ATOMIC SPECTROSCOPY WITH OPTOGALVANIC DETECTION

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ATOMIC SPECTROSCOPY WITH OPTOGALVANIC DETECTION

P. Camus

To cite this version:

P. Camus. ATOMIC SPECTROSCOPY WITH OPTOGALVANIC DETECTION. Journal de

Physique Colloques, 1983, 44 (C7), pp.C7-87-C7-106. �10.1051/jphyscol:1983708�. �jpa-00223265�

JOURNAL DE PHYSIQUE

Colloque C7, supplement au n O 1 l , Tome 44, novernbre 1983 page C7-87

ATOMIC SPECTROSCOPY WITH OPTOGALVANIC D E T E C T I O N

Laboratoire Aim6 cottoni, CNRS II, Campus dlOrsay, BCtiment 505, 91405 Orsay Cedex, France

Resume

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Ce p a p i e r p o r t e s u r l ' a p p l i c a t i o n de l ' e f f e t optogalvanique ( l e changement d'impedance, dans un gaz ou une vapeur metal l i q u e f a i b l e m e n t e x c i t e e e l e c t r i q u e m e n t , dir l ' a b s o r p t i o n d ' u n rayonnement lumineux) en s p e c t r o s c o p i e atomique. Ce changement d'impedance e s t aisement e n r e g i s t r e e t f o u r n i t un s i m p l e d e t e c t e u r non-optique pour mesurer l a m o d i f i c a t i o n des p o p u l a t i o n s des niveaux atomiques en presence du phenomene r a d i a t i f d ' a b - s o r p t i o n . Associee avec l a venue du l a s e r 5 c o l o r a n t s accordable, l a d e t e c - t i o n optogalvanique f o u r n i t un i n s t r u m e n t spectroscopique v a l a b l e pour e x p l o r e r l e s p r o p r i e t e s de l a s t r u c t u r e atomique p a r t i r de niveaux e x c i - t e s , p r i n c i p a l e m e n t pour l e s etudes des niveaux de Rydberg eleves.

L ' u t i l i s a t i o n de l a lampe

a

cathode creuse comme i n s t r u m e n t de p u l v e r i s a - t i o n pour c r e e r une vapeur atomique ouvre une l a r g e p e r s p e c t i v e

a

c e t t e methode pour e t u d i e r l e s elements r e f r a c t a i r e s .

A b s t r a c t

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T h i s paper d e a l s w i t h t h e a p p l i c a t i o n o f t h e o p t o g a l v a n i c e f f e c t ( t h e e d a n c e change i n a f e e b l y e l e c t r i c a l l y e x c i t e d gas o r metal vapour due t o t h e a b s o r p t i o n o f an i r r a d i a t i n g l i g h t ) i n a t o m i c spectroscopy. T h i s impedance change i s e a s i l y r e c o r d e d and p r o v i d e s a s i m p l e n o n - o p t i c a l d e t e c t o r t o measure t h e m o d i f i c a t i o n o f t h e atomic l e v e l p o p u l a t i o n s under t h e r a d i a t i v e a b s o r p t i o n process. A s s o c i a t e d w i t h t h e advent o f t h e t u n a b l e dye l a s e r , t h e o p t o g a l v a n i c d e t e c t i o n p r o v i d e s a v a l u a b l e s p e c t r o s c o p i c t o o l t o i n v e s t i g a t e t h e a t o m i c s t r u c t u r e p r o p e r t i e s f r o m t h e e x c i t e d l e v e l s , m a i n l y f o r h i g h l y i n g Rydberg l e v e l s t u d i e s . Use o f t h e h o l l o w - cathode lamp as a s p u t t e r i n g d e v i c e t o generate atomic vapours, opens t h e f u t u r e t o t h i s method f o r s t u d y i n g r e f r a c t o r y elements.

I

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INTRODUCTION

The o p t o g a l v a n i c e f f e c t (O.G.) a r i s e s when a r e s o n a n t o p t i c a l i n t e r a c t i o n w i t h t h e atomic o r m o l e c u l a r species i n a f e e b l y i o n i z e d plasma changes i t s e l e c t r i c a l impe- dance. T h i s change i n impedance i n response t o t h e a b s o r p t i o n o f t h e i r r a d i a t i n g l i g h t can be c o n v e n i e n t l y m o n i t o r e d w i t h and w i t h o u t i r r a d i a t i o n and l e a d s t o a v e r y s i m p l e and s e n s i t i v e n o n - o p t i c a l d e t e c t o r i n a b s o r p t i o n spectroscopy. The b a s i c p h y s i c s o f t h i s e f f e c t depends o f t h e p r o p e r t i e s o f t h e c o n s i d e r e d plasma b u t i t i s g e n e r a l l y due t o s i g n i f i c a n t p e r t u r b a t i o n s o f t h e steady s t a t e p o p u l a t i o n o f t h e e x c i t e d l e v e l s induced by a r e s o n a n t r a d i a t i v e a b s o r p t i o n process and r e l a t e d t o t h e i r charge c o l l i s i o n a l p r o d u c t i o n r a t e s .

Two v e r y d i f f e r e n t t y p e s o f l o w p r e s s u r e environments have been shown t o y i e l d O.G.

s i g n a l s . The e a r l i e s t r e p o r t o f t h e e f f e c t u t i l i z e d a t h e r m i o n i c diode e x h i b i t i n g space charge a m p l i f i c a t i o n i n a caesium vapour by FOOTE and MOHLER / l / . The second t y p e of low p r e s s u r e gas environment g i v i n g impedance changes i s a glow d i s c h a r g e of neon and has been used by PENNING /2/ t o observe an impedance i n c r e a s e i n a neon d i s c h a r g e when i t was i l l u m i n a t e d w i t h t h e l i g h t f r o m a second neon discharge. Since these two p i o n e e r works, t h e method u s i n g c o n v e n t i o n a l l i g h t sources has n o t been w i d e l y used e x c e p t f o r a l k a l i vapour s t u d i e s u s i n g t h e r m i o n i c d i o d e by POPESCU e t a l . / 3 / .

*~ssociB B 1'UniversitB de Paris-Sud

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983708

C7-88 JOURNAL DE PHYSIQUE

D e s p i t e t A e i r l o n g h i s t o r y n e i t h e r o f t h e methods assumed much importance u n t i l t h e advent o f t u n a b l e dye l a s e r d i s c o v e r y which has y i e l d t o t h e powerful l a s e r s p e c t r o s - copy. R e v i v a l o f t h e o p t o g a l v a n i c method i s due t o GREEN e t a l . /4/ which have shown t h a t t h e use o f a C.W. dye l a s e r c o u l d enable h i g h s e n s i t i v i t y i n an h o l l o w cathode discharge.

Since t h i s extended use o f l a s e r s i n 1976, t h e f a c t t h a t t h e f i e l d o f O.G. research and a p p l i c a t i o n s i s r a p i d l y expanding can be a p p r e c i a t e d by n o t i n g t h a t s e v e r a l r e v i e w a r t i c l e s have a l r e a d y been p u b l i s h e d r e c e n t l y / 5 t o 8/ and I w i l l r e f e r t h e l e c t u r e r t o them f o r h i s t o r i c a l b i b l i o g r a p h y . The aim o f t h e p r e s e n t a r t i c l e i s t o d e s c r i b e O.G. g e n e r a t i o n and r e c e n t a p p l i c a t i o n s i n a t o m i c spectroscopy f i e l d n o t covered i n d e t a i l i n t h e o t h e r r e v i e w a r t i c l e s mentionned above.

I 1

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OPTOGALVANIC EFFECT I N LOW PRESSURE GAS OR METAL VAPOURS

The atomic a b s o r p t i o n o f photons w i l l be c o n s i d e r e d i n c o n n e c t i o n w i t h c o l l i s i o n a l processes i n gaser o r metal vapours. Since t h e development o f t h e c o n t i n u o u s o r p u l s e d t u n a b l e dye l a s e r s , l a r g e changes o f t h e e l e c t r i c a l p r o p e r t i e s o f low-pressure gas o r metal vapours can be r e s o n a n t l y enhanced by an i n t e n s e monochromatic l a s e r i r r a d i a t i o n . Recording o f these changes w h i l e scanning t h e l a s e r wavelength generates a p o w e r f u l n o n - o p t i c a l d e t e c t i o n method f o r s p e c t r o s c o p i c i n v e s t i g a t i o n s o f species i n t h e plasma. Such o p t o g a l v a n i c spectroscopy shows s i m i l a r c h a r a c t e r i s t i c s w i t h f l u o r e s c e n c e l a s e r spectroscopy b u t has a decided advantage when an e x c i t e d atom has a g r e a t e r p r o b a b i l i t y t o i o n i z e t h a n t o decay by spontaneous emission.

I t i s p a r t i c u l a r l y t r u e f o r m e t a s t a b l e l e v e l s and na Rydberg l e v e l s ( t h e i r r a d i a - t i v e l i f e t i m e s i n c r e a s e as n 3 ) .

The s i m p l e e x p e r i m e n t a l set-up o f a dye l a s e r o p t o g a l v a n i c spectrometer (DYELoGS) i s i l l u s t r a t e d on F i g . 1.

DISCHARGE TUBE

BEAM

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CHOPPER

LLA U C H A R T

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U+ 1 I 1

^RECORDER

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POWER

S l l P P l V CAPACITDR

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_AMP. ^IN

I

REFERENCE

l

F i g . 1 : Experimental arrangement f o r O.G. spectroscopy (DYELOGS)

The apparatus i s c o n t a i n i n g a t u n a b l e dye l a s e r , a weakly e x c i t e d plasma i n a c e l l and a l o c k - i n a m p l i f i e r t o measure t h e impedance changes o f t h e i r r a d i a t e d plasma.

Fig.' 2 shows t h e v a r i o u s gas d i s c h a r g e plasmas which have been used f o r l o w p r e s s u r e gas o r metal vapours s t u d i e s : t h e r m i o n i c d i o d e ( h o t cathode f i l a m e n t ) , h o l l o w - cathode ( c o l d cathode) o r R.F. d i s c h a r g e ( e l e c t r o d e l e s s t u b e ) .

I

THERMOlONlC D I O D E IN THE SPACE CHARGE REGIME

HOLLOW -CATHODE

+ 1 ANODE^

^EATHODE

F i g . 2 : V a r i o u s gas d i s c h a r g e plasmas f o r O.G.

spectroscopy

R.F DISCHARGE WITH INDUCTIVE COUPLING

R . F POWER SUPPLY

11.1

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Space a m p l i f i c a t i o n method u s i n g t h e r m i o n i c d i o d e

T h i s new o p t o g a l v a n i c spectroscopy has been demonstrated w i t h s p a c e - c h a r g e c o n t r o l l e d a l k a l i vapour d i o d e s f i l l e d w i t h l o w p r e s s u r e vapours o f caesium, r u b i d i u m a n d p o t a s - sium u s i n g c o n v e n t i o n a l monochromatic l i g h t source by POPESCU e t a l . /3/. Since t h e development o f t u n a b l e dye l a s e r s , t h e p o t e n t i a l o f t h e space charge a m p l i f i c a t i o n method was i n c r e a s e d u s i n g two-photon o r two-step e x c i t a t i o n s .

I n t h e e a r l i e s t r e p o r t / l / e l e c t r o n i c a l l y e x c i t e d CS atoms were f o u n d t o f o r m mole- c u l a r i o n s by t h e mechanism o f a s s o c i a t i v e i o n i z a t i o n :

C S * + C S + CS: + e-

m o l e c u l a r a s s o c i a t i o n f o l l o w e d by d i s s o c i a t i v e a t t a c h m e n t : CS* + CS + CS; + CS+ + cs-

o r d i r e c t p h o t o i o n i z a t i o n :

CS + 3hv + CS+ + e-

The appearence o f i o n s i n t h e i n t e r e l e c t r o d e s p a c i n g i s d e t e c t e d b y a v a r i a t i o n o f t h e p o t e n t i a l b a r r i e r formed by t h e l o w k i n e t i c energy e l e c t r o n s w i t h subsequent i n c r e a s e i n t h e e m i s s i o n c u r r e n t due t o t h e n e u t r a l i z a t i o n o f t h e e l e c t r o n space charge c l o u d . S i n c e an i n d i v i d u a l i o n remains u n n e u t r a l i z e d f o r a t i m e i n t e r v a l seve- r a l o r d e r s o f magnitude l a r g e r t h a n f o r an e l e c t r o n , i t r e l e a s e s I D 5 t o 106 e l e c t r o n s a c c o u n t i n g f o r t h e extreme s e n s i t i v i t y o f t h e method /g/. Due t o t h e f a c t t h a t a l a r g e s a t u r a t i o n e m i s s i o n o c c u r s f r o m a l o w t e m p e r a t u r e e m i t t e r covered by an a l k a l i l a y e r , space c h a r g e ~ w o r k i n g c o n d i t i o n s a r e reached f o r v e r y l o w p o l a r i s a t i o n v01 t a g e of t h e cathode ( S 100 mV) and thermal e l e c t r o n s e m i t t e d have l e s s than 0.1 eV k i n e t i c energy.

The p o t e n t i a l breakdown o f t h e gas i s n o t reached a t these l o w p o l a r i z a t i o n v o l t a g e and p o p u l a t i o n s o f t h e a t o m i c l e v e l s a r e v e r y c l o s e d t o these g i v e n by t h e Boltzmann t h e r m a l e q u i l i b r i u m .

C7-90 JOURNAL DE PHYSIQUE

I n t h e s e c o n d i t i o n s , o n l y t h e t r a n s i t i o n s f r o m t h e ground l e v e l can be observed.

A b i a s f i e l d o f a b o u t 2V on t h e a l k a l i - v a p o u r d i o d e produces a l o w power d i s c h a r g e ( g l o w i l l u m i n a t i o n o f t h e gas) which a l l o w s t h e h e a t i n g o f e l e c t r o n gas i n t h e p l a s - ma and g i v e a s i g n i f i c a n t p o p u l a t i o n o f t h e l o w l y i n g e x c i t e d atomic s t a t e s .

D i f f u s e , sharp and fundamental s e r i e s were s i m p l y recorded as v o l t a g e changes a c r o s s t h e d i s c h a r g e /10/.

F i g . 3 : Space charge a m p l i f i c a t i o n

F i g . 3 shows t h e p r i n c i p a l Rydberg s e r i e s of Rb and CS r e c o r d e d by t h e Romanian p i o n e e r i n g group / 3 / .

Noise l i m i t a t i o n s a r e e s s e n t i a l l y due t o thermal f l u c t u a t i o n o f t h e e l e c t r o n emis- s i v i t y o f t h e h o t c a t h o d e o r f r o m t h e h e a t i n g system needed t o m a i n t a i n t h e gazeous phase o f t h e s t u d i e d atom.

To a r e s o n a n t l i g h t p u l s e (10-8s) corresponds a f a s t i n c r e a s e i n t h e c u r r e n t o f t h e d i o d e f o l l o w e d by a s l o w l e r decrease as shown on F i g . 4.

F i g . 4 : Diode p u l s e . P o l a r i z a t i o n voltage, 300 mV, v e r t i c a l , lOmV/division, h o r i z o n t a l , 5 m s e c / d i v i s i o n /11/

T h i s d e c r e a s i n g s i g n a l i s analyzed as t h e sum o f two e x p o n e n t i a l c u r v e s w i t h two d i f f e r e n t t i m e decays. The f i r s t p a r t , w i t h a t i m e c o n s t a n t T , S l W 3 s would be g i v e n by t h e d i r e c t c a p t u r e o f n e g a t i v e charges by t h e anode and p o s i t i v e charges by t h e f i l a m e n t . The second p a r t , w i t h a l o n g e r t i m e c o n s t a n t -C, 2 1 W 2 s would g i v e evidence o f imprisonment o f p o s i t i v e i o n s i n t h e space charge /11/.

F o r t h e a l k a l i n e - e a r t h elements, space charge a m p l i f i c a t i o n method has been e x t e n s i - v e l y used a t I.B.M. b y WYNNE e t a l . /12/. As most o f t h e a l k a l i n e - e a r t h vapours r e a c t w i t h t h e windows, i t i s necessary t o p r e v e n t them a g a i n chemical r e a c t i o n s by u s i n g a b u f f e r gas.

A c y l i n d r i c a l h e a t - p i p e c e l l i s t h e n used t o c o n t a i n t h e gazeous phase o f t h e m e t a l and forms t h e anode o f t h e c y l i n d r i c a l t h e r m i o n i c diode. To p r e v e n t c o r r o s i o n pheno- mena o r chemical r e a c t i o n s a11 t h e p a r t s c o n t a i n i n g t h e sample a r e made o f s t a i n l e s s

s t e e l . The i n n e r w a l l s o f the p i p e a r e covered by l a y e r s o f f i n e mesh screen t o i n s u r e r e c y c l i n g o f t h e m a t t e r i n t h e l i q u i d phase from b o t h sides c o o l e r extremi- t i e s by c a p i l l a r y a c t i o n .

When the heat-pipe working c o n d i t i o n s /13, 1 4 1 are s a t i s f i e d , t h i s c e l l presents two advantages from c l a s s i c a l absorption c e l l :

-

one i s t h a t i t minimizes t h e pressure e f f e c t s on t h e h i g h l y i n g Rydberg s t a t e s by c o l l i s i o n s w i t h t h e buffer-gas atoms ;

-

t h e second i s t h a t i t s t a b i l i z e s i n time and keep u n i f o r m on i t s u s e f u l l e n g t h t h e thermoelectronic emission o f the f i l a m e n t by keeping i t temperature s t r i c t l y cons- t a n t (any f l u c t u a t i o n o f t h e h e a t i n g power o f t h e oven producing o n l y an increased o r decreased l e n g t h o f t h e vapour column i n the heat-pipe working c o n d i t i o n s ) .

Fig. 5 : Two-photon spectrum o f Ba I : h i g h members o f t h e 6sns 'S, and Gsnd

'D,

s e r i e s

m S m 05

,

^{m R}

I I I I I I I I I 1 I I I I I 1 1 1 1 1 1 1 1 1 1 1 6snd ' j D Z 5s

U L U

-L-JLL-

^{6sns ' S ,}

F i g . 5 shows t h e two-photon spectrum from the ground l e v e l 6s' 'So o f Ba recorded w i t h a thermionic heat-pipe diode /11/.

11.2

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Optogalvanic e f f e c t w i t h d.c. discharges

The continuous regime o f an e l e c t r i c discharge i s determined by t h e constant c u r r e n t I and t h e constant v o l t a g e drop V maintened between t h e electrodes. These two values g i v e a working p o i n t o f the s t a t i c c u r r e n t - v o l t a g e c h a r a c t e r i s t i c o f t h e e l e c t r i c a l discharge. I n f a c t , the continuous regime traduces t h e dynamic e q u i l i - brium o f t h e e l e c t r i c discharge i n the plasma and t h e s u s t a i n i n g d.c. power supply.

Such s t a t i c c h a r a c t e r i s t i c curve o f a d.c. discharge i n a c y l i n d r i c a l p i p e i s given on Fig. 6.

S t a t i c c h a r a c t e r i s t i c o f a d.c. discharge i n a c y l i n d r i c a l p i p e

C7-92 JOURNAL DE PHYSIQUE

The f i r s t p a r t o f t h e curve OD t r a d u c e s t h e regime where t h e c o n d u c t i o n i s c o n t r o l e d by t h e p r i m a r y mechanism of spontaneous c r e a t i o n o f e l e c t r o n - i o n p a i r s i n t h e plasma.

I o n i z a t i o n i s produced by t h e c o l l e c t i v e a c t i o n o f t h e e l e c t r i c f i e l d , cosmic r a y s o r any o t h e r i o n i z a t i o n process. F l o w i n g o f charges i s t r a d u c i n g t h e c u r r e n t obser- ved : e l e c t r o n s toward t h e anode, p o s i t i v e i o n s t o t h e cathode. As f a r as t h e poten- t i a l i s growing up, t h e k i n e t i c energy o f t h e charged p a r t i c u l e s i s increased. I n t h e r e g i o n AB, a l l t h e charged p a r t i c u l e s a r e c o l l e c t e d by t h e e l e c t r o d e s which i s c h a r a c t e r i z i n g t h e s a t u r a t i o n regime. A t B, t h e k i n e t i c energy o f t h e e l e c t r o n s i s enough t o i o n i z e by impact t h e n e u t r a l p a r t i c l e s . Along t h e curve C, t h e e l e c t r o n secondary emission i s produced by c o l l i d i n g charged p a r t i c u l e s w i t h t h e e l e c t r o d e s . Around D, t h e v o l t a g e d r o p between t h e e l e c t r o d e s has reached t h e breakdown p o t e n t i a l

o f t h e d i s c h a r g e and we have a glow i l l u m i n a t i o n o f t h e plasma which c h a r a c t e - k z e s t h e d i s r u p t i v e regime.

I n t h i s case, t h e steady s t a t e regime r e s u l t s f r o m t h e s t a b i l i z a t i o n o f numerous coupled phenomena, m a i n l y as r a d i a t i v e and e l e c t r o n impact processes i n v o l v i n g atoms o r molecules p r e s e n t i n t h e discharge. Then, t h e plasma i s composed o f f r e e e l e c - t r o n s , n e u t r a l o r i o n i z e d elements w i t h e n e r g i e s spread o v e r a l l t h e p o s s i b l e e x c i t e d t a t e s . When t h e steady s t a t e regime i s obtained, t h e p o p u l a t i o n d e n s i t y i s s t a t i o n - n a r y o v e r a l l t h e l e n g t h o f t h e d i s c h a r g e which i s l e a d i n g t o a t o t a l r a t e o f c r e a t e d i o n s and t o a c o n s t a n t charge d e n s i t y . Then t h e plasma p r e s e n t s an i n t e r n a l c o n s t a n t impedance Z f i x e d by t h e experimental c o n d i t i o n s :

-

n a t u r e o f t h e gas ( l e v e l diagram, pressure, temperature)

-

geometry o f t h e d i s c h a r g e c e l l

-

e l e c t r i c power energy d i s s i p a t e d i n t h e c i r c u i t f e e d i n g t h e discharge.

The secondary emission a t t h e cathode i s more i m p o r t a n t a t t h e b e g i n n i n g o f t h e d i s r u p t i v e regime than i n t h e abnormal mode where i o n i z i n g c o l l i s i o n s were respon- s a b l e f o r t h e l a r g e enhancement o f t h e charges. I n t h e d i s r u p t i v e regime, t h e p o s i - t i v e column i s t h e p r i n c i p a l l i g h t source o f t h e glow discharge, t h e e l e c t r i c f i e l d i s q u i t e u n i f o r m w i t h no space charge e f f e c t . The dynamic e q u i l i b r i u m i s then d e s c r i - bed by t h e S c h o t t k y model.

I l l u m i n a t i o n o f a glow d i s c h a r g e (d.c. d i s c h a r g e o r c o o l e d hollow-cathode) w i t h a l a s e r i r r a d i a t i o n c o r r e s p o n d i n g t o an atomic t r a n s i t i o n p e r t u r b s t h e steady s t a t e p o p u l a t i o n o f t h e l e v e l s . I n g e n e r a l , t h e c o l l i s i o n a l i o n i z a t i o n r a t e s f r o m d i f f e r e n t l e v e l s ( l o w energy m e t a s t a b l e l e v e l o r Rydberg ones) a r e u n i q u a l , so t h e i o n i z a t i o n balance o f t h e d i s c h a r g e i s d e s t r o y e d and an a n o t h e r e q u i l i b r i u m corresponding t o a d i f f e r e n t s t a t i c c u r r e n t - v o l t a g e p o i n t i s set-up, i n presence o f t h e l a s e r i r r a d i a - t i o n .

Three processes will make i m p o r t a n t c o n t r i b u t i o n s t o i o n i z a t i o n as shown i n Ne by SMYTH and SCHENCK /15/ :

-

e l e c t r o n impact i o n i z a t i o n Ne

+

e- + ~ e + + 2e-

-

e l e c t r o n impact m e t a s t a b l e Ne* + e- + ~ e ' + 2e-

-

metastable-metastable Ne* + Ne* ⁺ ~ e + + Ne + e-

-

a s s o c i a t i v e i o n i z a t i o n Ne** + Ne ⁺ ~ e+ le-

G e n e r a l l y , t h e d i s c h a r g e i n metal vapour a r e s u s t a i n e d b y a f o r e i g n gas, so chemioni- z a t i o n i s a l s o p o s s i b l e when an e x c i t e d atom o f one species c o l l i d e s w i t h a ground s t a t e atom o f a second species h a v i n g an i o n i z a t i o n p o t e n t i a l l e s s t h a n t h e e x c i t a - t i o n energy o f t h e f i r s t . The second atom i s i o n i z e d when t h e f i r s t i s r e t u r n e d t o t h e ground s t a t e .

When t h e o p t i c a l e x c i t a t i o n i s modulated a t l o w frequency ( 5 1 KHz) by measuring t h e change o f c u r r e n t , t e n s i o n o r impedance between t h e two s t a t i c p o i n t s ( w i t h and w i t h o u t l a s e r i r r a d i a t i o n ) , we a r e making an 0.G. d e t e c t i o n o f t h e o p t i c a l resonance w i t h t h e species i n t h e discharge.

A t an oven temperature around 190°C and a caesium vapour p r e s s u r e about 50 mTorr, a s t a b l e d i s c h a r g e has been o p e r a t e d a t 20 mA, 400 V by BRIDGES /16/ and F i g . 7a shows a t y p i c a l o s c i l l o s c o p e t r a c e o f t h e a.c. component o f t h e d i s c h a r g e v o l t a g e w i t h

F i g . 7a : Waveform o f t h e cesium d i s c h a r g e v o l t a g e w i t h

50 V l D I V t h e chopped dye l a s e r

i n p u t tuned t o t h e 6010

4

a b s o r p t i o n

a l m o s t 150 V peak t o peak v a r i a t i o n a t t h e 1 KHz chopping f r e q u e n c y . So mentionned by BRIDGES t h e rounded c o r n e r s do n o t a r i s e f r o m RC t i m e c o n s t a n t i n t h e e l e c t r o n i c s b u t a r e d i s c h a r g e t r a n s i e n t phenomenon c o r r e l a t e d t o t h e e s t a b l i s h m e n t o f t h e steady s t a t e regime w i t h and w i t h o u t l a s e r i r r a d i a t i o n . A t d i f f e r e n t d i s c h a r g e c u r r e n t s were a l s o observed n e g a t i v e and p o s i t i v e s i g n a l s i n a glow d i s c h a r g e o f neon /16/.

I f t h e dye l a s e r i s pumped by a p u l s e d l a s e r : n i t r o g e n , excirner l a s e r s o r neodymium Y.A.G. l a s e r ( Q 10-'s) w i t h r e p e t i t i o n r a t e s r a n g i n g f r o m 5 t o 50

Hz,

t r a n s i e n t o p t o g a l v a n i c s i g n a l s a r e observed and temporal f e a t u r e s o f t h e s i g n a l s g i v e evidence o f t h e d i f f e r e n t 5 r e t u r n s o f t h e p e r t u r b e d d i s c h a r g e t o i t s i n i t i a l e q u i l i b r i u m b e f o r e i n t e r a c t i n g w i t h t h e l a s e r l i g h t /17/.

EREZ e t a l . /18/ have shown t h a t e x c i t a t i o n o f atoms w i t h i n t h e d i s c h a r g e t o a h i g h e r l e v e l enhances t h e i o n i z a t i o n i n d u c i n g t h e n a v o l t a g e drop ( F i g . 7b) w h i l e e x c i t a t i o n f r o m a m e t a s t a b l e l e v e l s s t a r t s by a n e g a t i v e p a r t due t o t h e enhanced i o n i z a t i o n f o l l o w e d by a p o s i t i v e and s l o w l e r r e t u r n . T h i s r e t u r n shown on F i g . 7c i s determined by t h e t i m e r e q u i r e d t o r e f i n d t h e steady s t a t e m e t a s t a b l e l e v e l p o p u l a t i o n and i s i n t h e o r d e r o f 5 vs. We s h o u l d remember t h a t i n t h e same c o n d i t i o n o f p u l s e d l a s e r e x c i t a t i o n t h e answer o f t h e space charge t h e r m i o n i c d i o d e i s 1 000 t i m e s l a r g e r .

Fig. 7b : OGE s i g n a l o f uranium F i g . 7c : OGE s i g n a l o f neon a t

a t 591.54 nm 558.19 nm.

E x c i t a t i o n f r o m t h e m e t a s t a b l e l e v e l 134043 cm-'

C?-94 JOURNAL DE PHYSIQUE

The t i m e d e l a y T f o r t h e t y p e o f d i s c h a r g e t u b e u s u a l l y used i s l i m i t e d b y t h e p a r a - s i t e i n d u c t a n c e L o f t h e plasma and t h e dynamic r e s i s t o r R : ^T = L X R-'.

I n t h e goal t o r e a c h t h e r e a l l i m i t where r i s r e f l e c t i n g t h e mechanisms o f t h e p h o t o e m i s s i o n and p h o t o i o n i z a t i o n , i t i s necessary t o m i n i a t u r i z e t h e d i s c h a r g e tube.

I n f o r t u n a t e l y , as p r e d i c t e d by EYTAN and KOPEIKA /19/ t h e p a r a s i t e c a p a c i t o r i s a c t i n g i n t h e o p p o s i t e d i r e c t i o n t o i n c r e a s e t h e t i m e d e l a y ^T = RC.

F o r t h e v e r y small s i g n a l s , t h e s e n s i t i v i t y o f t h e d e t e c t i o n i s l i m i t e d by t h e n o i s e which c h a r a c t e r i z e s t h e regime o f t h e glow d i s c h a r g e i t s e l f . I n t h e subnormal regime, c o n d u c t i o n i s r e s u l t i n g f r o m t h e secondary e m i s s i o n b y t h e p o s i t i v e i o n bombardement on t h e cathode, and f o r a gas p r e s s u r e P a t t h e t e m p e r a t u r e T, t h i s regime i s c h a r a c t e r i z e d by a n o i s e spectrum /20/ w i t h an average f r e q u e n c y fi centered a t :

- i PO

T 1

f . = --- -

l b To P

w i t h

u i

: i o n m o b i l i t y

PO = 1 T o r r To = 273OK

b : average d i s t a n c e between t h e e l e c t r o d e s

c

⁼ average v a l u e o f t h e c o n t i n u o u s e l e c t r i c f i e l d between t h e e l e c t r o d e s .

I n t h e abnormal regime, a m p l i f i c a t i o n o f t h e charges i s due t o cascade i o n i z i n g c o l - l i s i o n s and t h e hazardous f l u c t u a t i o n s o f t h e a r r i v a l r a t e o f t h e e l e c t r o n s a t t h e anode determine t h e observed n o i s e s i m i l a r t o a thermal n o i s e . KOPEIKA /21/ g i v e s t h e f o r m u l a f o r produced a m p l i t u d e f l u c t u a t i o n s o f t h e p o t e n t i a l a t t h e e l e c t r o d e s

w i t h B : bandwidth o f d e t e c t o r which i s used t o measure t h e n o i s e V, R, : e q u i v a l e n t r e s i s t o r o f t h e t u b e

= kTe : average k i n e t i c energy of t h e e l e c t r o n s f o r e l e c t r o n i c t e m p e r a t u r e Te

Pdc : average e l e c t r i c power d i s s i p a t e d i n t h e t u b e N : t o t a l number o f f r e e e l e c t r o n s

V : e l a s t i c c o l l i s i o n frequency.

KOPEIKA and ROSEMBAUM /20/ mentioned t h a t i t was b e t t e r t o work i n t h e subnormal regime t h a n i n t h e abnormal regime f o r i n c r e a s e t h e s e n s i t i v i t y o f t h e o p t i c a l t r a n - s i t i o n O.G. d e t e c t i o n .

Gas d i s c h a r g e s a r e o f t e n c o n s i d e r e d t o be e l e c t r i c a l l y n o i s y d e v i c e s and i n many cases t h i s i s t r u e because o f t h e s p u t t e r i n g problems. However, p r o p e r l y designed d i s c h a r g e o r commercial h o l l o w cathodes have been o p e r a t e d w i t h a c u r r e n t n e a r t h e Shot n o i s e l i m i t .

W i t h n i c k e l e l e c t r o d e s , an h e a t p i p e d i s c h a r g e c e l l /22/ shown on F i g . 8 has been used f o r two-step p u l s e d o p t o g a l v a n i c spectroscopy i n Ba /23/ and has g i v e n a p o s i - t i v e column o f b a r i u m vapour w i t h a v e r y l o w n o i s e l e v e l .

I n d.c. d i s c h a r g e s o r h o l l o w - c a t h o d e lamps t h e e l e c t r o n t e m p e r a t u r e i s i n t h e o r d e r o f 3 0 eV and most o f t h e i n t e r m e d i a t e l e v e l s o f t h e atom a r e populated. So; i t i s v e r y i n t e r e s t i n g i n t h e s e cases t o do a b s o r p t i o n spectroscopy f r o m e x c i t e d l e v e l s .

F i g . 8 : P r o d u c t i o n and e x c i t a t i o n by an e l e c t r i c d i s c h a r g e o f a h e a t - p i p e b a r i u m vapor

ELECTRIC WINDOW

7 oooooo FURNACE

--- - - - -

---

p

-

- - - -

-

f---

-. ,,.-

^{- - p}

- -

He

^I

^{2~ T H ~}

11.3

-

O p t o g a l v a n i c spectroscopy w i t h r.f. d i s c h a r g e s

I

O p t o g a l v a n i c spectroscopy w i t h r.f. d i s c h a r g e has one advantage o v e r O.G. s p e c t r o s - copy w i t h d.c. d i s c h a r g e : t h e n o i s e caused by e l e c t r o d e s p u t t e r i n g can be avoided.

Besides a l o w p r e s s u r e r.f. d i s c h a r g e plasma i s h i g h l y homogeneous and has a r e l a t i - v e l y h i g h e l e c t r o n temperature. It i s a l s o a p p l i c a b l e f o r s t u d i e s o f s p e c i e s which a r e r e a c t i v e t o metal e l e c t r o d e s i n t h e d i s c h a r g e t u b e . T h i s method has been proposed as o p t i c a l impedance spectroscopy by STANCIULESCU e t a l . /24/ and used f o r m o l e c u l a r spectroscopy by SUZUKI 1251.

oooooo

I t c o u l d be expected t h a t , a t l e a s t i n t h e l i m i t o f v e r y s m a l l s i g n a l s , t h e f r a c - t i o n a l change o f t h e impedance nZ/Z observed i n t h e S.C.A. method and i n t h e d.c.

d i s c h a r g e becomes AZ/(Z-R) which much l a r g e r e f f e c t r e s u l t i n g f r o m t h e n e g a t i v e r e s i s t o r - R w h i c h c h a r a c t e r i z e s t h e c o u p l i n g o f t h e microwave o s c i l l a t o r w i t h t h e medium and l o w e r s t h e t o t a l impedance o f t h e system.

T N i

i : ELECTRODES

TEMPERATURE A ^I

I

I 'EQUILIBRIUM LIQUID-GAS LIMIT OF PHASE GAS-SOLID

NO EQUILIBRIUM

I

, L ~ m

10 20 30 4 0

There a r e t h r e e d e t e c t i o n schemes f o r t h e o p t o g a l v a n i c e f f e c t i n t h e r.f. d i s c h a r g e :

-

m o n i t o r i n g t h e r e f l e c t e d r.f. f i e l d i n t h e o s c i l l a t o r power u n i t /24/

-

p i c k - u p c o i l o f t h e r.f. c u r r e n t /25/.

The d i s c h a r g e p l a y s t h e p a r t of a t r a n s m i t t i n g antenna and t h e c i r c u i t , t h e r e c e i v i n g antenna as shown on F i g . 9a.

-

two e l e c t r o d e s measuring t h e f l u c t u a t i o n s o f t h e c u r r e n t /26/ as shown on F i g . 9b.

R . F TRANSMITTER

I

R.F TRANSMITTER

&

F i g . 9a F i g . 9b

C7-96 4OURNAL DE PHYSIQUE

The l a s t d e v i c e a l l o w s LABASTIE e t a l . /26/ t o e x t r a p o l a t e t h e l i n e w i d t h observed a t z e r o d i s c h a r g e i n t e n s i t y f o r m i n i m i z i n g i n t h e i r measurements t h e s h i f t and broade- n i n g w i t h p r e s s u r e o f t h e a b s o r p t i o n l i n e s .

A l l t h e s e experiments u s i n g r.f. d i s c h a r g e have been made u s i n g chopped C.W. dye l a s e r s .

A t y p i c a l o p t o g a l v a n i c spectrum o f argon has been observed i n a d i s c h a r g e t u b e o f 30 mm 0.0. a t a p r e s s u r e o f 0.4 T o r r a 10 Watt-13.56 MHz r.f. power u n i t /25/.

A l l a t o m i c l i n e s l i s t e d i n t h e l i t t e r a t u r e i n t h e r e g i o n 580-630 nm has been o b s e r - ved as shown on F i g . 10.

F i g . 10 : O p t o g a l v a n i c spectrum o f A r d i s c h a r g e

f i O S Torr

* l o w

-

q-

630 620 610 660 590 500 (nn WAVELENGTH

"

v-

.p

$

^-C."

S e n s i t i v i t y o f t h e r.f. o p t o g a l v a n i c spectroscopy i s l i m i t e d b y t h e d i s c h a r g e n o i s e as shown by SUZUKI /25/. Noise spectrum on F i g . 11 shows a r e g i o n above 5 KHz where t h e m o d u l a t i o n i s s u i t a b l e f o r m i n i m i z i n g it.

F i g . 11 : N o i s e power spectrum o f d i s c h a r g e i n a r g o n t a k e n w i t h a bandwi t h o f 8 Hz

11.4

-

Advantages and disadvantages of O.G. d e t e c t i o n

The mean advantages of t h e O.G. d e t e c t i o n r e s u l t from t h e f a c t t h a t i t i s a non- o p t i c a l d e t e c t i o n of an o p t i c a l t r a n s i t i o n :

- ~ I I s o l i d e angle d e t e c t i o n ,

-

no background from s c a t t e r e d e x c i t a t i o n l a s e r l i g h t ,

-

high s e n s i t i v i t y meanly f o r t h e S.C.A. method because of t h e l a r g e a m p l i f i c a t i o n f a c t o r ,

- use in l a r g e wavelength s c a l e ,

-

slow t r a n s i e n t s i g n a l with pulsed l a s e r e x c i t a t i o n .

The p r i n c i p a l disadvantages a r e due t o t h e discharge i t s e l f :

-

low noise l e v e l l i m i t e d by t h e s p u t t e r i n g in t h e hollow-cathode lamps,

-

s e n s i t i v i t y t o r . f . n o i s e s ,

-

time delay l i m i t e d by t h e c i r c u i t feeding t h e d i s c h a r g e ,

-

non l i n e a r i t y of t h e O.G. signal depending of t h e p o s i t i o n of t h e l a s e r beam in t h e plasma between t h e e l e c t r o d e s and from t h e discharge c o n d i t i o n s .

I11

-

OPTOGALVANIC SPECTROSCOPY WITH ATOMS

The very s i m p l i c i t y of t h e O.G. d e t e c t i o n i s one of t h e most a t t r a c t i v e a s p e c t s of i t use i n l a s e r spectroscopy. Optogalvanic d e t e c t i o n provides a convenient means of mea- suring an o p t i c a l resonance without r e q u i r i n g any conventional o p t i c a l d e t e c t o r . The space charge a m p l i f i c a t i o n method has been prooved t o be extremely s e n s i t i v e f o r high Rydberg l e v e l s t u d i e s because t h e r a d i a t i v e l i f e t i m e of these l e v e l s i n c r e a s e s a s n3 (n i s t h e p r i n c i p a l quantum number) p r o h i b i t i n g t h e f l u o r e s c e n t d e t e c t i o n method. B u t a g r e a t advantage i n O.G. spectroscopy proceeds from t h e use of glow discharges and r . f . discharges where t h e e l e c t r o n temperature i s s u f f i c i e n t l y high t o c r e a t e s i g n i - f i c a n t populations in t h e e x c i t e d s t a t e s of t h e discharge medium ( t h i s i s s p e c i a l l y t r u e f o r t h e metastable o n e s ) . This method opens a l a r g e f i e l d of i n v e s t i g a t i o n s in l a s e r absorption spectroscopy f o r thermally vapourized elements from e x c i t e d l e v e l s i n a c e s s i b l e by d i r e c t r a d i a t i v e t r a n s i t i o n (metastable l e v e l ) o r r e q u i r i n g vacuum u l t r a v i o l e t r a d i a t i o n ( r a r e gas s t u d i e s ) , but a l s o f o r r e f r a c t o r y elements by using t h e hollow-cathode lamps which a r e r e a d i l y a p p l i c a b l e set-up f o r t h e i r s p u t t e r i n g /27/. The high s e n s i t i v i t y of t h e method /7/ i s a l s o used in t r a c e element a n a l y s i s in flames.

The p r i n c i p a l l i m i t a t i o n s of t h e method a r e due t o t h e gas pressure and t h e i n t e n s i t y of t h e c u r r e n t needed t o o p e r a t e a q u i e t regime of t h e discharge i t s e l f which a r e responsable f o r t h e pressure and t h e Stark broadening of t h e l i n e s meanly f o r t h e high lying Rydberg l e v e l s .

I11 . l

-

Doppler l imited O.G. spectroscopy

Laser O.G. spectroscopy r e q u i r e s tunable dye l a s e r s which can be C.W. o r pulsed. In t h e v i s i b l e wavelength region (near I.R. t o 480 nm) most of t h e experiments have been performed i n i t i a l l y using broad-band operated C . W . dye l a s e r s pumped by ion l a s e r s . In t h e s h o r t v i s i b l e wavelength region ( u n t i l 380 nm) pulsed dye l a s e r s o f f e r t h e advantage of l e s s expensive c o s t , wider t u n a b i l i t y , high peak power and e a s i e r frequency mixing o r doubling in c r i s t a l s . Multiphotons and m u l t i s t e p e x c i t a - t i o n s can be e a s i l y performed t o reach high-lying l e v e l s and increased t h e s e n s i t i v i - t y of t h e O.G. d e t e c t i o n because of t h e i r e a s i e r i o n i z a t i o n .

C7-98 JOURNAL DE PHYSIQUE

I n g e n e r a l , l a s e r l i n e w i d t h s (1.5 t o 3 GHz) a r e i n t h e o r d e r o f t h e D o p p l e r w i d t h much l a r g e r t h a n t h e p r e s s u r e o r S t a r k broadening o f t h e l i n e s . These s t u d i e s a r e s u i t a b l e f o r wavelength c a l i b r a t i o n and t e r m a n a l y s i s o f t h e a t o m i c s p e c t r a .

Table I g i v e s t h e p r e s e n t s t a t u s o f elements where O.G. e f f e c t has been observed and t h e c o r r e s p o n d i n g r e f e r e n c e s can b e e a s i l e r f o u n d i n t h e b i b l i o g r a p h y a t t h e end o f t h e p r o c e e d i n g book.

R.F.

*

^{0 . c .} _DISCHARGE

H . C A DIODE

Table I : P r e s e n t s t a t u s o f elements where O.G. e f f e c t has been observed

Three s p e c t r a r e c o r d e d i n t h e l a b o r a t o r y and reproduced on F i g . 12a, b and c summa- r i z e t h e s e p r o p e r t i e s o f t h e D o p p l e r O.G. spectroscopy.

111.2

-

Doppler f r e e O.G. spectroscopy

S i n c e t h e a v a i l a b i l i t y o f monomode t u n a b l e dye l a s e r s , v a r i o u s o p t i c a l e x c i t a t i o n t e c h n i q u e s have been used i n f l u o r e s c e n c e spectroscopy as quantum b e a t s , l e v e l c r o s - s i n g , two-photon and s a t u r a t e d a b s o r p t i o n spectroscopy t o e l i m i n a t e t h e D o p p l e r b r o a - dening.

111.2.1

-

Two-photon a b s o r p t i o n spectroscopy

The use o f two-photon a b s o r p t i o n (one photon from each o f two c o u n t e r p r o p a g a t i n g beams) t o e l i m i n a t e D o p p l e r b r o a d e n i n g o f s p e c t r a l l i n e s was suggested by VASILENKO e t a l . /30/ and CAGNAC e t a l . /31/.

I t i s r e a d i l y seen, f r o m t h e r e f e r e n c e frame o f any atom (moving w i t h a v e l o c i t y vx i n t h e d i r e c t i o n o f t h e l a s e r s beams) t h a t t h e photon f r e q u e n c i e s absorbed f r o m t h e two c o u n t e r - p r o p a g a t i n g l i g h t beams a r e ^U ( I + ( v x / c ) ) and ( l - ( v x / c ) ) , t h e i r sum b e i n g ZU, independant o f atomic motions a t t h e f i r s t o r d e r o f a p p r o x i m a t i o n . Thus a b s o r p t i o n o f two photons by one atom, one f r o m each o f t h e opposed l i g h t beams l e a d s t o t h e o b s e r v a t i o n o f D o p p l e r - f r e e s p e c t r a l l i n e s . I t s h o u l d be p o i n t e d o u t t h a t i n t h i s case a l l t h e atoms o f t h e d i f f e r e n t group v e l o c i t i e s a r e c o n t r i b u t i n g t o t h e

F i g . 12a : Two-step o p t o g a l v a n i c spectrum o f barium. ( a ) Fabry-Perot f r i n g e s

~ a = 1 . 3 1 4 6 7 ( 8 ) cm-'; ( b ) Spectrum o f Ba w i t h 5d6s 3D,-5d6p 3F: : 7280.3

a

f o r t h e f i r s t - s t e p t r a n s i t i o n i n t h e v i c i n i t y of t h e 5d7d 'D, and 3 ~ , ; ( C ) Same spectrum w i t h o n l y t h e second-step l a s e r /28/

F i g . 12b : Two-step o p t o g a l v a n i c spectrum o f k r y p t o n f r o m t h e 5s13/21, m e t a s t a b l e s t a t e . ( a ) K r I spectrum, o b t a i n e d w i t h t h e 2p, i n t e r m e d i a t e l e v e l , i n t h e range 680.88-680.58 nm ; (b) Fabry-Perot f r i n g e s ( t h e d r i f t i s due t o an a c c i d e n t a l temperature change o f t h e photomul t i p 1 i e r ) . ( c ) M o l e c u l a r i o d i n e a b s o r p t i o n spectrum 1291

F i g . 12c : Two-photon spectrum i n europium u s i n g a d i o d e w o r k i n g i n t h e space charge regime showing .a two-photon e l e c t r i c dipo- l a r t r a n s i t i o n and a one- photon e l e c t r i c quadru- p o l a r t r a n s i t i o n

(CAMUS

P.,

DAMASCHINI R.

and WYART J.F., unpubl i- shed m a t e r i a l

,

1981 )

"-.J&

TWO-PHOTON D E RESONANCE ONE PHOTON 0 E RESCNbNCE

4t1 6 9 - 4 1 7 ( 8 ~ b 7 3 'sov2 4t76r2 8 7 , 2 - 4 1 7 ~ 8 ~ 1 566s 80912

C7-100 JOURNAL DE PHYSIQUE

s i g n a l . Two-photon a b s o r p t i o n has been used i n O.G. spectroscopy w i t h t h e space charge a m p l i f i c a t i o n method i n r u b i d i u m b y

HARVEY

and STOICHEFF /32/ t o show t h e S t a r k b r o a d e n i n g and s p l i t t i n g o f t h e Rydberg l i n e s ( n 2 40) g i v i n g a s t r o n g l i m i t a - t i o n t o t h e t h e r m i o n i c d e t e c t o r . They used an e l e c t r o s t a t i c a l l y s h i e l d e d d e t e c t o r and observed sharp, f i e l d - f r e e components even a t n = 85. R e s u l t s a r e shown on F i g . 13a and 13b

Rb 5s -47d

TWO

-

PHOTON TRANSITION STARK SPLITTING E.0.5 V/cm

l

F i g . : 13a : Spectrum o f t h e 5s-47d t r a n s i t i o n i n Rb showing t h e S t a r k s p l i t t i n g a t a f i e l d o f 0.5 V/cm i n t h e t h e r m i o n i c d e t e c t o r

I l l I J

0 I 2 3 4

FREQUENCY (GHz)

F i g . 13b : Spectrum o f t h e 5s-75d t r a n s i t i o n i n Rb showing t h e sharp, f i e l d - f r e e components observed w i t h t h e e l e c t r o s t a t i c a l l y s h i e l d e d t h e r m i o n i c d e t e c t o r

Very few experiments where performed a t h i g h r e s o l u t i o n using pulsed l a s e r l i g h t on account f o r t h e d i f f i c u l t i e s encounted t o g e t a monomode intense tunable pulsed dye l a s e r . Using a Hansch h i g h r e s o l u t i o n apparatus, t h e Zeeman s t r u c t u r e o f t h e 6s54d 'D, Rydberg l e v e l has been observed i n barium u s i n g a thermionic diode and a l i n e a r l y p o l a r i z e d l a s e r l i g h t perpendicular t o a magnetic f i e l d B o f 0.125 T . The zeeman s p l i t t i n g reproduced on t h e F i g . 14 shows c l e a r l y t h e paramagnetic s h i f t E p r o p o r t i o n a l t o B and t h e diamagnetic s h i f t A p r o p o r t i o n a l t o B,.

F i g . 14 : Two-photon Doppler-free spectrum o f barium : Zeeman spl i t t i n g o f 6s54d 'D, Pydberg l eve1 (P. CAMUS and C. MORILLON, unpublished m a t e r i a l , 1979)

Since t h i s time, narrow-band C.W. r i n g dye l a s e r was a p p l i e d i n combination w i t h space-charge l i m i t e d thermionic diode f o r powerful h i g h r e s o l u t i o n spectroscopy and r e p o r t s w i l l be given a t t h i s conference on magnetic and e l e c t r i c f i e l d s t u d i e s f o r h i g h Rydberg s t a t e s (n

*

150) i n rubidium by GAY e t a l . /34/ and i s o t o p i c s h i f t s and h y p e r f i n e s p l i t t i n g s i n calcium by BEIGANG e t a l . /35/.

Two-photon optogalvanic spectroscopy (T.O.G.S.) has a l s o been done i n a helium-neon discharge tube by GOLDSMITH e t a l . /36/. They have placed t h e He-Ne discharge i n s i d e t h e C.W. l a s e r c a v i t y t o take advantage o f t h e h i g h c i r c u l a t i n g power and have observed t h i r t e e n two-photon neon t r a n s i t i o n s i n t h e tunning range o f Rhodamine 6G.

They performed t h e f i r s t Doppler-free observation o f two-photon t r a n s i t i o n s o r i g i n a - t i n g from energy l e v e l s o t h e r than ground o r metastable l e v e l s . Doppler-free obser- v a t i o n has a l s o been done i n helium f o r t h e 2 3~

-

5 3~ and 2 3S

-

5

3 D

two- photon t r a n s i t i o n s by GOLDSMITH and SMITH /37/.

But t h e more spectacular Doppler-free spectroscopy o f atoms i n e x c i t e d s t a t e s has been demonstrated i n an helium d.c. discharge u s i n g a technique c a l l e d intermodula- t e d optogalvanic spectroscopy (I.M.O.G.S.) by LAWLER e t a l . /38/ which i s d e r i v e d from t h e very s e n s i t i v e s a t u r a t i o n spectroscopy.

111.2.2

-

S a t u r a t i o n spectroscopy

I n t h e intermodulated optogalvanic spectroscopy, the o u t p u t o f a s i n g l e mode dye l a s e r i s s p l i t i n t o two beams o f equal i n t e n s i t y and sent through a p o s i t i v e column discharge i n opposite d i r e c t i o n s as shown on F i g . 15.

One beam i s chopped a t a frequency f, and the o t h e r a t frequency f,. They a r e amplitude modulated. I n general, because o f t h e Doppler s h i f t , t h e two beams w i l l i n t e r a c t w i t h d i f f e r e n t v e l o c i t y classes of atoms and no atom w i l l see b o t h beams and t h e r e s u l t i s a Doppler-broadened p a i r o f optogalvanic s i g n a l s modulated a t the frequencies f, and f,

.

JOURNAL DE PHYSIQUE

Fig. 15 : Experimental apparatus f o r intermodulated optogalvanic spectroscopy

However, i f the l a s e r i s tuned t o the c e n t r e o f t h e t r a n s i t i o n , t h e two beams w i l l i n t e r a c t w i t h t h e same group o f atoms, those t h a t have no v e l o c i t y along the beam a x i s and consequently f o r t h i s c l a s s o f atoms o n l y , no Doppler s h i f t i s observed.

The s a t u r a t i o n o f t h e t r a n s i t i o n caused by t h e two beams a c t i n g on t h e same group o f atoms produce Doppler-free s i g n a l s modulated a t the frequencies f, + f, and f,

-

^f,. The l o c k - i n a m p l i f i e r , w i t h a reference pick-up a t fl + f, g i v e s t h e Doppler-free s i g n a l modulated a t f, + f2.

Fig. 16 shows t h e 2 3P

-

3 3D 3He l i n e a t 587.5 nm recorded w i t h I.M.O.G.S. by LAWLER e t a l . /38/.

F i g . 16 : Intermodulated optogalvanic spectrum o f p a r t o f t h e 3He 2 3 ~ - 3 3 ~ t r a n s i t i o n a t 587.5 nm

L . I ' I L ' ' ' ' I ' I ' I v l

0 1 2 8 4 5 6 7 8

LASER FREOUENCY DETUNING ( G H Z ) Y

-

I.M.O.G.S. has been used f o r r e f r a c t o r y m a t e r i a l s e x c i t e d i n a hollow-cathode lamp f o r molybdenum by SIEGEL e t a l . /39/ b u t t h e recorded Doppler-free spectrum shows sharp peaks Doppler-free on t o p o f broad pedestals when a background gas pressure i n t h e order o f 0.1 T o r r o r g r e a t e r i s used. These pedestals a r e a t t r i b u t e d t o velocity-changing r e l a x a t i o n c o l l i s i o n s which tend t o r e d i s t r i b u t e t h e p o p u l a t i o n o f t h e ground o r metastable l e v e l atoms over t h e o r i g i n a l Maxwel l i a n d i s t r i b u t i o n andereduce the v e l o c i t y s e l e c t i o n o f t h e s a t u r a t i n g beams broadening consequently t h e Doppl e r - f r e e spectrum. This problem has been b e a u t i f u l l y overcome by t h e method o f p o l a r i z a t i o n - i n t e r m o d u l a t e d - e x c i t a t i o n (POLINEX) developed by HANSCH e t a l . /40/.

I n POLINEX t h e p o l a r i s a t i o n o f one o r both o f t h e beams i s modulated r a t h e r than t h e i n t e n s i t y o f t h e beams i n 1.P.I.D.G.S.. I n t h i s way, POLINEX u t i l i z e s t h e normally unwanted process o f c o l l i s i o n a l d e p o l a r i z a t i o n . By modulating the p o l a r i z a t i o n r a t h e r than t h e i n t e n s i t y , t h e detected Doppler-free s i g n a l s then a r i s e from l i g h t - induced atomic o r i e n t a t i o n w i t h t h e r e s u l t t h a t atoms which have s u f f e r e d v e l o c i t y -

changing c o l l i s i o n s w i l l no l o n g e r c o n t r i b u t e a signal i f t h e i r o r i e n t a t i o n a r e des- troyed. POLIXEX can take advantage o f t h e s e l e c t i o n r u l e s f o r t h e absorption of p o l a r i z e d l i g h t t o y i e l d i n f o r m a t i o n on t h e angular momenta o f t h e p a r t i c i p a t i n g energy l e v e l S, as w e l l as on t h e r e l a x a t i o n o f l ight-induced a1 ignnent. This has been demonstrated f o r neon where second o r d e r pumping e f f e c t induces a modulated d e p l e t i o n o f t h e metastable Zp5 5s l e v e l /41/.

The intermodulated s i g n a l does n o t have t o be detected on a s t r o n g l y modulated background, i f t h e t o t a l e x c i t a t i o n r a t e i s observed because n e i t h e r beam alone can produce a mo- d u l a t e d s i g n a l i n an i s o t r o p i c mediun;. The experiment can be s i m p l i f i e d as shown on F i g . 17, because t h e intermodulated s i g n a l S

a r e observed w i t h a good s e l e c t i v i t y even i f o n l y one o f the beams i s modulated.

MODULATOR

I

Fig. 18a, b, c and d show comparisons b e t -

AMPLIFIER ween d.c. and r.f. optogalvanic POLINEX and

T-- I intermodulated fluorescence records on neon

A M P : ~ ~ E R l i n e s made by HANSCH e t a1

.

^/40/.

F i g . 17 : Setup f o r Doppler-free ON^

POLINEX spectroscopy o f he1 ium

2 0 ~ e

a

2 2

Ne ( C )

- l

L A S E R T U N I N G ( G H z )

'ON*

L l I I I I I I I ~ I I I ~ I I I

0 I 2 3 4

LASER DETUNING [ ~ n r ]

LASER TUNING ( G H z l

F i g . 18a,b : ( a ) Doppler-free POLINEX spectrum o f t h e Ne ls,-2p, t r a n s i t i o n . ( b ) Same spec- trum recorded by the i n t e r - modulated fluorescence method

F i g . 18c,d : Doppler-free spectrum o f t h e Ne 2p,-Is, t r a n s i t i o n a t 585.2 nm, recorded by

( c ) intermodulated r.f.

optogalvanic detection, ( d ) intermodul ated f lu o - rescence d e t e c t i o n

I t can be seen on Fig. 18a t h a t t h e O.G. POLINEX r e c o r d i s f r e e o f the broad pedes- t a l s which appear on t h e intermodulated fluorescence record o f t h e same neon l i n e . I n t h i s case, because o f the q u i e t regime o f t h e discharge, we should n o t i c e t h e comparable s i g n a l t o n o i s e r a t i o o f t h e O.G. and fluorescence spectra.

C7-104 JOURNAL D€ PHYSIQUE

Doppl e r - f r e e optogal vanic spectroscopy (I .M.O.G.S. ) as a l s o been used f o r Zeeman e f f e c t s t u d i e s i n Ne and Ca by BEVERINI e t a l . /42/.

111.2.3

-

Level c r o s s i n g spectroscopy

Optogalvanic spectroscopy has a l s o been extended t o the d e t e c t i o n o f l e v e l c r o s s i n g phenomena, s i m i l a r t o t h e Hanle e f f e c t , by HANNAFORD and SERIES i n Z r I /43/ and l e a d the authors t o measurements o f Land6 g - f a c t o r s /44/. They have a l s o measured u s i n g the same l e v e l - c r o s s i n g optogalvanic spectroscopy h y p e r f i n e s t r u c t u r e s i n Y I /45/.

I V

-

CONCLUSION

I n t h e e a r l y days o f optogalvanic spectroscopy a l l t h e spectra observedwereDoppler- l i m i t e d because o f t h e l a c k o f monomode tunable dye l a s e r s . I n t h e r e c e n t years s i n c e t h e i r discovery, power f u l l and s o p h i s t i c a t e d techniques o f Doppler-free l a s e r spectroscopy are j u s t beginning t o be a p p l i e d and adapted t o optogalvanic d e t e c t i o n as i t w i l l be demonstrated by t h e f o l l o w i n g r e p o r t s presented a t t h i s colloquium.

The l a r g e increase i n t h e q u a l i t y o f t h e s i g n a l t o n o i s e r a t i o can be p o i n t e d o u t i n Fig. 19 which reproduces the h y p e r f i n e s t r u c t u r e of t h e 591.5 nm U I l i n e r e c o r - ded i n 1979 by KELLER e t a l . /46/ and the same l i n e recorded by GAGNE e t a l . /47/

and presented a t t h i s colloquium.

The l a r g e number o f atomic resonances o f r e f r a c t o r y elements t h a t hollow-cathode lamps render accessible, combined w i t h t h i s new wave o f h i g h r e s o l u t i o n techniques make optogalvanic spectroscopy one o f t h e most important t o o l s t o i n v e s t i g a t e t h e i n t e r a c t i o n o f l a s e r r a d i a t i o n w i t h l i g h t gas media.

"5985

X l00

Fig, 19 : O.G. h y p e r f i n e s t r u c t u r e o f 591.5 nm U 1 l i n e i n H.C. lamp

X I0

?

? f l 1

a b < d h

KELLER R.A., EEIGLEWN R. J r and ZALENSKI G.F., 1979 /46/

---p

G 01 0 2 0 3

, . 3 t 2 5

-

^{> E} 20

151jbbcK

-

l0 D E F G H

5 0

-IJ - l I )

0 5 10 GAGNE

J.M.,

DEMERS Y., PIANAROSA P.,

v (GHZ) 1: DREZE C., t h i s colloquium /47/

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