COMPARISON OF FIELD IONIZATION YIELDS FROM FOIL AND GAS EXCITED RYDBERG STATES IN FAST IONS

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COMPARISON OF FIELD IONIZATION YIELDS FROM FOIL AND GAS EXCITED RYDBERG

STATES IN FAST IONS

D. Schneider

To cite this version:

D. Schneider. COMPARISON OF FIELD IONIZATION YIELDS FROM FOIL AND GAS EXCITED

RYDBERG STATES IN FAST IONS. Journal de Physique Colloques, 1987, 48 (C9), pp.C9-279-C9-

284. �10.1051/jphyscol:1987949�. �jpa-00227367�

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JOURNAL DE PHYSIQUE

Colloque C9, supplement au n012, Tome 48, decembre 1987

COMPARISON OF FIELD IONIZATION YIELDS FROM FOIL AND GAS EXCITED RYDBERG STATES I N FAST IONS

D. SCHNEIDER

~ a h n - M e i t n e r - I n s t i t u t , P O Box 390 128, 0-1000 B e r l i n 39, F - R - G .

ABSTRACT

Recent resu1.l.s o f an i r l v e o t i g a t i o n o f e f f e c t s o f e x t e r n a l e l e c t r i c f i e l d s or) liiyh Rydberg s t a t e s which a r e forcned i n f o i l and y a s i n t e r a c t i o n s O F f a s t (MeV) i o n s a r e r e p o r t e d . A b s o l u t e f i e l d i o n i z a t i o n y i e l d s o f t h e e x c i t e d Rydberg s t a t e s a r e measured f o r a v a r i e t y o f beam i o n s . Comparison between t h e q u a n t u r ~ s t a t e p o p u l a t i o n f o r t h e g a s and f o i l e x c i t a t i o n and model c a l c u l a t i o n s a r e c o n s i s t e n t with t h e a s s u ~ n p t i o r i t h a t t h e Rydbery s t a t e s a r e p r e d o m i n a n t l y f o r r ~ ~ e d i n t h e l a s t l a y e r s o f t h e s o l i d t a r g e t . The e f f e c t o f a v a r i a b l e weak s t . a t i c e l e c t r i c f i e l d a p p l i e d t o tlie Rydberg s t a t e s , c a u s e s rnarked o s c i l l a t o r y s t r u c t u r e s i n llle i o r l i z a l i o n y i e l d a s a ru11cL ion o f Lhe f i e l d s t r e n y t t , . l t l e s e o s c i l l a t i o r i s , o b s e r v e d f o r g a s a r ~ d f o i l e x c i t a t i o n , a r e i r i t e r p r e l e d i n t e r ~ ~ ~ s OF cluantur~ i n t e r f e r e n c e s anlong S t a r k s l a t e s c o l ~ e r e ~ l t l y d e v e l o p i n g i r l t h e weak r i e l d .

Near t h r e s h o l d e x c i t a t i o n , wilere e l e c t r o n c a p t u r e i n t o p r o j e c t i l e c e n t e r e d corltinuun o r Rydberg s t a t e s o c c u r s i n i n t e r a c t i o r ~ s o f f a s t h i g h l y c h a r y d

e

p r o j e c t i l e s w i t h s o l i d s h a s been i n t e n s i v e l y i n v e s t i g a t e d f o r sorne time

.

Yet, t h e r e is s t i l l no s u f f i c i e n t t h e o r e t i c a l model t o account f o r t h e va- r i o u s experirnerital f i n d i n g s . S t u d i e s c o n c e r n e d w i t h t h e f o r m a t i o n o f Rydberg s t a t e s ( e . g . r1>5U) i n f a s t i o n s emerging fro111 f o i l t a r g e t s h e l p t o p r o v i d e an u n d e r s t a r l d i n g o f l h e p r o c e s s e s irivolved i n t h e t h r e s h o l d e x c i t a - t i o n i n i o n - s o l i d i n t e r a c t i o n s .

Q u e s t , i o r ~ s , which a r e a d r e s s e d , a r e c o n c e r n i n g t h e e f f e c t o f v a r i o u s b u l k arld s u r f a c e p r o c e s s e s i n Llie i o n - s o l i d i n t e r a c t i o n or1 t h e p o p u l a t i o n o f Rydbery s t a t e s , t h e arlgular d i s t r i b u t i o n o f convoy e l e c t r o n s , and t h e i r mean f r e e p a t h . R e c e n t l y d e v e l o p e d t h e o r e t i c a l t n o d e l s 2 ' 3 f o r e l e c t r o n s n e a r Lhreshold under t h e i n f l u e t ~ c e o f s t o c h a s t i c s c a t t e r i n g p r o c e s s e s i n t h e s o - l i d and a s t r o i i g i o n i c Couloinb r i e l d t o g e t h e r witir new experimental r e s u l t s rnay i l l u r n i t l a t e t h e proble~rl o f tlie s t a b i l i l y of Rydbery o r b i t s under t h e i n - f l u e l l c e or s t o c h a s t i c p e r l u r b a t i o n s a s w e l l a s t h e t r a n s i ~ l i s s i o r l o f convpy e l e c t r o r l s llirough s o l i d s .

The p r o d u c t i o n ~~reclianisrr~s of F o i l - e x c i t e d Rydberg s t a t e s (125 MeV s 1 4 + o n [:-foil) were d i s c u s s e d p r e v i o u s l y on t h e b a s i s o f ~ n e a s u r e ~ n e n t s u s i n g t h e f i e l d - i o ~ l i z a t i o n t e c h n i ~ ~ u e . ~ ' ~ 1 t was a r g u e d t h a t t h e r e s u l t s a r e cornpa- tib.Le w i t t i t h e a s s u ~ n p t i o n t h a t l a s l - l e y e r c a p t u r e is-: t h e dominant produc- t i o n ~ n e c h a n i s ~ n . The r e s u l t s o f ~ r ~ e a s u r e ~ n e n t s o f d e l a y e d - x-ray e m i s s i o n f o l - lowing beam-foil e x c i t a l i o n 6 have been a n a l y z e d by means o f c a s c a d e c a l c u - l a l i o r l s . C o n f . l i c t i n g r e s u l t s r e g a r d i n g t h e i n i t a l p o p u l a t i o n o f t h e Rydberg s t a t e s have e ~ a e r y e d fro111 t h e s e a n a l y s e s . I t h a s been c o n c l u d e d t h a t f o i l e x c i t a t i o n l e a d s t o a p r e f e r e n t i a l p o p u l a t i o n o f s t a t e s with h i g h o r b i t a l a n y u l a r ino~~leritu~il I . Such a b e h a v i o r o f t h e 1 d i s l r i b u t i o n is a t v a r i a n c e wit.h t h e u s u a l assurtiption t h a t d i r e c t c a p t u r e o f t a r g e t e l e c t r o n s ( " l a s t - l a y e r c a p t u r e " ) is t h e doir~inarit p r o d u c t i o n rnechanisrrt o f h i g h Rydbery s t a t e s . Measure~r~ent o f t h e l i g l ~ t e ~ ~ ~ i s s i o r i from f o i l e x c i t e d Rydberg s t a t e s

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987949

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C9-280 JOURNAL DE PHYSIQUE

n e a r tlie Yrast l i n e i n f a s t F- and S i - bean1 i o n s have a l s o been r e p o r t e d r e c e n t l y . In b o l h c a s e s t h e d a t a were d e s c r i b e d q u i t e w e l l w i t h an e m p i r i - c a l p a r a ~ n a l r i z a t ion f o r t h e a v e r a q e p o p u l a l i o n p e r r i l Rydbery s t a t e , w i t h tile l i y l l t e ~ s i s s i o n showing a deperidence on t h e c o r e c h a r y e o f t h e p e n e t r a - t i n y i o n s a s w e l l a s on t h e n u c l e a r cliarqe. l h i s is c o n s i s t e n t w i t h a p i c - t u r e where e l e c t r o n c a p t u r e dominates t h e p r o d u c t i o n o f R y d b e r g - s t a t e s . llere new r e s u l t s 01 a n i n v e s t i g a t i o n o f a b s o l u t e f i e l d i o n i z a t i o n y i e l d s

Following g a s and f o i l e x c i t a t i o n a r e p r e s e n t e d . 8

I t s h o u l d be n o t e d t h a t Lhe a b s o l u t e p r o d u c t i o n y i e l d s o f Rydbery s t a t e s lor~rled i n f a s t c o l l i s i o n s p r o v i d e s i n f o r ~ ~ ~ a t i o n needed i n f o r exarrlple t h e d e s i y n arid develop~ltent OF s t o r a g e r i n y s , f o r Iieavy i o n s and i n t h e d e s i y n o f s l ~ o r t wave-length l a s e r scfierrres i n v o l v i r ~ g h i g h Rydberg s t a t e s .

11, o r d e r t o s t u d y t l ~ e lo r ~ n a t i n r i 01 Rydbery s t a t e s i n more d e L a i l c o n t i n u u ~ ~ ~ a r ~ d R y d b e r y - s t a t e e l e c l r o n c a p t u r e a c c o ~ ~ r l ~ a n y i n y b o u n d - s t a t e c a p t u r e h a s been i n v e s l i q a t e d v i a c o i r i c i r l e r ~ c e ~ r l e a s u r e r ~ ~ e ~ ~ t s between e l e c t r o n s l o l l o w i r ~ y f i e l d i o n i z a t i o n and p r o j e c t i l e cllarge s t a t e s . The very r e c e n t r e s u l t s s u y q ~ s l l h a t e l e c t r r ~ ~ l c o r r e l a t i o r l rllay b e r e s p o n s i b l e l o r a r e l a t i v e l y Iiigh [ ~ r o b a l ~ i l i t y 01 o c c u r a n c e o r t w o - e l e c t r o n c a p t u r e p r o c e s s p o p u l a t i r l y low arld Iiiyli 11-slate ( I l e l . 9 ) .

In Lhe s c o p e o f t l l e s e r r ~ e a s u r e ~ r ~ e ~ i t s , s y s t ~ ~ l ~ a l i c s t u d i e s o f t h e i n t l u e n c e o f d i f f e r e n t t y p e s o f e l e c t r j c r i e l d s or1 r o i l - e x c i t e d Rydbery s t a t e s i r i F a s t iorls have heell perforrrrerl. l o ' l 1 A r e ~ ~ l a r k a l I l e r e s u l t o f t h e s e s t u d i e s was t h e o b s e r v a t i o t i o f o s c i l l a t o r y s L r u c L u r e , a s a f u n c t i o r i 01 t h e sLrenyLh o f a weak l o ~ i q i t u d i n a l f i e l d , i n t h e i r i l e g r a l e l e c t r o n y i e l d f o l l o w i n q F i e l d i o r ~ i z a t i o n . l h e o s c i l l a t iorls were a s s o c i a l e d w i l t 1 quantclr~ i r l t e r r e r e n c e s ( " S t a r k b e a t s " ) alrionq S t a r k s t a t e s c o h e r e ~ i t l y d e v e l o p i n g i n t h e weak l i e l d . l h i s work h a s beer1 c o n t i n u e d and h e r e r e s u l t s a r e r e p o r t e d where t h e lielrl-ionizat.ior1 technique Itas been o ~ l p l o y e d i n a s t u d y o f neon Rydbery s t a t e s f o r ~ r ~ e d i n i n t e r a c t i o ~ r s 01 85-MeV ~ e i o n s w i t h a c a r b o n - f o i l t a r g e t ~ + and w i t h a tie1iurr1-yas t a r g e t . ' *

The p r e s e t i t e d r e s u l t s a r e fro111 ~ r l e a s u r e ~ ~ l e n t s c o n d u c l e d a t t h e VlCKSl Iieavy- i o n f a c i l i t y a t tlle t l a h r i - M e i t n e r - l n s t i t u t , B e r l i n . The e x p e r i m e n t a l s e l u p

Deflector Plates

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F i y . 1: Sclienlatic diagram OF t l ~ e e x p e r i ~ ~ ~ e r i t a l s e l u p .

( F i g . 1 ) is s i a ~ i l a r t o t h e one d e s c r i b e d i n d e t a i l i n Ref. 12. A t i g h t l y collimated beam o f i o n s l r a v e r s e s a c a r b o n - f o i l o r g a s - t a r y e t c e l l . A f t e r h a v i n g t r a v e l e d a c e r t a i n d i s t a n c e Froill t h e g a s t a r g e t - c e l l e x i t o r from t l l e f o i l t a r g e t , t h e i o n beam t r a v e r s e s t h e f i r s t a t a y e o f a n e l e c t r o s t a t i c 4 9 p a r a l l e l - p l a t e taridern e l e c t r o n s p e c t r o n ~ e t e r . The e l e c t r i c f i e l d o f t h e F i r s t s t a q e 01 t h e spectrometer s e r v e s t o F i e l d i o n i z e e l e c t r o n s from pro- j e c t i l e Rydbery s t a t e s and t o s e p a r a t e t h e i o n i z e d e l e c t r o n s from t h e beant. The e l e c t r o n s a r e a n a l y z e d and a r e F i n a l l y r e c o r d e d by cneans o f an e l e c t r o n r r ~ u l t i p l i e r . In a d d i t i o n t h i s s e t - u p h a s been m o d i f i e d by i n s t a l - l i n g a p a i r o f d e f l e c t o r p l a l e s a f t e r t h e t a r g e t . With l h a t a r r a r i g e ~ n e n t t h e f i e l d i o ~ l i z a t i o n y i e l d h a s been rr~easured a s a f u n c t i o n o f a weak e l e c t r o - s t a t i c f i e l d i n t h e d e f l e c t o r .

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r i g . 2: Elect.ru11 s p e c l r a r o l l o w i r ~ g r o i l a r ~ d y a s e x c i t a t i o n .

T y p i c a l e l y c t r o n s p e c t r a f o l l o w i n g 140 MeV Ne g a s and f o i l e x c i l a - t i o r i a r e shown i n F i g . 2. The s p e c - l c a a r e d o ~ n i n a t e d by t h e c u s p peak c e n t . e r e d sl an energy which c o r r e s - ponds t o e l e c t r o n s t r a v e l i n g w i t h t h e beam v e l o c i t y . E l e c t r o n s r e s u l - t i n g from f i e l d i o n i z a t i o n o f p r o - j e c t i l e Rydberg s t a t e s i n t h e f i r s t s l a y e o f t h e s p e c t r o m e t e r a r e ex- p e c t e d l o be o b s e r v a b l e a t a n "ap- p a r e n t " e n e r g y somewhat l a r g e r t h a n t h e c u s p e n e r g y s i n c e f i e l d i o n i z a - t i o n t a k e s p l a c e o n l y a f t e r t h e Rydbery ioris have t r a v e l e d a s h o r l d i s t a n c e i n t o t h e s p e c t r o i n e t e r

.

T h i s s h i f t and t h e s h a p e o f t h e s p e c t r a h a s been c o n f i r~ned by model c a l c u l a t i o n s f o r tlie e l e c t r o t 1 t r a - j e c t p ; i e s t h r o u g h t h e s p e c t r o m e - t e r . The y i e l d s from e l e c t r o n s r : o n l r i b u t i n q l o f i e l d i o n i z e d Rydbery s t a t e s a r e d e t e r m i n e d frorn t h e i r i t e y r a t e d c o u n t r a t e a r t e r

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C9-282 JOURNAL DE PHYSIQUE

l l l a t t l i e y i e l d s Y I ~ o f e l e r l r o l l s rrorrr t l i e f i e l d i o n i z a t i o r i o f h i q h r i s t a t e s arP c 0 1 1 1 p ~ r a b 1 e Tor Llie cases o f yas and r o i l e x c i t a t i o n . T h i s i s s i y r l i f i c a r i t s i r i c e t h e y i e l t l s f o l l o w i r i g gas e x c i t a t i u r i were ~rleasured under s i r i y l e c o l l i s i o r ~ c o n d i t i o r i s . Ari e r r o r o f a b o u t 50% rrlust be taker1 i r i t o account f o r t l i e a b s o u l u t e f i e l d i o r i i z a t i o r i y i e l d s arid i s n i a i r i l y due Lo t l i e u r ~ c e r t a i r ~ L y o f t h e t r a r i s ~ ~ l i s s i o n o f t h e Larideln s p e c L r u r r ~ e t e r and due t o colie- rprlce e r f e c t s . F u r l l i e r i r l o r e T a b l e 1 sliows v e r y good ayreernent betweeri t h e t h e o r e t i c a l arid e x p e r i r r l e r i t s l P(n)n3 qr~ar)tlsrl s t a t e p o p u l a t i o n s f o r t l i e gas as w e l l f o r t l i e f o i l case. I l i e e x c e l l e n t a y r e e i l ~ e n t between L l i e o r e t i c a l arid t : x p ~ r i ~ s ~ n t a l quarit~lrll s l a t e p o p u l a t i o n rimrlbers i n d i c a t e s t h a t L a s L - l a y e r (~rlealiirly t l i e l a s t 50 l a y e r s o r so) e l e c l r o r i c a p t u r e i s r e s p o r t s i b l e f o r po-

~ ~ u l a t i n y n e a r t h r e s l i o l d s t a t e s i n i o r i - s o l i d i n t e r a c t i o n s . l h e s e r e s u l t s suggest t h a t Rydbery s t a t e s seer11 Lo s u r v i v e inore c o l l i s i o n s i n t h e s o l i d t l ~ a r i r r e e e l e c l r u r i s would, arid, a d d i t i o r ~ a l l y t h a t h i g h e r o r d e r s o l i d e f - fect s a r e r i e y l i g i b l e . A n o n - s p h e r i c a l wake p o t e n t i a l c o u l d e f f e c t t h e I l y d b e r g - s l a t e s ( S t a r k - n l i x i ~ i g ) arid l e a d t o a s t a t i s t i c a l l y p o p u l a t i o n o f t h e o r b i L a l arigular rnorr~er~tu~rr s t a t e s .

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Deflector F i e l d (V/CILI) Field F r e q u e n c y (V/CIII-' ) * Ti 3: " S t a r k Beats" f o l l o w i r i g 85 MeV F i g . 4.: F o u r i e r power spec- Ne'; on gas and T o i l . L r u n f o r F i g . 3 .

W i t l i l n Lhe scope o f Lliese i n v e s t i g a t . i o n s Llie e f f e c t O F a weak electrostatic f i e l d on t h e p r u j e c t i l e Rydbery s t a t e s has been e x p l o r e d . Downstrea~n from Ltie t a r g e t t h e bea~ri passes Ltirouyh a t r a n s v e r s e d e f l e c t o r f i e l d , i . e . a s t a t i c hol~logeneous e l e c t r i c f i e l d o r i e n t e d perpendicular t o t h e bearn a x i s . D e t a i l s of' Llie e x p e r i l r c e ~ i t a l arrangernerits a r e q i v e r i i n R e f . 12. I n t l i e expe- r i r i l e n t a l p r o c e d u r e t h e a n a l y z i r i y vo l l a g e o f Llie s p e c l r o m e t e r i s k e p t f i x e d f o r a n a l y z i n g t l i e f i e l d i o n z e d p r o j e c t i l e Rydbery s t a l e s w h i l e t l i e d e f l e c - t o r v o l t a g e i s scanned. F i g . 3 stiows t h e d e f l e c t o r f i e l d dependence o f t h e e l e c t r o n y i e l d rrleasured f o r 85-MeV ~ e i o n s i n t e r a c t i n g w i t h a t h i n c a r - ~ + b o n - f o i l t a r g e t and a heliurn-yas t a r g e t . The spectrcnn shows a r a t h e r p r o - riounced o s c i l l a t o r y s t r u c l u r e . A r o u r i e r a n a l y s i s o f Lhe s p e c t r a g i v e s t h e F o u r i e r power s p e c t r a ( F i g . 4) c o r r e s p o n d i n g t o t h e b a c k g r o u n d - s u b t r a c l e d y i e l d c u r v e s .

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A q u a l i t a l i v e i n t e r p r e t a t i o n o f llie o s c i l l a t o r y s t r u c t u r e o b s e r v e d iri t h e f i e l d - i o n i z a t i o t i y i e l d c a n be g i v e n by e s s e n t i a l l y f o l l o w i n g t h e i d e a s o u t - l i n e d by Vager e t a 1 . l " and f o l l o w i n g t h e d e s c r i p t i o n o f llie 111ode1 g i v e n i n Ref. 12. There i t is used l l i a t tlie e n e r g y s p r e a d a s s o c i a t e d w i t h t h e tirne s c a l e (IT t h e e x c i t a t i o n p r o c e s s e s is l a r y e i n cornparison with t h e energy s p l i t t i n y s o f t h e Rydbery s t a t e s s t u d i e d , l h e r e f o r e t h e bearn-target i n t e r - a c t ion w i 11 i n g e n e r a l r e s u l t i r i ( f u l l y o r p a r t i a l l y ) c o h e r e n t e x c i t a t i o n of l h e Rydbery s l a t e s .

Tlle 1i111e e v o l u t i o n of t h e s l a t e i n t h e d e f l e c l o r f i e l d may b e c a l c u l a t e d i n f i r s t o r d e r i n l h e f i e l d s l r e r i y t h o f l h e d e f l e c t o r . Tlii,S f o l l o w s from t h e r a c t l h a t l h o s e Rydbery s t a t e s which g i v e t h e main c o n l r i b u t i o n t o t h e f ~ e l d i o n i 7 a t i o r i y i e l d i n tile s t r o r l y s p e c t r o r r ~ e t e r f i e l d a r e orlly weakly a f - f e c l e d by llie d e f l e c t o r f i e l d .

Tlie t o t a l field-ionization y i e l d c o w p r i s e s c o n l r i b u t i o n s from a c e r t a i n band OF n v a l u e s (by cornparison t o t h e o r e t i c a l v a l u e s 1 4 ) . Because o f t h e n depender~ce o f t h e r u n d a ~ r ~ e n t a l p e r i o d o f llie o s c i l l a t i o n s , t h e p o s s i b i l i t y t o o b s e r v e d e f l e c t o r - f i e l d - i n d u c e d o c i l l a t i o n s i n t h i s y i e l d depends on t h e width of t h e band, Marked o s c i l l a t i o n s a r e e x p e c l e d t o show up o n l y i f t h i s width i s s u P r i c i e n t l y s n ~ a l l . The n band i s l i r n i l e d or1 t h e low-rl s i d e by t h e

V1 2 0 - C ,

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F i g . 5: 3"Slark Beats" r o l l o w i n g 72 MeV C ⁺i n c i d e n t on tle ( b a c k -

a - 1 0 - ground s u b t r a c l e d ) .

T I 3

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t h r e s h o l d f o r f i e l d i o n i z a t . i o n i n t h e s p e c t r o ~ n e t e r f i e l d ; i t s e x t e n s i o n t o Irigh ri-values depends on tlie ~ n e c l i a n i s ~ n s of R y d b e r y - s t a t e f o r m a t i o n .

F i g . 5 shows S t a r k b e a t s f o r t h e c a s e o f 72 MeV 03+ i n c i d e n t on ~ e a g a i n , ' ~ t h e a m p l i t u d e s o f tlie o s c i l l a t i o n s a r e r a t h e r s t r o n g l y pronounced i n t h i s c a s e . Although t h e f i e l d p e r i o d e ( 2 2 V/c~n) a g r e e s w i t h p r e d i c t i o n s f o r t h e Z-dependence from l h e p r e s e n t rnodel; t h e 111odel d o e s n o t j e t a c c o u n t f o r t h e s t r o n y a ~ l r p l i t u d e s o f t h e o s c i l l a t i o r i s . The o b s e r v a t i o n s s o f a r seern t o i n d i c a t e t h a t t h e a ~ ~ ~ p l i t u d e s a r e enhar1ced f o r h i g h Z p a r t i c l e .

I t is sliown l h a t by e ~ r ~ p l o y i n y t h e f i e l d - i o n i z a t i o n t e c h n i q u e t h e i n f l u e n c e of a weak e l e c l r i c f i e l d or1 h i y h Rydbery s t a t e s forrned i n g a s and f o i l i n t e r a c t i o n s o f f a s t i o n s car, be s t u d i e d . For b o t h t y p e s o f t a r g e t s , t h e 111easured i o n i z a t i o n y i e l d e x h i b i t s r e g u l a r o s c i l l a t i o n s a s f u n c t i o n o f t h e weak f i e l d . l h e F o u r i e r a n a l y s i s o f t h e o s c i l l a t i n g p a r t o f t h e y i e l d r e v e a l s t h e p r e s e n c e o f a narrow band o f fundamental f r e q u e n c i e s a s w e l l a s o f second-harmonic c o n t r i b u t i o n s . The t h e o r e t i c a l i n t e r p r e t a t i o n o f t h e da- t a h a s p r o v i d e d f a i r l y s t r o n y e v i d e n c e t h a t q u a n t u n i n t e r f e r e n c e s arnong S t a r k s l a t e s c o h e r e n t l y d e v e l o p i n g i n t h e weak f i e l d ( S t a r k b e a t s ) a r e r e s p o n s i b l e f o r l l i e o c s i l l a t i o n s i n l h e y i e l d . S t a r k b e a t s among Rydbery s t a l e s w i t h p r i n c i l ~ a l quantun n u ~ n b e r s l a r g e r t h a n 100, a s t h e y have been o b s e r v e d i r i llie p r e s e n t e x p e r i m e n t s , had n o t been o b s e r v e d p r e v i o u s l y . The p r e s e r ~ t r e s u l t s c l e a r l y d e ~ n o n s t r a t e tlie p o t e n t i a l o f t h e d e s c r i b e d neth hod l o i r i v e s t i y a t e hiyli Rydbery s t a t e s forrned i n e x c i l e d F a s t bear11 i o n s .

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JOURNAL DE PHYSIQUE

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