INFRARED LASER OPTOGALVANIC SPECTROSCOPY OF NH3 AND NO2

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INFRARED LASER OPTOGALVANIC SPECTROSCOPY OF NH3 AND NO2

C. Webster, R. Menzies

To cite this version:

C. Webster, R. Menzies. INFRARED LASER OPTOGALVANIC SPECTROSCOPY OF NH3 AND NO2. Journal de Physique Colloques, 1983, 44 (C7), pp.C7-429-C7-437. �10.1051/jphyscol:1983741�.

�jpa-00223298�

I N F R A R E D L A S E R O P T O G A L V A N I C SPECTROSCOPY O F NH3 AND NO2

C .R. websterrand R.T. Menzies

California I n s t i t u t e o f Z'eehnology, J e t Propulsion Laboratory, 4800 Oak Grove Drive, California 91109, U.S.A.

Resum@ - Les s p e c t r e s optogalvaniques de p o r t i o n de l a bande v de NO2 d 5,2 um e t de l a bande v2 de NH3 d 9,5 um ont e t 6 e n r e g i s t r e e s d ?a l i t n i t e Doppler avec des l a s e r 2 diodes, continus e t accordables. Une nouvelle c e l - l u l e u t i l i s a n t des e l e c t r o d e s amovibles e t une georn6trie orthogonale e n t r e le l a s e r e t l a decharge a' permis d ' i d e n t i f i e r deux c o n t r i b u t i o n s au signal optogalvanique dont l 'une e s t associee uniquement a l a decharge lumineuse negati ve.

A b s t r a c t - I n f r a r e d l a s e r optogalvanic (LOG) s p e c t r a of p o r t i o n s of t h e NO2 v3 band a t 6.2 um and t h e NH3 v;, band a t 9.5 m have been recorded a t Doppler-limi t e d r e s o l u t i o n using cw tunable diode l a s e r s . A novel cel l design, using a d j u s t a b l e e l e c t r o d e p o s i t i o n s and an orthogonal geometry between t h e probe l a s e r and discharge a x i s allows two c o n t r i b u t i o n s t o t h e LOG signal t o be i d e n t i f i e d , one a s s o c i a t e d only with t h e negative glow.

1. Introduction

In t h e study of m01 e c u l a r s p e c i e s using optogal vanic spectroscopy, v i s i b l e l a s e r sources have been s u c c e s s f u l l y used t o record t h e s p e c t r a of neutral and r a d i c a l s p e c i e s i n discharges and flames 11-31. The high s e n s i t i v i t i e s achieved have r e s u l t e d i n p a r t from t h e a v a i l a b i l i t y of r e l a t i v e l y high dye l a s e r o u t p u t powers ( > l 0 0 mW, cw) and from t h e nature of t h e optogalvanic e f f e c t i t s e l f . For molecules i n d i f f e r e n t e l e c t r o n i c s t a t e s , l a r g e d i f f e r e n c e s may be expected i n t h e c r o s s s e c t i o n s f o r several e l ectron-neutral /electron-ion processes c r u c i a l t o t h e discharge operation.

A1 though e a r l y s t u d i e s of t h e optogal vanic e f f e c t within CO2 l a s e r p1 asmas /4/

i n d i c a t e d t h e o b s e r v a b i l i t y of i n f r a r e d molecular optogalvanic s o e c t r a , measur- a b l e changes i n t h e discharge impedance following e x c i t a t i o n i n t h e mid-infrared region were not expected with t h e low output powers a v a i l a b l e ( Q mW) from lead- s a l t tunable diode l a s e r s (TDLs).

We have r e c e n t l y reported t h e extension of molecular optogal vanic spectro- scopy t o t h e i n f r a r e d region /5/ where portions of t h e y band of NH3 and t h e v3 band of NO2 were recorded by probing low-pressure dc glow discharges i n pure ammonia and i n a nitrogen dioxidelhel i u m mixture. In t h i s paper we review t h a t work, p r e s e n t some new s p e c t r a recorded using an improved c e l l , and include new data on t h e s p a t i a l dependence of t h e magnitude, s i g n , and time evolution of t h e observed s i g n a l S.

2. Experimental d e t a i l S

The dc discharge c e l l used i n t h i s work i s shown schematically i n Figure 1.

Cross-connecting 0-ring j o i n t s f i t t e d with Mac a windows a1 l owed t h e TDL t o probe t h e i n t e r e l e c t r o d e region a t 90" t o t h e discharge a x i s . A tungsten rod anode and a molybdenum hollow cathode were each mounted s o t h a t t h e i n t e r e l e c t r o d e s e p a r a t i o n and t h e region of l a s e r e x c i t a t i o n could be e a s i l y adjusted by s l i d i n g 0-ring

%r. Webster was unfortunately unable to come to the Colloquium because of a re- scheduling of an air balloon experiment but has sent the paper he would have pre- sented and which is published in the Proceedings.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983741

C7-430 JOURNAL DE PHYSIQUE

connections. A flow of e i t h e r pure ammonia gas or a 50/50 nitrogen dioxide/helium mixture a t a t o t a l c e l l pressure of -1 t o r r was necessary t o keep the discharge s t a b l e over long periods.

FINE-METERING

wmL

^:,

JLYBDENUM '

m o w CATHODE

11

II II

GAUGE

Figure 1. The dc discharge cell w i t h adjustable electrode positions.

A negative polarity dc voltage supply provided 400-700 V across the c e l l , the anode being a t earth potential. For the two discharges studied, the discharge resistance was measured t o be i n the range 1-2 Mn. W i t h a 30 k n b a l l a s t r e s i s t o r i n s e r i e s with the discharge tube, the c i r c u i t current was limited by the discharge impedance t o currents i n the range 0.2-0.7 mA. A 0.01 LIF coupling capacitor allowed laser-dependent ac changes on the high-voltage side t o be monitored by a preamp connected t o a lock-in amplifier. Usually, the TDL beam was mechanically chopped and the lock-in amplifier referenced t o the chopping frequency. In an a l t e r n a t i v e ac detection scheme, first-harmonic detection was employed by removing the me- chanical chopper and modulating the TDL wavelength using a sine-wave current modulation and referencing the lock-i n amp1 i f i e r t o t h i s modulation frequency.

The lead-salt TDL used f o r NH excitation a t 1046 cm-l produced 0.8 mW output, mainly in a single longitudinal moje, a t a TDL current of 1.7 A. For NO2 excitation a t 1610 cm-l, a second TDL produced 1 mW output power, i n a single longitudinal mode, a t a TDL current of 0.7 A .

3. The LOG spectrum of NO7

In Figure 1 a portion of the NO2 spectrum near 1609.9 cm-' i s shown, re- corded i n both d i r e c t absorption using a HgCdTe detector and using the optogalvanic technique, with t h e TDL beam chopped a t 400 Hz.

The optogalvanic spectrum was recorded by probing the positive column of the discharge, where the measured linewidths are within 20% of the Doppler width a t 300 K (1.3 X 10-3 cm-I HWHM)

.

Figure 2. ( a ) The d i r e c t absorption spectrum and ( b ) the LOG spectrum of NO2 near 1609.9 cm-l. The variation of the 0% absorption l i n e i n the upper trace i s due t o the monochromator transmission and not the TDL output power. Total c e l l pressure i s = 1.2 t o r r .

4. LOG signal polarity and t h e NH3 spectrum

The NH3 spectra obtained near 1046 cm-l f o r c e l l pressures of 0.25 and 0.4 t o r r and discharge current = 0.2 mA are shown i n Figure 3. The l ine-centers may be iden- t i f i e d from Table l. For t h i s spectral scan, the TDL beam i r r a d i a t e d t h e negative glow region of the discharge, producing LOG signal S corresponding t o a decrease in the discharge current.

C7-432 JOURNAL DE PHYSIQUE

Table 1. The l i n e frequencies, t r a n s i t i o n s , and s t r e n g t h s of l i n e s observed i n t h e LOG s p e c t r a of NO2 and NH3. Information i s taken from reference 6 f o r NO2, and reference 7 f o r NH3.

Line Frequency T r a n s i t i o n

Molecule (cm-l )

Linestrength (cm. molecul e - l )

N02 1609.900 235 18 - 225 19 ( - 1 3.4 X I O - ~ ~

( v3 band) 1609.925 625 - 726 ( - 1 5.9 X I O - ~ O

1609.936 fj25 - 726 ( + ) 6.8 X ~ O - ~ O

1609.955 707 - 808 (-1 8.3 X ~ o - ~ O

1609.956 707 - 808 ( + ) 9.4 X ~ o - ~ O

tdH3 1046.3760 43 - 33 ( S ) 1.86 X 10-l'

! v2 band) 1046.3898 42 - ^{( S )} 1.45 X 10-l'

1046 -4026 a1 - 31 ( 5 ) 1.72 X 10-l'

1046.4075 40 - 30 ( 5 ) 3.60 X 10-l'

The design of t h e glow discharge c e l l allowed t h e LOG e f f e c t a t d i f f e r e n t d i s - charge regions t o be observed by changing t h e point of i r r a d i a t i o n of t h e TDL beam, which probed t h e discharge a t 90" t o t h e i n t e r e l e c t r o d e a x i s .

From a d e t a i l e d study of t h e NH3 discharge, several observations were made: ( i ) The LOG spectrum changes signal p o l a r i t y a t t h e negative glow; ( i i ) d i p s a t t h e l i n e - c e n t e r p o s i t i o n of s t r o n g l i n e s a r e seen f o r i r r a d i a t i o n a t t h e negative glow;

( i i i ) No e f f e c t s due t o t h e TDL beam s t r i k i n g e i t h e r cathode o r anode were seen;

( i v ) The phase ( s e t a t t h e lock-in a m p l i f i e r ) of a l l s i g n a l s a t t h e 400 Hz chopping frequency i s t h e same t o within a few degrees; ( v ) The a b s o l u t e signal s i z e of t h e negative glow s i g n a l s was t y p i c a l l y twice t h a t a t o t h e r discharge l o c a t i o n s , where t h e signal S have opposite p01 a r i t y ; ( v i ) A p o s i t i v e ( i n c r e a s e i n discharge c u r r e n t ) signal spectrum coul d be recorded by probing a region o u t s i de t h e i n t e r e l e c t r o d e region; ( v i i ) Lowering t h e c e l l pressure generally s e r v e s t o i n c r e a s e t h e negative (decrease i n discharge c u r r e n t ) signal s i z e a t t h e negative slow r e l a t i v e t o t h e p o s i t i v e signal s i z e near t h e anode.

We t h e r e f o r e i d e n t i f y two c o n t r i b u t i o n s t o t h e optogal vanic signal S observed i n t h e NH3 discharge: t h e f i r s t , corresponding t o an i n c r e a s e i n discharge c u r r e n t , is seen when probing any l o c a t i o n within t h e discharge o r even o u t s i d e t h e i n t e r - e l e c t r o d e region; t h e second, l a r g e r s i g n a l , corresponding t o a decrease i n d i s - charge c u r r e n t , i s seen only when probing a narrow region c l o s e t o t h e negative glow. These s i g n a l s w i l l be r e f e r r e d t o a s p o s i t i v e and negative s i g n a l s , respec- t i v e l y .

The negative s i g n a l s , which correspond t o a decrease i n discharge c u r r e n t , a r e believed t o be generated by t h e absorption of TDL r a d i a t i o n only i n a small region c l o s e t o t h e discharge a x i s which we i d e n t i f y with t h e negative glow. The dip seen i n Figure 3 ( b ] a t t h e c e n t e r of t h e 1046.4075 c p i l NH3 l i n e r e s u l t s from l o s s i n TDL power reaching t h e negative glow caused by absorption i n t h e gas volume between t h e entrance window and t h e discharge region. Not only does t h i s produce a drop i n t h e negative s i g n a l , but i t a1 so produces a p o s i t i v e signal c o n t r i b u t i o n a t l ine-center.

1

- ^-

-

1 I

Figure 3. A portion of the LOG spectrum of NH3 i n the 9.5 micron region, recorded by i r r a d i a t i n g the negative glow of a pure NH3 discharge a t ( a ) 0.25 t o r r and ( b ) 0.4 t o r r

.

A t lower c e l l pressures, t h i s e f f e c t i s reduced and the negative LOG signal a t the 1046.4075 cm-l l ine-center increases re1 a t i v e t o the neighboring l i n e s . In the low- pressure l i m i t the r e l a t i v e magnitudes of a l l four l i n e s should follow the line- strengths given i n Table 1. I t i s unlikely t h a t the mechanism responsible f o r signal production will show any measurable differences in cross-section f o r the d i f f e r e n t t r a n s i t i o n s involved. The c e l l used t o produce these spectra was similar t o t h a t used in our e a r l i e r study /5/ except the window t o discharge axis distance was greatly reduced, producing, as predicted 151, much stronger negative g1 ow sig- nals.

5. Frequency-modulated optogalvanic spectroscopy

The output frequencies of tunable diode l a s e r s are fine-tuned by changing (increasing) the injection current t o the TDL junction. Modulation of the TDL current may therefore be used t o frequency-modulate the TDL output. By using a 400 Hz sinusoidal modulation of the TDL, removing the mechanical chopper, and referencing the lock-in ampl i f i e r t o the modulation frequency, f i rst-harmonic detection of optogalvanic signals was demonstrated.

Figure 4 shows t h e f i r s t harmonic spectrum of the same NH3 quartet near 1046.4 cm-l a s t h a t recorded i n Figure 3, resulting in t h i s case from irradiation of the positive column.

The lower t r a c e of Figure 4, the first-harmonic LOG spectrum, r e s u l t s from laser- induced impedance changes a t the modulation frequency of 400 Hz. Following i t s traversal through the gas c e l l , the TDL beam f a l l s on a HgCdTe detector t o bllow the upper t r a c e of Figure 4, the first-harmonic (conventional) absorption spectrum, t o be recorded f o r comparison. Defining a modulation c o e f f i c i e n t m a s the r a t i o of ha1 f t h e peak-to-peak modulation ampl i tude (cm-l), a modulation c o e f f i c i e n t of m = 1.2 was used in t h i s work. T h i s i s about 75% the m value of 1.65 which produces the maximum first-harmonic signal size. Loss of resolution from modu- l a t i o n broadening i s therefore small i n these spectra.

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DIODE LASER FREQUENCY (cm-')

I I I I I

(a) Absorption Spectrum

F i g u r e 4. ( a ) The d i r e c t absorption and ( b ) t h e LOG spectrum o f NH3 near 1046.4 cm-l recorded u s i n g f i r s t harmonic detection. A l o c k - i n a m p l i f i e r time constant o f 1s i s used i n both cases.

A

6. Mechanisms producing I R LOG s i g n a l s

The i n f r a r e d r e s u l t s r e p o r t e d here show s t r i k i n g s i m i l a r i t i e s t o t h e v i s i b l e

1

(b) L O G Spectrum ' h

J I ; ' / ~

1 1

glow region. Indeed, f o r TDL i r r a d i a t i o n of NH3 a t 9.5 m, the absolute positive signal s i z e , expressed i n mV change in discharge voltage per mW absorbed l a s e r power, i s of a similar order of magnitude t o t h a t produced by dye l a s e r irradiation of Ip a t 0.59 m.

I t i s proposed t h a t the positive signals observed i n these infrared LOG studies r e s u l t from an increase in k i n e t i c energy of species located in the negative glow of the discharge. For i r r a d i a t i o n outside the interelectrode region o r away from the negative glow the energy absorbed as rovibrational quanta i s degraded t o trans- lational energy through V-T t r a n s f e r i n a time f a s t compared t o the bulk energy t r a n s f e r time t o the discharge region. The positive signal S produced therefore r e s u l t from an increase in the translation energy (temperature) of species in the negative glow. However, when d i r e c t l y i r r a d i a t i n g t h i s region, vibrational l y excited species will be produced i n the discharge, the mean concentration depending on the relaxation times involved. I t i s believed t h a t the neqative signals observed r e s u l t from an increase in ( p r i n c i p a l l y ) the vibrational energy of species in the negative glow.

Electrons a r e l o s t in a glow discharge through negative ion formation (attach- ment), electron diffusion, mutual repulsion and recombination. Electron attachment

~ n d recombination are particr1l arly important in t h e rlegative g1 ow, perturbation of t h i s region, we propose, being crucial t o the generation of the LOG signal. In t h e reeion comprising the negative glow, the cathode dark space, and the cathode glow, the voltage gradient, the net charge density, and the electron and ion den- s i t y a l l have t h e i r maximum values. The slow electrons entering the negative glow produce t h i S luminous region by undergoing numerous excitation col l i s i ons which leave the electrons with very low k i n e t i c energies. In t h e negative glow the corn- bination of the high electron and ion densities and the low e l e c t r i c f i e l d allows recombination and electron attachment reactions t o be important processes in t h i s region of the discharge, even though i n the low current ( < l m A ) discharges employed i n t h i s study, ambipolar diffusion may dominate the overall l o s s processes f o r charged species. I t i s l i k e l y t h a t the positive signals observed r e s u l t from the reduced efficiency, with increasing gas kinetic temperature, of ion-ion or elec- tron-ion recombination processes. Another possible cause of the positive signal S

which were observed i S the increased efficiency with increasing electron temperature of electron impact-ionization. This mechanism suggests t h a t , in a similar manner t o t h a t proposed1* f o r atomic systems, the energy supplied t o the molecular system by i r r a d i a t i o n i s ultimately transferred t o the electron gas through the numerous electron col l isions. However, f o r i r r a d i a t i o n outside the discharge region, the i n i t i a l t r a n s f e r of energy must proceed via intermolecular col l i sions.

The negative signals which correspond t o an increase i n the discharge impedance may r e s u l t from a large enhancement in the electron attachement r a t e when the negative g1 ow region i s i r r a d i a t e d by the resonant infrared power. Electron attach- ment t o molecules, which i s an important process occuring i n discharges of electro- negative gases l i k e NO?, 12, and NH3, i s known t o be p a r t i c u l a r l y sensitive t o thermal effects.. The attachment cross-sections may increase or decrease with in- creasing electron energy depending on the mean electron energy, which may be l e s s than one eV a t t h e negative glow or a few eV i n the positive column of low-current glow discharges. For molecules, the dependence on internal vibrational energy content may be large.

In order t o f u r t h e r identify the two, opposite polarity, contributions t o the LOG signals, the dependence on chopping frequency of the LOG signals was investi- gated. To do t h i s , the TDL was tuned t o the center of the 1046.376 cm-' NH3 l i n e . Then, w i t h the lock-in amplifier phase s e t t i n g re-optimized f o r each chopping fre- quency, the signal s i z e variation a t the negative glow was measured i n the range 0.1-5 KHz using a variable-speed chopper. These measurements were repeated f o r TDL i r r a d i a t i o n of the positive column, The r e s u l t s a r e shown i n Figure 5.

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A. NEGATIVE GLOW IRRADIATION

0 P = 0.4 torr P = 0.5 torr

0.0

0 1 2 3 4 5

CHOPPING FREQUENCY (kHz)

B. POSITIVE COLUMN IRRADIATION

I l

-

\

0 P = 0.4 torr

I

I

0.0

-

CHOPPING FREQUENCY (kHz)

Figure 5. The v a r i a t i o n with chopping frequency of t h e LOG signal i n NH3 a t 1046.376 cm-' r e s u l t i n g from i r r a d i a t i o n of ( A ) t h e negative glow and ( B ) t h e p o s i t i v e column region of a low-pressure ammonia discharge. Note t h a t while t h e s i g n a l s a r e i n each c a s e normalized t o t h e low chopping frequency signal s i z e , t h e s i g n a l S a r e of o p p o s i t e p01 a r i ty.

I t i S immediately evident t h a t i r r a d i a t i o n of t h e two regions produces LOG s i g n a l s not only of d i f f e r e n t s i g n but a l s o of d i f f e r e n t time evolution. Further- more, t h e f a l l - o f f with chopping frequency of t h e p o s i t i v e column s i g n a l , and i t s v a r i a t i o n with gas pressure, i s i n semi-quantative agreement with t h e known vi- b r a t i o n a l r e l a x a t i o n r a t e s of t h e NH3 molecule.

The LOG signal a t t h e negative glow is seen i n Figure 5 t o f a l l o f f much f a s t e r with chopping frequency and show a much l e s s pronounced dependence on gas pressure. Signal generation from t h e negative g1 ow probe i ^S t h e r e f o r e a s s o c i a t e d with a time-constant of a few hundred microseconds.

Much work remains t o unambiguously i d e n t i f y t h e mechanisms responsible f o r t h e observed optogal vanic e f f e c t i n molecules e x c i t e d with IR r a d i a t i o n . How- ever, once such an i d e n t i f i c a t i o n i s made, t h e s e same mechanisms may themselves be s t u d i e d q u a n t i t a t i v e l y . That i S , molecular optogal vanic spectroscopy may one day f i n d a p p l i c a t i o n t o t h e measurement o f , f o r example, i o n i z a t i o n , attachment, detachment, recombi n a t i on, and re1 axation r a t e s .

Acknowledgement

The research described i n t h i s paper was performed a t t h e J e t Propulsion Laboratory, C a l i f o r n i a I n s t i t u t e of Technoloqy, under c o n t r a c t with t h e National Aeronautics and Space Administration.

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