In trod u ction

The ch an ge i n i o n i s a t i o n i n a d is c h a r g e , c a u se d by i l lu m i n a t i n g one d is c h a r g e w ith l i g h t from a n o t h e r , h a s b e e n known f o r a lm o s t s i x t y y e a r s . I n th e p a s t e i g h t y e a r s , w ith th e u s e o f tu n a b le dye l a s e r s to i r r a d i a t e d i s c h a r g e s , a re n e w a l o f i n t e r e s t i n t h i s so c a l l e d o p to g a lv a n ic e f f e c t h a s le d to i t s a p p l i c a t i o n to a l a r g e num ber o f s p e c tr o s c o p ic s t u d i e s . T h ese w i l l be o u t l in e d w ith p a r t i c u l a r r e f e r e n c e to s t u d i e s i n n e o n . I n t h i s w ork e x p e r im e n ta l s t u d i e s h av e b ee n c a r r i e d o u t i n a neon p o s i t i v e colum n d i s c h a r g e . The o p to g a lv a n ic l i n e p r o f i l e f o r th e ls ^ - 2 p ^ t r a n s i t i o n (P a s c h e n n o t a t i o n ) h a s b e e n s t u d i e d a s a f u n c tio n o f dye l a s e r pow er (0-300mW) and d is c h a r g e c o n d itio n s (2 -2 0 mA, 1 - 2 .5 T o rr) f o r a v a r i e t y o f e x p e rim e n ta l a r ra n g e m e n ts , in c o r p o r a tin g s im u lta n e o u s a b s o r p ti o n and e m is s io n s t u d i e s . U nder c e r t a i n d is c h a r g e c o n d itio n s th e s ig n o f th e o p to g a lv a n ic s i g n a l ch a n g es a s a f u n c ti o n o f dye l a s e r po w er.

I n an a tte m p t to m odel th e above b e h a v io u r a s e t o f fo u r r a t e e q u a tio n s d e s c r ib in g th e p o p u la tio n s o f th e I s y , I s ^ and 2p^ l e v e l s in n e o n , w ith and w ith o u t r e s o n a n t l a s e r i r r a d i a t i o n on th e ls ^ - 2 p ^ t r a n s i t i o n a t 5 8 8 .2nm, a r e p ro p o se d and s o lv e d i n th e s te a d y s t a t e . The s te a d y s t a t e v a lu e s a r e u sed a s i n i t i a l v a lu e s f o r n u m e ric a l i n t e g r a t i o n o f th e r a t e e q u a tio n s to show th e tim e e v o l u t i o n o f th e v a r io u s p o p u la tio n s a s th e l a s e r i s s w itc h e d on and o f f . A q u a l i t a t i v e i n t e r p r e t a t i o n o f th e e x p e c te d o p to g a lv a n ic s i g n a l may be made from th e r e s u l t s . I n p a r t i c u l a r , i t i s s e e n t h a t b u ild - u p i n th e

I s ^ m e ta s ta b le d e n s i t y w ith l a s e r i r r a d i a t i o n on th e 5 8 8 .2nm t r a n s i t i o n ca n e x p la in th e o b s e r v a tio n o f a s ig n r e v e r s a l i n th e

— 68 — o p to g a lv a n ic s i g n a l i n a n eo n p o s i t i v e colum n d is c h a r g e , 4 .2 R eview o f O p to g a lv a n ic S p e c tro s c o p y When a d is c h a r g e i s i r r a d i a t e d w ith l i g h t o f a w a v e le n g th c o r re s p o n d in g to a n a llo w e d t r a n s i t i o n o f th e d is c h a r g e s p e c ie s th e p o p u la tio n s w i t h i n t h e d is c h a r g e ch an g e th u s g iv in g r i s e to ch a n g e s i n t h e r a t e o f i o n i s a t i o n . The m ost common e f f e c t i s an i n c r e a s e in th e io n d e n s i t y c a u se d by in c r e a s e d p o p u la tio n d e n s i t y o f h ig h ly e x c it e d s t a t e s due to o p t i c a l pum ping . T h ese s t a t e s h a v e h ig h e r c o l l i s i o n a l i o n i s a t i o n r a t e s a s a l a r g e r p e r c e n ta g e o f th e e l e c t r o n s i n th e d is c h a r g e h av e s u f f i c i e n t e n e rg y to c a u se i o n i s a t i o n . An i n c r e a s e i n t h e io n (an d e l e c t r o n ) d e n s i t y le a d s to a d ro p i n t h e v o lta g e a c r o s s th e d is c h a r g e tu b e r e q u ir e d to m a in ta in th e d is c h a r g e . T h is d ro p in v o lta g e may be m easu red a s a n i n c r e a s e i n v o lta g e a c r o s s a b a l l a s t r e s i s t o r i n s e r i e s w ith t h e d i s c h a r g e , a t i t s a n o d e , i n a c o n s ta n t v o lta g e s u p p ly re g im e . T r a n s i t i o n s w ith a m e ta s ta b le lo w e r l e v e l g iv e r i s e to e i t h e r i n c r e a s e s o r d e c r e a s e s in th e v o lta g e a c r o s s th e b a l l a s t r e s i s t o r d e p e n d in g upon d is c h a r g e c o n d itio n s and th e s p e c i f i c c o u p lin g m echanism s f o r th e p a r t i c u l a r a to m ic o r m o le c u la r s p e c i e s .

The f i r s t o b s e r v a tio n o f t h i s e f f e c t , c a l l e d t h e o p to g a lv a n ic e f f e c t , was made by P e n n in g [ l] who o b se rv e d a n i n c r e a s e i n th e v o lta g e a c r o s s a neon d is c h a r g e when i t was i l lu m in a te d by a seco n d n eon lam p . T h is o b s e r v a tio n was p r e d a te d by a r e l a t e d e x p e rim e n t [2] w here io n f o r m a tio n was d e t e c t e d by em p lo y in g th e rm io n ic d io d e s . S u b se q u e n t e x p e rim e n ts s tu d y in g th e ch an g e i n v o lta g e a c r o s s a d is c h a r g e tu b e i l lu m i n a t e d by a n o th e r a r e d e t a i l e d i n r e f e r e n c e s [ 3 - 9 ] . I n p a r t i c u l a r , D ro u e t and Novak [7] n o te d t h a t i l lu m i n a t i n g a neon d is c h a r g e r e s u l t s i n r e d u c t i o n s o f b o th th e e l e c t r o n d e n s i t y and th e

m e ta s ta b le d e n s i t y . They a l s o m easu red th e e l e c t r o n e n e rg y d i s t r i b u t i o n and fo u n d t h a t i l l u m i n a t i o n c a u se d a s h i f t o f e l e c t r o n s t o lo w er e n e r g i e s .

The r e c e n t r e v o l u t i o n in o p to g a lv a n ic s p e c tr o s c o p y was s t a r t e d when G reen e t a l [1 0] u s e d a co m m ercial CW dye l a s e r ( l i n e w i d t h o f a p p ro x im a te ly 0 .003nm , t u n a b le o v e r a ra n g e 540-675nm , w ith t y p i c a l pow ers o f 100-200mW) t o i n v e s t i g a t e th e o p to g a lv a n ic s i g n a l in d u c e d on t r a n s i t i o n s o f l i t h i u m , so d iu m , c a lc iu m , b a riu m , u ra n iu m , n eon and h e liu m i n co m m ercial h o llo w c a th o d e la m p s. The e x p e rim e n ta l a rra n g e m e n t, w h ich i s a s ta n d a r d on e f o r l a s e r l i n e w i d t h - l i m i t e d , o r D o p p le r - lim ite d s t u d i e s , i s shown i n f i g u r e 4 .1 . The l a s e r beam i s ch opped a t fre q u e n c y f and t h e o p to g a lv a n ic s i g n a l p ro d u c ed a t t h i s fr e q u e n c y , d e v e lo p e d a c r o s s r e s i s t o r R , i s d e te c te d by a l o c k - i n a m p l i f i e r . C a p a c ito r C i s u s e d to b lo c k th e dc ano de v o l t a g e . The a d v a n ta g e s o v e r f lu o r e s c e n c e s p e c tr o s c o p y a r e t h a t no o p t i c a l d e t e c t i o n a p p a r a tu s i s r e q u i r e d , th e s i g n a l o b ta in e d i s i n s e n s i t i v e to s c a t t e r e d e x c i t a t i o n l i g h t and b ac k g ro u n d l i g h t , and n o n - v o l a t i l e s p e c ie s c a n be s tu d ie d i n h o llo w c a th o d e s . The o p to g a lv a n ic e f f e c t h a s b ee n u se d to f r e q u e n c y - lo c k a CW dye