(T T(E))2 r2(E)] (3.19)

is from Green and Sawada (1972) with

A(E) 5.30 ln
SE n1.74 eV

To(E) = 4.71 eV 1000 eV)2
o (E + 31.2 eV)


r(E) = 13.8 eV E/(E + 15.6 eV)

a = 1 x 10 -16 cm2

Equation (3.18) may seem highly complicated; however, integration

over the solid angle is given very simply as Eq. (3.19) which is the
singly differential ionization cross section. The total ionization cross
section is then

N(E) dT (3.20)

T = (E I)
M 2
so that

OION(E) = A{tan"[(TM To)/r] + tan- (Tor)} (3.21)

The fit to Opal, Beaty, and Peterson's (1972) data is given in Figure 3.7

at several primary and secondary energies. The x's represent the ex-
perimental data and the solid line represents the analytic expression,

Eq. (3.18).
Other inelastic processes include the simple excitation events.

The scattering of the incident electron due to the excitation of a par-
ticular state has been studied by Silverman and Lassettre (1965), and

more recently by Cartwright, Chutjian, Trajmar, and Williams (1977) and
Chutjian, Cartwright, and Trajmar (1977).
In order to account for this scattering, the Silverman and Lassettre
(1965) generalized oscillator strength data for the 12.85 eV peak (cor-
responding to the optically allowed b 1,u state)were fit with the use of
a phase function similar to model 1. The very sharply forward scattering



10 -



90 180



0 90 18

0 (Degrees)

0 90 180

Figure 3.7

N2 experimental ionization doubly differential cross sections
from Opal, Beaty, and Peterson (1972) are represented by x's.
The solid line (-) denotes the cross section resulting from
the use of Eq. (3.18). The incident electron energy is de-
noted by E (eV) and the secondary electron energy is denoted
by T (eV).


peak indicated in these data was used in a MC calculation. The electron

scattering which results using this inelastic scattering approximation

in a computation was so small as to be virtually undetectable.

Cartwright et al. (1977) and Chutjian et al. (1977) have studied a

more comprehensive list of states and have observed significant electron

scattering (especially due to the optically forbidden states) in the

range from 10 eV to 60 eV. Characterizing this data in some way appears

to be a rather endless task.

Dealing with this type of inelastic scattering is thus still a

problem. Above 100 eV the optically allowed excitations are the most

important; thus it is safe to conclude that the inelastic scattering

events will not affect the energy degradation process. Below 100 eV,

as a first approximation, it is assumed in this work that the inelastic

excitation events scatter as much as the elastic events. This is

probably a reasonable approximation to the very complex inelastic ex-

citation scattering. In section VI.B the effects of this assumption

are discussed.

C. Total Cross Section (Elastic Plus Inelastic)

Elastic and inelastic processes have been considered in sections

III.A and III.B. Another aspect of the cross sections is the total

(elastic plus inelastic) cross section.

Blaauw, de Heer, Wagenaar, and Barends (1977) have recently pub-

lished experimental data on the total cross section values of N2. These

experimental values are compared with the cross section values from this

work in Figure 3.6.


Throughout the energy range the cross sections used in this study

compare favorably with those of Blaauw et al. (1977). For an easy

reference, the total inelastic and total elastic cross sections are

also given in Figure 3.6 as separate curves.

All the major influences on the IEE energy loss and scattering have

been accounted for in this chapter. The next chapter presents the MC

energy deposition scheme which employs these cross sections.



A Monte Carlo calculation is used here for energy degradation by

energetic electrons in N2. This stochastic process is probably the most

accurate method for obtaining the energy loss of particles in a medium.

The generalizations about electron impact on N2 that are made through the

use of this technique can be applied in other energy deposition approaches

to electrons impinging on the atmosphere. (This is true even though the

magnetic field is neglected in these MC calculations. The three dimen-

sional yield spectrum U(E,z,E ) [see Chapter VII] is most useful for

applications to the atmosphere and changes in the magnetic field will

not affect the yield spectra greatly below altitudes of about 300 km

where the gas density is fairly high [see Berger, Seltzer, and Maeda,

1970 and 1974].)

Building on the MC work done in this paper, more exact models of

auroral electrons and photoelectrons can be established. In the first

section, IV.A, a brief discussion of the MC calculation outlines the

general procedure involved in the degradation process. The computer

program and machinery used are briefly described in section IV.B. Sec-

tion IV.C relates in detail the various aspects of the calculation.

Finally, section IV.D discusses the statistical error that arises from

the use of the MC calculation.



A. Brief Discussion of the Monte Carlo Calculation

In Figure 4.1 a short version of the MC calculation is presented.

Briefly, each electron is degraded in a collision by collision manner

down to 30 eV. Below 30 eV the electrons are degraded with the use of

a multiple scattering distribution. This multiple scattering approach

characterizes the resultant coordinates of the electron which goes

through several elastic collisions between each inelastic collision.

The incident electron has an energy E To begin with, the running

total of the electron energy, E, is set equal to E At the position

START, this energy E is checked against a cutoff energy, E If the E

is more than Ec then, first the distance traveled by the electron to

the collision is calculated.

Second, the type of collision which occurs is determined. If a

collision is elastic then the electron is scattered with the use of a

phase function, the appropriate energy AEElas is lost, and the electron

goes back up to the START of the degradation process. Whether collision is

inelastic it is determined if the collision is an ioniz tion event or an

excitation event. In the excitation process, scattering occurs if the

energy E is less than 100 eV, E is reduced by the threshold, W, for

excitation of this state, and the electron goes back up to the START of

the degradation process.

Ionization collisions are the most complex occurrences to compute.

The energy loss, W, by the incident electron is equal to the kinetic

energy, T, of the secondary electron produced plus the ionization thresh-

old, I. The primary electron is then scattered and reduced in energy by

W. If the secondary electron has a kinetic energy greater than E then,

Figure 4.1 Flowchart of the Monte Carlo degradation of an incident
electron of energy Eo.



Calculation Complete
Electrons Thermalize

Calculate Path Length
and Coordinates of
Next Inelastic
Collision with the


it is scattered and sent back to the START to be degraded further. In

the meantime, the primary electron's properties are stored.

If a secondary produces a tertiary electron with a kinetic energy

greater than E then that tertiary is completely degraded before any

further degradation of the secondary is considered. Like the primary,

the secondary electron's properties are stored in the meantime. No other

generations were included in this study as their contribution would be,

at most, a couple of tenths of a percent of the incident electron's


After the tertiary is entirely degraded below Ec then the secondary

is again sent back to the START to be degraded further. The secondary

is next entirely degraded below E and, finally, the primary is again

sent back to the START to be further degraded. This whole process may

then again repeat itself.

B. Computer Programs and Machinery Used in the
Monte Carlo Calculation

In the previous section a brief discussion was given of the electron

energy degradation process. A brief discussion will be given below about

the MC computer codes and the computing machinery used. The MC computer

program employed in this work evolved from an original MC code written

by R.T. Brinkmann (see applications in Brinkmann and Trajmar, 1970). This

program was revised for use in Heaps and Green (1974), Kutcher and Green

(1976), and Riewe and Green (1978). The author has further modified this

MC technique for energetic electron impact into N2 to be used in the

energy range from 2 eV to 5 KeV.


This MC technique was applied to several incident electron energies.

The vast majority of the MC program runs used the Amdahl 470/175 computer

at the Northeast Regional Data Center at the University of Florida.

There were, however, several MC runs using the PDP 11/34 of the Aeronomy

group of the University of Florida.

It should be noted here that running the same program on both

machines at the same energy, Eo = 1 KeV, showed a factor of 240 dif-

ference in the execution time. Thus a program that takes four hours on

the PDP 11/34 will take one minute on the Amdahl 470/175. This time

advantage plus the ability to store each collision of the electrons on

magnetic tape does make the Amdahl 470/175 a more desirable "number-

crunching" machine. The PDP 11/34 is only able to produce intensity plots

in the longitudinal direction. This mini-computer is thus mainly useful

in deriving a range (to be described in the next chapter).

Two programs were used in deriving the MC results. The first pro-

gram (listed in Appendix A), the modified version of Brinkmann's code,

degraded the electrons in energy from their initial E down to the Ec

and recorded each collision and its properties on the tape. The second

program (listed in Appendix B) coalesces the data from the tape into an

array of ordered output. This output contains information for three

dimensional intensity plots, energy loss plots, and yield spectra.

C. Detailed Discussion of the Monte Carlo Electron
Energy Degradation Technique

Now, a more detailed discussion is given for the MC method of

degrading an electron's energy. An electron will start off with an

energy of Eo and coordinates x y z e and o The symbols x, y,


and z are the Cartesian coordinates of the electron. The polar angle e

is measured with respect to the z-axis and the 0 denotes the azimuthal

angle measured with respect to the x-axis (see Figure 4.2). In this

approach, the initial coordinates xo, y, z e and 0o were all set

equal to zero. The coordinates xb, yb' zb' b', and Ob of the electron

before starting on its journey to a collision are, therefore, initially

established as xb = x yb = Yo, zb = zo' eb = eo, and Ob = o'
The MC approach relies on the random number, R, between 0.0 and 1.0

to aid in the deposition calculation. For each collision several R's

are needed and for each R a new property of the collision is gained. In
order to explain this MC approach, an accounting of the random numbers

and their subsequent usefulness is now made. The multiple elastic scat-
tering distribution used below 30 eV and the lowest energy cutoff 2 eV

are also described.

1. First Random Number, R1

The first random number, R1, is used to find the path, PT, traveled

by the electron before it collides with a molecule of N2. Calculation

of PT proceeds in the following manner. The mean free path, A, is
defined as

= (4.1)

where n is the density of N2 molecules in #1cm3 and aT(E) is the total

(inelastic plus elastic) cross section of N2 in units of cm2 at an energy
E. The densities used at the various initial input energies are given
in Table 4.1.

(Xb, Yb, Zb)

(Xo, Y, Z


Figure 4.2

Schematic representation of the coordinates and directions
of motion of the electron in its travel between collisions
with the N2 molecules.



The energy E is
density n, used
second column.

E (KeV)

presented in the first column with the number
in the MC calculation, being given in the
(8.0 E+ 14 means 8.0 x 1014)

n (#/cm3)

0.1 8.0 E+14

0.3 2.0 E+15

1.0 8.2 E+15

2.0 2.8 E+16

5.0 1.2 E+17

Table 4.1


All electrons are forced to be degraded in a 30 cm long cylinder;
thus an increase in the density is required for an increase in the energy.
There are 10 cm allowed in the negative z direction and 20 cm allowed in
the positive z direction. The x and y axes extend to infinity. Some
electrons actually escape from the cylinder, but the energy lost due to
these electrons is only a few tenths of a per cent of the incident elec-
tron energy. The path length PT is then given as

PT = -x In(Rl) (4.2)

using the relation that

R1 = e T (4.3)

Figure 4.2 represents a schematic of the electron traveling and
colliding with three N2 molecules. The PTl' PT2' and PT3 are the path
lengths traveled by the electron between the initial coordinates and
the first collision, the first and second collisions, and the second and
third collisions, respectively.
The xa, y,, and za coordinates at this collision can now be found
from PT, xb, Yb' Zb, eb' and %b using

Xa = Xb + PT sineb cosCb (4.4)

ya = Yb + PT sineb sintb (4.5)

Za = Zb + PT coseb (4.6)

In Figure 4.2 the coordinates of the first and second collisions are
represented to illustrate how the electron's direction of motion might
change during its collisions with N2. So far emphasis has been only on


the Cartesian coordinates. Now, calculate the azimuthal angle *a and the

polar angle ea of the electron after a collision.

2. Second and Third Random Numbers, R2 and R3

In actuality the type of collision must be specified before the

scattering can be calculated. It is assumed, however, that the type of

collision is already known (see subsection IV.C.4). The second, R2, and

third, R3, random numbers are not chosen if the collision is an excita-

tion event and E is greater than 100 eV. They are chosen for all other


The R2 is used to calculate the azimuthal scattering angle, 0', of

the electron from its direction of motion. The premise is that the

azimuthal scattering is isotropic; therefore,

= R2 2i (4.7)

(Note that the 0' angle is the only angle not represented in Figure 4.2.

Inclusion of 0' adds too much complication to an already cluttered


The third random number, R3, is employed to calculate the polar

scattering angle e' of the electron from its direction of motion. (The

angle e' is represented twice in Figure 4.2: Once as the scattering due

to the first collision and once as the scattering due to the second


For elastic collisions, Eq. (3.1), (3.12), (3.13), or (3.14) are

used in determining o'. In all but one of these phase functions, an

analytic expression can be used to determine e' from the random number,

R3. These analytic expressions are given below.


Using the screened Rutherford differential cross section form (see

Eq. (3.1)), it follows that

e' = cos-1 [1 + 2n 2n(+n) ] (4.8)

For model 1 (see Eq. (3.12))

= cos-1 [ -1- + 1 + a] (4.9)
R3[(2 + a) a ] + a

and for model 2 (see Eq. (3.13))

S= cos-1 [-B B2 4AC (4.10)


A=R3 + f (1 f)
3 a[(2 + a)-l a"1] (2 + c)[(2 + c)- c

f (1 f)
B =-A(a -c) + + (2+c f)
[(2 + a)-1 a-1] [(2 + c)-1 -


C =-A(l +a)(l +c) + f(l +c) (l 1 -+
[(2 + a)"- a ] [(2 + c)- c ]

Model 3 (Eq. (3.14)) is not so easy to write in such a convenient
form. The equation for primary scattering after an ionization event

(Eq. (3.17.)) is, also, not easily inverted.
For these two differential cross sections, the following approach
is taken. The angular range from 00 to 1800 is divided up into angular
intervals. A certain probability for scattering at angles less than the
end of each angle interval is calculated from the differential cross


section form. The angle e' is then found through the correct placement

of R3 into an angular segment whose beginning and ending point scattering
probabilities bracket R3.
For this work twenty-four angular segments were chosen. Their end-

points are given in Table 4.2. With twenty-four angular intervals, the
results from the Monte Carlo calculation came out to be the same as with
the use of forty angular intervals. If sixteen or even twenty segments
were used, the MC computation gave results that were 5% to 10% different.
The 0' and e' are not the scattering angles from the original

coordinate system, but represent the azimuthal and polar scattering of
the scattered electron from the direction of travel of the incident elec-
tron. In order to calculate 0a and ea, the azimuthal and polar angles
representing the motion of the electron after the collision, some spheri-
cal trigonometry must be used. The following relations hold in this

cOSAa = [coseb cosb sine' cos'

sin b sine' sine' + sineb cos^b cose']/sinea (4.11)

+ cosb sine' sin(' + sineb sineb cose']/sinea (4.12)

;' = cos- (cosia) (4.13)

cosea = coseb cose' sineb sine' coso' (4.14)

sinea = l cos ea (4.15)
a a


Twenty-four angle intervals are given here that were used in
the Monte Carlo calculation. First column lists the index of
the segment and the second and third columns give the begin-
ning and end points for each segment with units of radians

Index Beginning End





Table 4.2


ea = cos-1(cose ) (4.16)

Now the azimuthal angle *a and the polar angle ea have been established
for the collision with respect to the fixed coordinate system. These
angles are also represented in Figure 4.2. The two angular coordinates

Ob and eb of the electron before traveling to the next c61ision are then
set as Ob = Oa and eb = ea.

3. Fourth Random Number, R4

A fourth random number, R4, is required if a secondary is produced
and if that secondary has an energy above the cutoff energy, E This
R4 is chosen to determine the polar angle, e', of scattering of the
secondary. Again, an analytic formula can be employed to define e'.
This equation was derived from Eq. (3.18) and is written as

1 C -1 -v-(1 +cose )
e' = cos [- tan [R4{tan ( C

-1 ((1- cose )
tan- C}

-1 (l coseo)
+ tan ( C )] + chose ] (4.17)

The 0' for the secondary is found with the use of Eq. (4.7) and ea
and 0a result from the use of Eqs. (4.11) through (4.16).

4. Fifth Random Number, R5

The fifth random number, R5, determines the type of collision that
occurs. Here, the type may be either elastic or inelastic. If the


type is inelastic then the individual excitation or ionization event is

found as well.

There are cross sections for thirty-four states of N2 employing the

papers of Jackman, Garvey, and Green (1977) and Porter, Jackman, and

Green (1976). Using all these states in the MC calculation would greatly

increase the cost. It was therefore decided to reduce these thirty-four
1 1 +
states to nine states. Two allowed states, the b 1 and the b' 1 ,

and the six ionization states were kept the same as given in the papers.

For the ninth state, all the Rydberg and forbidden states were combined.

Above 200 eV, the forbidden states are contributing only a minuscule

amount to the total cross section. Since the other states have roughly

the same In E/E fall-off at high energies, it is assumed that the pro-

babilities for excitation to any of these states will be constant. These

probabilities were simply found from the ratio of the cross section of

the state in question to the total inelastic cross section at the elec-

tron energy of 5 KeV.

In Table 4.3 these states, their probabilities, and thresholds are

presented. The probability, pc, of the composite state is simply
P,= Pi (4.18)

where m = the total number of Rydberg and forbidden states and pi is the

probability for excitation of the ith Rydberg or forbidden state. The

average threshold, W for exciting the composite state is found easily

with the following equation

Wc = Pi-- (4.19)


Table 4.3 N2 inelastic states, their probabilities, p, and thresholds,
W, taken for electron energies above 200 eV are presented

State p W (eV)

N2 bl u 0.092 12.80

N b'I 0.042 14.00

N2 Composite 0.233 15.40

N2 X2 0.289 15.58
2 g

N2 A2ru 0.127 16.73

N+ B2 + 0.066 18.75
2 u

N2 D2w 0.044 22.00

N+ C2 + 0.044 23.60
N2 40 eV 0.063 40.00
N 40 eV 0.063 40.00


with Wi being the threshold of the Rydberg or forbidden
Below 200 eV, the probabilities for excitation to the various
inelastic states are changing quite rapidly. The parameters for the
eight individual states are taken from Jackman et al. (1977b) and
Porter et al. (1976). The composite state's properties are found in

the same manner that they were above. In these lower energy regimes
the probability and energy loss are changing fairly rapidly, thus Table
4.4 illustrates these probabilities and threshold values at several
With the background on the inelastic cross sections and their
subsequent probabilities, consider now the collision type. The R5
random number determines the type of collision that occurs in the
following manner: If

Rg aT(E) for all electron energies (4.20)

where cTE(E) is the total elastic cross section, then the collision is
elastic. If

OTE(E) plaTI(E) + OTE(E)
aT(E) Rs a (E) T and E > 200 eV (4.21)

where aTI(E) is the total inelastic cross section and p, is the proba-
bility for exciting the first inelastic state (in Table 4.2 the first
state is the b lu thus p, = 0.092), then the inelastic collision results
in the excitation of the first state.
A relation follows from Eq. (4.21) that holds true for j = 2 to 9
such that: If


Table 4.4 N2 inelastic composite
ability, p, and average
energies between 2 and

state with its characteristic proba-
energy loss, W, given for several
200 eV.

E (eV) p W (eV)





SpiTi(E) + TE(E) J PiaTI(E) +TE(E)
i-R < R =1 and E > 200 eV (4.22)
oT(E) 5 aT(E)

then the inelastic collision results in the excitation of the jth state.
Thus the R5 random number for an electron of energy E > 200 eV will
determine which type of collision occurred when satisfying Eq. (4.20),
(4.21), or (4.22).
For energies below 200 eV, the following relations must be con-
sidered: If

aTE(E) ol(E) + aTE(E)
T- < R (5 aT(E) and E f 200 eV (4.23)

where ol(E) is the cross section for exciting the first inelastic state,
then the inelastic collision results in the excitation of this state.
A relation similar to Eq. (4.22) can now be established for j = 2
to 8 such that: If

J 0i(E)+oTE(E) aii(E)+o(TEE)
i=1 < R5 < -i=E) and E < 200 eV (4.24)
aT(E) 5 aT(E)

then the jth inelastic state is excited. If
I oi(E) + aTE(E)
Rg > T(E) and E < 200 eV (4.25)

then the excitation of the composite state is assumed and the energy loss,
Wc, in this case is found through a linear interpolation with the use of
the values given in Table 4.3.


5. Sixth Random Number, R6

The sixth random number, R6, is computed only if the collision type
is an ionization event. This R6 determines the energy lost by the
primary in creating a secondary of energy, Ts. Using the S(E,T) from
Eq. (3.19) the following relationship is established:
I S(E,T) dT
R6 0 (4.26)
6 oIoN(E)

Integrating the numerator in Eq. (4.26) and using Eq. (3.21) to solve
for Ts, Eq. (4.27) is derived.

Ts = r(E)[tan{R6tan-[(TM To(E))/r(E)]

+ (R6 1)tan-1[To(E)/r(E)]}] + To(E) (4.27)

The energy loss, W, is then found by the relation:

W = Ik + Ts (4.28)

where Ik is the ionization threshold for the kth ionization state.

6. Multiple Elastic Scattering Distribution Used Below 30 eV

The MC calculation can be used to degrade an electron down to
practically any energy. Even below the lowest threshold for excitation
to any vibrational level, the electron will still lose energy via elastic
collisions with molecules of nitrogen as well as other electrons. This
energy loss to other electrons is fairly low unless a substantial frac-
tion of the gas has been ionized (see Cravens, Victor, and Dalgarno,


1975). In this study the fraction of ionization is assumed to be

negligible; therefore, this loss is ignored.

Unless there is a very large amount of money available for computer

time, an electron can not be followed to its thermal energy with any

practicality. This implies that a multiple elastic scattering distri-

bution (hereafter referred to as MESD) must be used below some given

energy. In this work the MESD will be used below 30 eV.

Bethe, Rose, and Smith (1938) used the Fokker-Planck differential

equation, neglecting energy loss, to consider the penetration of elec-

trons through thick plates. This, however, leads to a Gaussian solution

in the small-angle approximation so that the tail of the angular dis-

tribution was omitted. The large-angle multiple scattering has been

studied by Goudsmit and Saunderson (1940) [hereafter referred to as GS]

who used a series of Legendre polynomials to determine the resultant

angle of scattering.

Lewis (1950) studied the integro-differential diffusion equation of

the multiple scattering problem in an infinite, homogeneous medium,

without the usual small-angle approximation. He obtained the GS solu-

tion for the scattering angle and also derived certain moments for the

longitudinal and transverse distributions.

Berger (1963) applied a MESD for condensed case history MC cal-

culations. His application of the MESD is at the energies above 200 eV

and probably is not accurate for electrons with energies less than about

100 eV. Furthermore, Berger's (1963) work contains approximations that

are only good for the sharply forward peaked cross sections of higher

energy electrons.


In this work a different problem exists. The MC calculation is

used to degrade electrons in a collision by collision manner all the way

down to 30 eV. At this energy, the elastic collisions are occurring

with twice the frequency of the inelastic events, and at energies below

30 eV the number of elastic collisions between inelastic events may be

up to several hundred or thousand. Keeping track of all these elastic

collisions would be very costly.

Kutcher and Green (1976) [hereafter referred to as KG] studied the

radial, longitudinal, and polar angle distributions for elastic scatter-

ing by H2 in the energy range from 2 to 50 eV. An approach similar to

KG's could be applied to N2. Since such a project would require a

substantial amount of time and computer money, the possibility of adapt-

ing the KG results was first considered.

With this in mind, consider the differences between N2 and H2.

First of all, there are some dissimilarities between the differential

cross sections. There is more backscatter observed experimentally in

N2 at all energies. Secondly, the total inelastic and elastic cross

sections are different. The second difference is no real problem because

the MESDs are given in terms of the mean free path lengths (hereafter

referred to as MFPs). The first dissimilarity does pose a minor problem

which is solved in a simplistic way below.

Above 5 or 6 MFPs the polar angle is approximately random. At most

energies below 30 eV, the number of MFPs between inelastic collisions

is above 5 or 6. Since the distribution found in KG is not easily in-

verted, a reasonable assumption is that the polar angle is oriented



Knowledge of the radial distribution is not crucial for our pur-
poses. The most interesting radial distribution output from this MC
calculation is that of the 3914 A emission. Electrons below 30 eV make
little contribution to this profile because the cross section for ex-
citation to this N2 B2 state is fairly low (see Figure 5.1). Thus
knowledge of the radial distribution of these electrons multiply scat-
tered is not extremely important.
An approximation, however, is employed in most MC computations to
calculate a fairly reasonable radial distance. The average radial
distance, as observed from the calculations in KG, for most energies and

at the longer path lengths is approximately one-sixth of the total path
lengths, thus

Pave = s/6 (4.29)

The most important spatial displacement is the longitudinal dis-
tance z. In order to calculate z, the total path length s must be known.
This length s is calculated from the random number, R1, the total elastic
cross section, oTE(E), and the total inelastic cross section, oTI(E),
by using


The ratio oTE(E)/oTI(E) is simply a fairly accurate approximation
of the number of elastic collisions occurring per inelastic collision.
The value -In(R1) [see Eq. (4.2)] is the path length (in units of MFPs)

traveled by the electron between collisions. Thus knowing the number
of elastic collisions occurring and the path length traveled between
collisions allows one to write Eq. (4.30) as the expression for the


total path length s (in units of MFPs) traveled between inelastic
rn KG an equation which can be easily inverted to calculate the

z distance (in units of MFPs) from some random number, R2, and path
length s, is written

[R2/ 1]

z = ln [F(O)/v 1 (4.31)

v(s) = 1 exp[-(s/sV)D]

F(0) = K{1 exp[-(s/s f)075]}


u(s) = (H + s )/s

where K = 0.425.
Since there is more backscatter during N2 elastic collisions
(because of its differential backscatter contribution), it seems reason-
able that the parameters for Eq. (4.31), which are useful for N2, are
different than those derived in KG. One approach to this dilemma might
be to correlate the elastic differential cross section (hereafter called
EDCS) from N2 at some energy E' with the EDCS from H2 at some energy E.
This would work if the H2 EDCSs showed more backscatter than the N2
EDCSs; however, the opposite is observed experimentally. Thus the N2
EDCSs from some E' (around 6-7 eV) values correlate with the H2 EDCSs
at E values less than 2 eV (where the Kutcher and Green, 1976, MESD is
not defined).


Another straightforward and simplistic approach is to do the

following. Calculate the approximate backscatter at three energies,

the two endpoints and the middle (2 eV, 15 eV, and 30 eV), from the KG

H2 EDCS form and the experimental data on N2 EDCSs (given in Sawada,

Ganas, and Green, 1974). At these energies the backscatter with the KG

H2 EDCS form is less than that of the N2 EDCS by the following values:

2 eV t 5%, 15 eV 10%, and 30 eV % 10%. An average of these three

values is about 8%. Since the major influences of the backscatter in

Eq. (4.31) is the value of K, this parameter is the only one that is

changed from the KG formulation. It is, therefore, increased by "8% so

that in these MC calculations K = 0.46. The other parameters in Eq.

(4.31) are listed in Table 4.5.

Actually it appears that the value of K makes little difference in

the MC computational results. Two MC calculations at an incident elec-

tron energy of 100 eV with K = 0.46 and with K = 0.425 were undertaken

(all other parameters and inputs were the same). The yield spectra

(described in Chapters II and VII) changes substantially only at fairly

large longitudinal distances (where the distances are about 1.5 times

the range). At these large distances there are relatively few electrons

anyway, thus there is little effect on the major aspects of the spatial

electron energy deposition process.

The Cartesian coordinates xa, Ya, and za are found from the coor-
dinates xb, Yb, and zb in the following manner. After z is calculated

in units of MFPs with the use of Eq. (4.31), it can then be written in

units of cm or km by multiplying by the MFP, x (calculated from Eq.

(4.1)), thus za = zb + zX.


Table 4.5

Parameters from Kutcher and Green (1976) for several energy
intervals used in Eq. (4.28).

(eV) H I J D s5 SF

2-5 12. 1.37 1.71 1.75 5.05 8.5

5-10 9.6 1.32 1.67 2.50 4.25 8.5

10-20 15.5 1.28 1.67 2.31 6.29 10.3

20-30 23.5 1.24 1.69 1.98 9.65 13.6


As established earlier, the polar angle ea and azimuthal angle a'

representing the motion of the electron after the collision, can be

chosen in a random way from the two random numbers, R3 and R4, using

a = rR3

Ca = 2rR4 (4.32)

A reasonable approximation of xa and ya can then be made using

Eqs. (4.29) and (4.32) such that

Xa = Xb + ave A cos a


a = Yb + Pave x sin a

In the MESD the fifth random number, R5, is used to determine the

inelastic collision type. A method similar to that illustrated in sub-

section IV.C.4 is employed, the only difference is the fact that the

collision is only inelastic.

7. Value of the Cutoff Energy, 2 eV

The Ec used in this work has been set at 2 eV because the lowest

threshold for excitation to an inelastic state is 1.85 eV. With this

cutoff energy the yield spectra can be defined down to 2 eV at all

longitudinal distances. Subsequently, a reasonable calculation of the

excitation to any N2 state may be made.


D. Statistical Error in the Monte Carlo Calculation

The statistical error inherent in the MC computation can be derived

by considering the following. Since the MC calculation is a probabilis-
tic method of degrading an electron in energy, the multinomial distribu-
tion can be used to find the statistical standard deviation for each bin

considered. This discussion of the statistical error employed the work
of Eadie, Dryard, James, Roos, and Sadoulet (1971).
The probability of getting an excitation of a certain state j in bin

k is Pjk. The pjk is normalized such that
m n
I Pjk = 1 (4.33)
k=l j=l

In this MC study the multinomial distribution is an array of histograms
containing N events distributed in n states and m bins with rjk events
in state j and bin k. The rjk values are normalized such that
m n
I I rjk = N (4.34)
k=l j=l

Thus, the rjk observations can be considered somewhat conditional
on the fixed observational value of N. The variance of the calculation
is represented as

V(rjk) = N Pjk (1 Pjk) (4.35)

In this work the m x n variables rjk can all be correlated. For the
specific example of electron deposition represented in Figure 5.2,

Pjk << 1. This is true because there are total almost 5 x 105 col-
lisions (i.e., N = 5 x 105) to consider in this degradation scheme and


at maximum rjk t 4000. Using this information, Eq. (4.35) can then be

approximated by

V(rjk) n N Pjk rjk (4.36)

and the statistical standard deviation of the number of N B2 E events
2 u
in a bin becomes

a O WF (4.37)
jk jk

Equation (4.37) holds true for the specific example represented in

Figure 5.2 and it also holds true for all the intensity plots, energy

loss plots, and yield spectra that were studied in this work. Thus,

in order to obtain the approximate standard deviation for any MC generated
number, the square root of this value is its standard deviation. The

error bars found in the rest of this paper are determined in this

Now that the MC calculational technique has been outlined, this

method will be used in the next three chapters to deal with the spatial

and energetic aspects of electron energy degradation.



Incident electrons with energies between 0.1 and 5.0 KeV are de-

graded in N2 using the MC method described in Chapter IV with the cross
sections given in Chapter III. The intensity plots of the 3914 A

emission are described in this chapter.

Emission intensity plots of the 3914 A radiation from the N2 B Z

state are used in describing the range (found by extrapolating the linear

portion of the longitudinal 3914 A intensity plot to the abscissa) for

incident electrons. Section V.A describes the excitation of the

N2 B2 E state. In section V.B the range of the electrons is defined

more completely. Previous experimental and theoretical work on the

3914 A emission of N2 is given in section V.C. The range results from

the MC calculation are then discussed in section V.D. Finally, section

V.E describes the intensity plots resulting when plotted as functions of

the radial direction.

A. Excitation of the N2B 2E State

The main concern of this chapter will be the intensity plots showing

the emission of the 0-0 first negative band (B 2 state) of N2 at 3914 A.

Experimentally (see Rapp and Englander-Golden, 1965; McConkey, Woolsey,

and Burns, 1967; and Borst and Zipf, 1969), it has been shown that the

number of photons at 3914 A produced for each ionization of N2 is



independent of the energy of the exciting electron for energies from

30 eV at least up to 3 KeV.

In Figure 5.1 the N2 total ionization cross section and cross sec-

tion for ionization and excitation to the B 2 state of N are presented.

The curves are approximately parallel thus even if the absolute values

for the two cross sections are slightly in error, the shapes of the in-

tensity plots that result from this MC calculation should be fairly


The total ionization curve lies nicely in the middle of an array of

experiments (namely, Opal, Beaty, and Peterson, 1972; Tate and Smith,

1932; Rapp and Englander-Golden, 1965; and Schram, de Heer, Wiel, and

Kistenaker, 1965) but the B 2 cross section values may be high when
compared to experiments (see Holland, 1967; and McConkey, Woolsey, and

Burns, 1967).

The threshold for excitation to this B 2 + state is 18.75 eV, thus
any electron above that energy can excite and ionize a ground state N2

molecule up to this level. The cross section for excitation and ioniza-

tion to the B 2+ state is not large when compared with the total in-
elastic cross section. In fact, the probability for exciting this state

is only 0.066 for electron energies above 200 eV. The accuracy of the

MC calculation is dependent on the number of excitations in each bin

(see section IV.D). In order to enhance the precision of the MC results,

excitations of the X and A states of N are added to the B
g u 2 u
excitations. The ionization cross sections for these two states are

found to be proportional to the B 2 E state for electron energies above
30 eV.
30 eV.


b ci'

IO .

Figure 5.1



Total loss function L(E) from N2, denoted by the solid line;
total ionization cross sections for NZ, denoted by the dash-
dot line; and the N B zy cross section, denoted by the
dashed line, are given as functions of energy, E.


10 >
0 )







Previous workers (Barrett and Hays, 1976; Cohn and Caledonia, 1970;
and Grun, 1957) have used the 3914 A emission as a measure of the energy

deposited. In these works it is assumed that since the 3914 A radiation

is proportional to the number of ionizations in a given volume and if

the number of ionizations is proportional to the energy deposited in

that volume, then the 3914 A intensity is proportional to the energy

deposited in that volume. These experimenters, therefore, measured the

3914 A radiation at several energies, extrapolated their intensity plots

to find a range (to be described in section V.B), and derived an empiri-

cal expression for the range that could be used to find the energy loss


This idea of using the 3914 A emission to derive the energy loss

scheme is useful for energies above 2 KeV. In Figure 5.1, compare the

loss function, L(E), used in this work and the N2 B 2u state cross


The two curves are not parallel below 2 KeV. This implies that the

energy loss function can not be derived directly from the range results

at incident energies below 2 KeV. The energy loss plots from this MC

study are given in section VII.A and more will be discussed in that

section about them.

B. Range of Electrons

The concept of the mean range must be defined next. For each

monoenergetic primary electron impinging into a gas, a range can be

calculated. In general (at least above 100 eV), the higher the electron

energy the further the electron will penetrate into the medium. If an

electron is incident along the z-axis, the excitations of the N2 B 2E


state can be graphed in an intensity plot with the z-axis as the

In Figure 5.2, the intensity plot from 5000 incident 1 KeV electrons

is graphed (the model used in this MC calculation should only be taken
as an illustrative example) in histogram form. Bins along the z-axis
are taken to be 0.5 cm in width for these incident electrons. The linear
portion of the curve may be extrapolated, as illustrated by the dashed
line, to define a mean range of the beam.
All the intensity plots are normalized in this paper so that the

beam starts out at z = 0 cm along the z-axis. The intensity in Figure

5.2 seen at negative values of z is brought about by backscattered
electrons. The error bars given near the peak of the histogram are
found simply from a method described in section IV.D.
From Figure 5.2, the range is seen to be 16 cm for these 1 KeV

electrons. Range values, R in units of gm/cm2 are written

Rg = Rc (5.1)

where R is the range in cm, p = n MN2 (in gm/cm 3), n is the number
density of N2 molecules (in #/cm ), and MN is the weight (in gms) of an

N2 molecule. In this case, n = 8.2 x 101 molecules of N2/cm MN2 =
4.651 x 10"23 gm/N2 molecule, and R = 16 cm; therefore, Rg = 6.06 x 106

C. Previous Experimental and Theoretical Work on the
3914 A Emission of N

GrUn (1957) measured for air the total luminosity of the 3914 A

radiation in planes perpendicular to the axis of the electron beam with

C 0
- *
o 5- E E

0 1MID U3
C 0) S- -

U U)VIcn

4-a) (n 0)
*r- 0 r U
, *r- *r- a
>3 > 0C

r- -in r

0 0 VI
to *r W a
C -Q C

0r- *r- *r- 0 "
0cn L0 a)

,- u- S-
oC C)
4) M10 ***r
*r-3* C

Cr- 4-) (n
4( 0) C4)C -
a) 0 *r- 0

CCO 4-;
*Dr- S- a
c C) C
S0 0>
C r- j= (

) 0 t

Cl r- C

V i Z CQ









IC)x Sol) Nuuau 0'I


an initial energy of 5 to 54 KeV. Cohn and Caledonia (1970) measured

intensity profiles of electron beams with incident energies from 2 to

5 KeV impacting into N2. Barrett and Hays (1976) then extended the

incident electron range down to 300 eV by measuring the radiation pro-
files of 3914 A resulting from electron beams with energies from 0.3 to

5.0 KeV impinging on N2.

Spencer (1959) used the Spencer and Fano (1954) method of spatial

energy deposition and found good agreement between his energy loss plots

and the 3914 A intensity plots of Grin (1957). The Berger, Seltzer, and

Maeda (1974) [BSM] MC calculation provided energy loss plots down to

2 KeV. These plots are also in fairly good agreement with the experi-

ments mentioned above.

Comparisons will be made in this paper between the available experi-

mental electron energy loss data and the MC calculations done here.

Since this MC calculation follows the incident electrons, as well as its

secondaries and tertiaries down to 2 eV, this MC computation is one of

the most detailed ever employed for electron impact energy degradation.

It is, therefore, of interest to compare the results from this study

with experimental results for incident electrons with energies from

300 eV up to 5 KeV.

D. Range Results and Longitudinal Intensity Plots
from the Monte Carlo Calculation

Range data at several incident electron energies are calculated

with the use of the screened Rutherford and the model 3 differential

elastic cross sections. The screened Rutherford model is used because

it is the most widely used form for elastic scattering in theoretical


studies and, also, because BSM were quite successful in using this form

down to incident energies of 2 KeV. Model 3 was used because of its

very close agreement with experimental differential cross section data

in the range from 30 eV up to 1 KeV.

Table 5.1 presents the range data (for perpendicularly incident

electrons) from three different experiments, the theoretical calculation

by BSM, and two sets of theoretical computations from this study. The

values in parentheses from BH (Barrett and Hays, 1976), CC (Cohn and

Caledonia, 1970), and G (GrUn, 1957) are simply calculated from the

empirical formulae given in these works.

For the rest of this chapter, the results of this work will be com-

pared with those of BH. This is the most recent experimental study and

is probably the most reliable experimental work. Theyalso use the same

incident electron energy regime as that employed in this work. In

Table 5.1 it is apparent that the BH values have the largest ranges of

the experimental studies.

The two separate MC calculations in this study seem to bracket the

BH results at all energies. The model 3 ranges are consistently larger

than those of BH. These results are 10% higher at 5 KeV and about 19%

higher at 0.3 KeV. The screened Rutherford ranges, on the other hand,

are about 9% lower at 5 KeV and about 10% lower at 0.3 KeV.

If it can be assumed that the BH results are indeed the most re-

liable data, then the following conclusion can be made: The screened

Rutherford phase function scatters the electron too much while the

model 3 phase function provides too little scattering. This conclusion

is made assuming that the total cross sections described in Chapter III

are fairly accurate.


Table 5.1 Range data (in 10-6 gm/cm2) at several energies, E (in KeV),
are given below. The second column M3 (model 3), third
column SR (screened Rutherford), fourth column BH (Barrett
and Hays, 1976), fifth column CC (Cohn and Caledonia, 1970),
sixth column G (GrUn, 1957), and seventh column BSM (Berger,
Seltzer, and Maeda, 1974) range values are presented.


0.1 0.37 0.34 (0.53) (0.07) (0.08) --
0.3 1.25 0.95 1.06 (0.51) (0.56) --
1.0 6.45 5.57 5.72 (4.17) (4.57) --
2.0 18.6 16.8 17.7 14.0 (15.4) 15.2
5.0 91.5 75.9 83.0 69.7 76.4 71.9


In this work model 3 is the result of a careful investigation of the

detailed molecular nitrogen cross sections. Therefore no attempt will

be made here to change the cross sections compiled in Chapter III. Model

3 will be used in most of the MC calculations in the rest of this

chapter and also in Chapter VII (BSM have, however, chosen n used in

the screened Rutherford cross section, to be a constant value whose value

was selected so as to obtain the best agreement between their MC calcula-

tion and the experimental results of G and CC).

In Table 5.1 the importance of the elastic phase functions is clearly

illustrated. Up to a 25% change in the range is observed when com-

paring the screened Rutherford with the model 3 phase functions. More

elaboration on the effects of various phase functions on the energy

deposition process will be given in Chapter VI.

Figures 5.3 and 5.4 give intensity plots for the 3914 A radiation

resulting from 2 KeV and 0.3 KeV incident electrons, respectively. The

experimental work of BH and the calculations using model 3 and the

screened Rutherford are presented in these figures. The shapes appear

to be somewhat similar; however, the BH results at both energies pre-

dict a range that is between the two theoretical calculations.

E. Intensity Plots in the Radial Direction

Most attention, so far in this study, has been concentrated on the

intensity plots in the longitudinal direction. There is experimental

data available on the intensity of the 3914 A radiation as a function

of p (the axis perpendicular to z). Experimentally, G, CC, and BH all

present data of this type.


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This study uses the experimental data of BH as a comparison with

the results of this study. The next three graphs, Figures 5.5, 5.6,

and 5.7,portray sample results for incident electrons with energies

5.0, 1.0, and 0.3 KeV, respectively. The z and p values given in these

three figures are in units of fractions of the total range.

Fairly good agreement between the MC calculation (using model 3

cross sections) and the experimental work of BH and Barrett (1975) is

observed at all three incident energies. The largest differences be-

tween the two sets of data are noted at 0.3 and 1.0 KeV.

For the 1.0 KeV case, the MC calculation tends to predict more in-

tensity at the lower values of p for z values of 0.3 and 0.4. A similar

result is apparent for the z values of 0.36, 0.48, and 0.60 for an

energy of 0.3 KeV. At a z value of 0.12, however, the experimental data

tend to predict more intensity at all values of p.

Two conclusions can be drawn from these comparisons, if it is

assumed that the experimental data of BH and Barrett (1975) are correct.

First, the cross section for excitation to the N2 B may be under-

estimated in the energy regime between 0.3 and 1.0 KeV. Raising this

cross section in this energy regime could bring about an increase in the

intensity observed early in the electron's degradation process with a

subsequent decrease in intensity later in the electron's degradation

process. Second, more scattering from the elastic collisions would

help to reduce the total intensity at low p values and raise it at the

higher p values. The screened Rutherford differential cross section has

more scattering than model 3. Use of this set of cross sections in the

MC calculation did result in a little better agreement at 1.0 KeV, but

only about the same type of agreement at 0.3 KeV.


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