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This is to certiig that the

thesis entitled

A HOLLON CATHODE DISCHARGE SOURCE

FOR SPECTRA 01" IONIZED MOLECULES

presented In]

Paul Tak-‘shing Chan

has been accepted towards fulfillment

of the requirements for

#3,— degree in Main:—

llate Mar—8,4952—

(BIKW

Major professur

 

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\

A HOLLOW CATHODE DISCEJRGE SCUnCE FOR
L1A OF IOLIZED LO EOULES

Paul Tak-saing Chen

A THESIS
Submitted to the School of Gre 'duete Studies of thAif”n
State College of Agriculture and.Applied Science
in partial fulfillment of the requirements
for the degree of

.ADr-A OF SCIE 1303

Department of Pn"sics

952

Grateful ecknonledgem*nt is made to

Dr. C. Duane Eause for the sugresticn of

1&4

this problem and his invaluable guidance.

@w 3. 6052‘“.

. ‘ “-‘f}sl\({;
~., ¢. 1,. f.

TAZLE

I. Introduction
II. Hollow Cathode Source

A.
B.
C.
D.

III. Ionized

A.

B.

C.

Introductory

f10\’j1'1fif$rl1
v ‘i.l~_‘.i. S

fiechanism of the Discharge-

General Design

Gas and Power Supplies

Spectrum of K0

Introductory

Electronic Configurat'
Allowed Energy r

‘ . +
Tne Unknown Spectrum of HO

IV. Exper'nental

A.

h

D.

Optical System

Results

V. Conclusion

18
18

“4
.g

I. Introduction

Since the turn of the last centry, investigation of
molecular spectra has become more and more important. For
from it, one can understand more about the structure of the

q
\

molecules, electron cictribution, nature of the binding of
the molecules, etc.

A part of the above problem is the develOpment of
light sources which rive controlled excitation and emit rad—

~~

iation suitable for hijh resolution work.

Among the several light sources deviSed which are
useful in the field of hiJh resolution spectroscOpy and cap-
able of controlled excitation, the hollow cathode discharge
tube is the simplest in design and most efficient in Operat-
ion.

The majority of hollow cathode sources are desir

‘u—r

ned

to excite the solid materials w.ith which the cathode is im-
pregnated. A variation in design is introduced here, such
that a permanent gas can be excited. A rather intensive study
of the molecular radiations emitted by the source was made
with small quantities of NO (nitric oxide) admitted to the

syste

._
*3
I

II. Hollow Cathode Source

A. Introductory
The hollow cathode source is essentially a modified

Geissler tube -— a source in which n electrical di we iarge is

maintained in a gas at low pl essure. It is g1n«ple in desiun

and ea sy to 03erate. It was first develOped by Paschen.l

5

He found that if the cathode was ma.e hollow, as the discharge

cot

pressure was reduced, the ne ive glow retreated into the

cathode cavity. When a pressure such that the mean free path
of the electron is apjrox m1ately the same as the diameter of

the hollow cathode was attained, the negative glow became

most intense. However, due to the high temperature, the DOp-

Q P!
~, c k) 0
pler ooardening ’

of the spectral lines became very serious.

4 , , h . ,
Schuler‘ redesigned tne tube by removing the ccwt ooe to tne
exterior where d; astic cooling could be emoloyed As a result,

this defect is almost completely eliminated. Hence, Irom this

source, sharp and intense spec r.l lines can be obtained.

3. hechanism of the Discharge
In the hollow cathode source, an inert pas, such as

J
helium, argon, etc., is passed throu'n th tube at a reduced
pressure. When a potential is applied between the anode and
cathode, a tyoical gas eous discllarge take splace. As the
Operational conditions of the 3a pressure and the aoolied

voltage are adjusted, the positive glow dwis=p:>ears and the

negative glow recedes inside the cathode and becomes very in—

tense. The positive ions of the gas, say helium, formed in
the discharge are urged by the field toward the cathode and
Sputter any material there. Since a gaseous sample is intend-
ed for investigation, a special desi;n and suitable construct-
ional Inaterial for the cathode are necessarV. This material
must have he miniznum or the lea st sputterring fac
he spectrum for investigation will not be complicated by it.
Aluminum is suitable for this purpose.
Due to the mechanism of the discharge, large numbers

e

r!"

of the helium atoms are excited to a metastable energ sta

93
:3
Q:
O
O
(n
m
(D
m
m
H
U‘
0
\7

ev (argon, 11.5 ev). Upon collisions with
the sample Las molecules, the energy of the metastable helium
atoms is transformed into the pctential energy of excitation
for the sample gas molecules. Hence the characteristic emis-
sion spectrum of the latter is ooserved wien the excited mole-
cules return to lower energy states. Since the metastable

helium atoms lose their energy by re -dia onless transitions,

the helium sgieCt; um, althouLh observed, is very weak.

n

C. General Design0

Fig. 1 shows the cross section of a hollow cathode

source for exciting the gaseous samgvle The main body of the
tube is made of brass. It consis of two sections which are

insulated by a rubber gasket on the edye and a pyrex tubing
inside. They are held together by the enternal atmosoheric
pressure. One section iicoioom es the anode and tne otlier

the cathode. They are m de of hard aluminum. The position

 

 

 

 

 

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7

 

“hon NM» Q 1 Kong

2

{h

(31
l

of the anode is adjustable such that th .1.
the discharge can be utilized. The cathode
.Hence its design ca n oe easily altered. Fig

of a cathode for exciting solids. The catnod

a cavity where water is circulated. A quart

rubber we 1:ets at one end allows the redia

An inlet for the carrier the ; seous

L,

n
C.

in front of the node, outlet is

all

0
‘...J

In this way

and have the maxium cpgortunity

POWEP Sup

subject is

4.

and the

so that

and NO can be the hollow cathode

valves with controlable devices are necess

+-
U

A cylinder of helium a

:3ly of carrier gas for the source. This pre

'to 4 psi by a Purox reduction valve. Again

1",?

zreduced by means of two tapered leci valves,

zand the other at the outlet of the system.

‘Valve consists of a male taper which may be

tion

at

U.

to control
carrier

desirable mi

1,700 psi furni

intensity of

is removeable.
. 2 is a design

e is inoedded in

2 window with

to be Observed.

is

SDD_31€ placed

the

gas, helium,

v. .1.

“ture of helium

Hence leak

ary.

shes the sup-

ssure is reduced
this pressure is
one at the inlet
The tapered eak

seated snugly or

Iloosely into a corresponding female taoer as indicated in
Phig. 5. Therefore the flow of gas can be adjusted. An 0-
R143: furnis Eies the seal. A H: nanometer is nlaoed by the
iJllet of the hollow cathode discharge tube to indicate the

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0-.

cuion o: a Iicro-Le

 

3k

r.__._--——-1
N \
H;—
I
0
F13. 5 Cross section of a Tavered Lea.
NO Hg
N00

Valve

pressure in the tube.
The helium gas Hf er release from the cylinder, is

passed through a carbon trap which is suomeryed in a d y ice

2.88.

Cu.

and acetone bath in order to sosorb the inouri
Since only a small portion of E0 is used in the hollow
cathode, a very sensitive type of leek valve is needed in order
to control the flow of NO to a minute amount. From the sug-
gesti-on of John J. hepfield, a gless veriaole micrO— leek

valve is built as indicated in Fir. 4. A capillary tube which

has been drawn to eoJroxin tely the size of a .01? inch plat-
ium Wire, is joined to a 12 mm pyrex tube and then fused into
a 15 mm pyrex tube. The platium wire is put inside the capil-
lary with tungsten wires as leads. A loop in the platium wire
is to prevent the s rain on the glass envelOpe due to the
thermo—exoension of the wires. When a current is passed through
the jlatium wire, the wire is heated and expands, and has a
tendency to fill up the space witldin the co oi lery. Hence,
the flow of H0 is decreased. The range of soolied current

4- ' ". .1.

can be from zero b0 eight amoeres without fear of damarine
the device. That rate flow is deoendent upon the pressures
at the reservoir of NO and at tfleW10ll r cathode, and the

rate of evsc ustion of tne gases. From exver inental results,

the rate i'low is as follows:

 

 

 

Pressure Lt Kgpiied Current Re ce flouw
ieservoir of E0 Hollow Cathode at tne licro- of H
(mm of E3) (mm of E5) Leak Valve (n icron
(angere) oer ec )
$3 4 O l7
l5 5 4 2

 

 

 

 

 

 

The No is supplied by the hatheson Co. in a cvlinde“
A small quantity is fed into a two liter pyrex flesh The
pressure is indicated by a H; manometer. The gas is allowed
to pass through the micro-leak valve and enters the hollow
cathode for excitation.

The power supply for the discharge tune is from s

('4.

2, 500 volt, 1.2 kw filtered full wave rectifier. The circuit

diagram is shown in Fig. 5. A resistance of 2,700 ohms is

 

 

 

 

JK

 

 

 

33E

x...)

Fig. 5 Circuit Die ram of the Power Supply

placed in series with the discharge tube in order to facili-
tate starting the cischer e and also to stcbilize the current
during Operation. For, at the beginning, the resistance of

the discharge tube is very high, hence the ayplied iotential is
entirely across the electrodes. When tne discharge commences,
the resistance of the tune decreases to approxiriately 8 00 ohms.
.As a result, almost three quarter of the applied potential is
Elcross the ballast resistance. Therefore, a smell fluctuation

A

not eifect the constant flow of

W
C).
’5
D
,_
SS
0‘
(D
*3
(Y)
(n
I.)
U)
6
:3
O
(D
‘)J
O
(D
m

current.

The applied potential for the micro— eak valve is
taken from a 12 volt DC voltage supply and is regulated by a
2.8 ohm rheostat.

The schematic diagram of the gas flow is represented
by Fir. 6. The helium gas after gassing through the reduction
valves and the carbon trap, mixes with the H0 at the outlet
of the micro-leak valve and enters to;ether with the KO into
the hollow cathode. From there this mixture passes through
another leak valve, comes to a two-liter pyrex flask, and

‘0

finally is pumped by a Cenco hegavac rotary oil pump into the

J

air. The flask serves to stebil

Ho

ze the pressure of the system.

-10-

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

L-w |lV
h
\
—‘ i \I‘?\ u
4____ tht
Vac“.
I
Zloov

 

 

 

III. Ionized Spectrum of NO

A. Introductory

In order to perdict vhat mby occur when IQ is intro—
{11236313, iTLJLC ”IL-c 13.175)? ..; .53."; C13 3. 23".. .C‘ , . £3 1171011‘10-113 01° {3:19
=lectronic configuration of the molecule and those molecules
which may be formed is desireable.

When the F0 is introduced, the molecules may be Cis—

?‘
C)
'b

\7 + '-
0 forms

sociated or ionized and N9, L. , FO+, 0?, or 09

course these formations are dependent on the enerjies avail-
able in the discharge an the stability of the molecules.

The energy available is closely controlled in the hollow cat-
hode being largely the energy of the metastable states of the
carrier :as. The latter is dependent on the nature of the

k.)

molecules. If the situation is favorable for a particular
molecule, its Spectrum will aooear and oe intens'fied, other-

wise it will be depressed or even absent.

B. Electronic Configurations and Allowed Energy Transitions
TV .0 a. K? + + . Y" o 0
Molecular spectra oi hg, mg , 02, 03 , and so in emis—
sion have been studied quite extensively. The nost prominent
bend -vstems attributed to each of them are as follows:

N :8 The electronic configuration is

I '- 2 “ ‘ ~ 3
The best established electronic levels for nitro en in order

f energy from the ground state are K2". A3: of“, 03“ c,“ 332.
l ' i l -

C)
:Fhe observed band systems are:

First Positive System:9

rt
9
,J
(D
Q
J
1'0
I

Occurrencez. In the positive column of

che r e tubes containin5 nitrogen. The bands
appee r very rend: ly.
Appearance: De r'ded to the violet. Under smell

of waves of re—

0)

dispersion the appearance 1
Qularly spaced trLDle-lze ded ”Wnds stron est
in the orange, the red and the yellow green.
Under large dispersion the rotation structure
is seen to be complex, and there are several
heads to each band.

Range: 10,000 - 5,000 A.

Transition: 33“ ———5 A32,

Excitation potential: 7.4 ev.

Second Positive System:10
Occurrence: In the positive column of dischsrg
tubes containing nitrogen or air and in arcs

at low pressure. The bands sopesr very T88R1-

'_J

y and are of frequent occurrence as an im—

t—k

ur ty.

’0

Appearance: Degre ded to shorter vs ve len5 ths.
Closed triole-lme ded bends formin5f f9 irly
obvious sequences.

Range: 4,900 - 2,800 A.

Transition: (SW ——> at“

Excite tion not ential: 11 ev.

Y + 1 J. o
to : 8 ll Tne eleCtronic configuration is:

I t t I “
(La 0‘3) (U T.) (9463) (um) cap it.) ( akG‘B)

The ground state is 'Zaeumlt;e only ooserved trensi tion is

A‘Z _«.x'2 in diSCZ'f 3 tuoes at very low pressure or at
moderate pressure in presence of excess nelium in hollow cath-
ode. Th strong bands are degraded to the violet, but a few

q

reek bands are de rs dec to the red. The sands entend from

‘

5,L00 to 5,200 A. Tne excitation jotential is 18.74 ev.

NO' The electronic con: i urction is:
(\N)‘Lu¢3(2,m)‘u,w)‘ (aym‘ awn“ ‘1‘? u)
The ground state is L“.
(3 Systemzl2
Occurrence: In discharge tube containing oxy;:en
and nitro en, end in nitroren sf ter-rloxv, and
especially strong wnen excess oxygen is intro-
duced into active nitrogen.

Apjeerance: Double-headed and degraded to the

Ranre: 5,500 - 2,300 A.
I I
Transition: 3 ‘fi. —* X 1‘

5.6 ev.

t1]
1.!
0’1
0
[Jo
C1"
52.)
d"
'_Jo
O
:3
0'
O
c
(D
,3
(”1‘
lJo
[1
i4

Occurrence: In discharge tube containing nitr Q ten
and oxygen, in active nitro
containing 30.

Appearance: Douole douole— needed sen ds and de-
*r'ded to the violet.

Range: 5,500 - 1,600 A.

.1, ' a
Trans1tion: A 2. 4 X 1:

Excitation potential: 5.48 ev.

_ .1 q 0 0
00:8”4 Tne electronic confirLration of 00 is:
(.1 RI
. . ‘ o‘(1¢‘)’(1 “3"(zwf
”’63) CUE.) (Lou-3H1» .3 \° 3 k a P g
. 3 , 1
Tne ground state is Z . Tne neutral xygen molecule does
not readily show an emission spectrum. However, an emission
a 3 , , ,
spectrum of oxygen (BX‘U‘ 1) use Observed oy Scnumann, and

Runge in the region 4,400 -5,100 A.

02+: 8 The electronic configuration is:

c m3): < mm‘ ( yeah to m‘ t 1 P “33‘ (Mg m 3“ < “(“33
The ground state is L“ . There are tw systems of bands at-
tribut F to the ionized oxygen molecules, the First Ke;ative

e
sands15 from the red to the green, and the Second fie“ative

C)

bands10 in the ultra-violet.

c. The Unknown Spectrum of mo+

So far the Spectrum of ionized nitric oxide molecules
has not been observed. A search for this spectrum has been
carried on here. The predictions for tne unknown are based
upon the iso-electronic relationship.

The electronic configuration of the 30+ molecule is:

cu ¢3‘(\..¢)‘Luws‘ (um‘ < am" up?

Since this is similar to that of the neutral nitr03en mole—

J-

Cule, then one would expect that the electronic sta

4‘
U

U)

e and
the band Spectrum should be resemble those of the neutral ni—

trogen molecule.

There are two possioili ties for the formation of the

+ ..
N0 molecules from tne 10 in the lollon cs thoCe discha

’3

J— W

(1) directly from the neutral H0 molecules 0 rougn ionization,
+ + , +

‘u

or (2) by combining I+ + O a . , or n + 0 4 10 , after col-
lisions of second.kind.

(1) How if the former did happen, tnen tne ionization
potential of IO is of importance. From the limit of ?Vberg

Series in band soectra, tne molecule; ionization potential of

k I.)

to 15.581 ev.

The only infoxxlation available for 30 is from electron impact
experimentslaw mi h gives an ionization potential of 0.5 ev.
ince the energy from the metastable helium is 19.? ev, then
there will be 10 to ll ev available to e1 :cite the E0+ mole-

cules.

(2) If the formation of the 30+ molecule is from
collisions of he second ki nd, that means cdfter the dissociat-
ion of K0, the neutral nitro;en atom or the neutral oxvgen
atom one or both excited, collides wi tn tile metastable, ener-
getic helium atom. Dur1nh the collision, tne ex itation ener—

gy of the latter is trensformed into the potential ener y for

tne form er and, as a result, becomes ionized. Sow, if the
ionized nitrogen or oxygen atom combines witn the neutral

xygen or nitrogen atom, resdectively, then tne ioniz d nitri

-J
>

P.
r »,

coxide molecule -orne . althou n this type of formation
seems in;robable, N0 is ooservei in dis cnarge tubes contain-

. . ... ,+ .
Zing kg and 02 as was mentioned and Alh was oota1ned witn

good intensity from

cathode and nelium
9

as an im;urity.l

n

C;.

517.“.

—18-

73'

.1soerimenta1

A

IV.

A. Optical System

soectrum

In order to search for tne

.nitric oxide molecules, various conditions f
cathode discharge are examined first cy case
with a constant deviation spectrometer, and

ing with a littrow spectrograoh vith quartz

Fig. 7 is a ,notograph of the Optica
soectrometer and the littrow spectrograph in

{odak SoectrosCOpic plates III F and

*r‘

,4-
~LJ

o1 es were Tne plates

(‘1

Analysis K0. 1

in D—19 for 4 minutes and fixed in D-VS.

B. Results
Two conditions of the hollow cathode
-investi;ated. One is with excess 50 and the

of the ionized

or the hollow
rvin: visually
then photograph-
optical train.
1 system with t'
position.
Soectrum

T, f: a" 1.
r» O “Ll-J“.

were develooed

discharge were

other with a

 

 

 

 

trace of NO. The operational conditions are as follows:
Operational Condition Excess KO Trace of K0
Operation-current for
discharge BOO ma ZOO ma
Pressure:
N & He 4 mm 5 mm
NO 52 mm 15 mm
Leak-valve current O amp 4 amp
Slit Time exposure
2mm Range 1 12 min. (Fe: 15 so) 25 min. (Fe: 15 sec)
2mm Range 2 50 min. (Fe: 15 sec) 1 hr. (Fe: 5 sec)
5mm Range 5 2 hrs. (Fe:150 sec) 2 hrs. (FezliO sec

 

 

 

 

 

 

-20-

in Fig. 8 I, II, and III with (a) representing the case having
“xcess HO and (c) for the case having a trace of HO. Their
identifications are made by the relative position 0? the Fe

A 4*
Spectrum.

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-24:—

V. Conclusion

The aim of this project is to design and construct

a hollow cathode source which gives controlled excitation for

(.1.

the spectra of ionized molecules. This purpose has been ful—

filled satisfactorily as indicated by the spectra shown in

F1.

r:-

. 8 I, II and III.

In case (a), the spectr m of N2 is prominent, because
with excess NO, the energy of the metastable helium atom is
distributed to either ionization or decomposition of the KO
+

molecules. As a result, NO , N and O are formed. The nitro-

gen atoms combine to form N2, and oxygen atoms, 03.
In case (b), the spectrum of N2+ is prominent, because
with a trace of EO, only a minute amount of N and O is formed.
With excess metastable helium atoms accumula.ing about the
cathode, Ng+ can be readily formed by collisions of the second
k nd.
After a careful searcn through the spectra shown in
Fig. 8 I, II, and III, no evidance for the presence of the
spectrum of HO+ has been found. Of course this does not mean
that there has been no formation of the KO+ molecules in the
discharge. According to the previous discussion (Section III
C, p. 14), the formation of the NO+ molecules are quite pos-
sible. Firstly, the availaole energy is more than enough for
the ionization of KO. Secondly, from the presence 01 0+, it

17111 be logical to believe that tnere are N and H+ also. Thus

-25-

NO+ could be forme , and the abundance of N3 and 39+ can not
apparently be accounted for on the basis of impurities. Hence,
by sayins that “he prominent transitions of no+

we can sum uo J
eme ultra-violet or extreme infra-

are most prooably in the extr

red which cannot be detected with our Shectevesuflh-

-25-

References

Paschen, F. Ann Cer Physih, g2, 901 (1316)
White, H. Introduction to Atomic DSpectra (IcGraw-Hill
1955) p. 419
YerQenau, H. and Natson, J. Rev. Lod. Pnys., §, 22 (1956)
Schuler, H. Zeits fur Pnysih, §§, 335 (1920)
Tolansky, S. High Resolution Soectroscooy (Pitman 1947)
p. 69

Tolansky, S. High Resolution S vectroscooy (Pitman 1347)
1). 50

HOpfield,.J. J. Rev. Sci. Instru., 91, 671 (1950)
Pease, R. X. B. and Gaydon, A. G. The Identification
of Iolecular Spectre (Kiley lOel)
Fowler, A. 5nd Strutt, 3. J. Prec. of Roy“1 Society,
85, 577 (1911)
. .. ll
Coster, D., Brons, H. ano Van der Ziel, A. Zei“s fur
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Ierton, i. R. and Pilley, J. G. Pn i. F:g., g9, ly5 (1995)
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~ ‘I 4- " ~ 1 r—
s~1n-e, R. ZleS Iur Physih, ea ee (10 o)
Lochte-Holtgreven and Disks, 3. H. Ann Physik, 5, 957
(1929)
Boz oky, L. and Schnid, R. Phys. Rev., 48, :65‘(13555.
fiulliken, R. S. and Stevens, D. S. Physical Review,
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Go_ydon, A G. Dissociation Enez gies and Spectra of
Diatonic ficlecules (Wiley 1947) o. 96 ; ‘\
f',‘ r \\
Iagstrun and Tate Phys. Rev., §§, 054 (1941) ‘x.
Alhy, G. I. and “atson, I. C. Phys. Rev., 45, 871 (1954)

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