FORCED AUTOIONIZATION OF THE Ba 5d7d STATE

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FORCED AUTOIONIZATION OF THE Ba 5d7d STATE

W. Sandner, K. Safinya, T. Gallagher

To cite this version:

W. Sandner, K. Safinya, T. Gallagher. FORCED AUTOIONIZATION OF THE Ba 5d7d STATE.

Journal de Physique Colloques, 1982, 43 (C2), pp.C2-177-C2-185. �10.1051/jphyscol:1982214�. �jpa-

00221825�

FORCED AUTOIONIZATION OF THE Ba 5 d 7 d STATE

W. Sandner, K.A. Safinya and T.F. Gallagher

SRI International, Department of Chemistry, Moleoular Physias Làboratory, Menlo Park, CA 94025, U.S.A.

Bésumé.- Nous avons f a i t l ' é t u d e de l ' i n t e r a c t i o n e n t r e l ' ë t a t 5d7d D2 s i t u é à 200 cm~l en dessous de l a première l i m i t e d ' i o n i s a t i o n du barium avec l a s é r i e de Rydberg 6snd. Nous d é c r i v o n s l ' é t u d e de l ' i n t e r a c t i o n de l ' é t a t 5d7d avec l e continuum Stark produit par des champs é l e c t r i q u e s de 5 kV/cm.

Abstract. - We report the study of the interaction of the Ba 5d7d D2 state lying 200 cm below the first ionization limit with the 6snd Rydberg series in which it is embedded. Specifically we describe the study of the interac- tion of the 5d7d state with the Stark continuum produced by electric fields of ~ 5 kV/cm.

1. Introduction. - If we can consider any atomic process which occurs when the valence electron is at a small orbital radius, it is expected that the ionization limit should have no particular significance. This is hardly surprising since, if the electron has an orbital radius of ~ 1 a , it has a potential energy of 1 au.

Therefore, whether the electron is slightly bound or slightly unbound makes little difference in its behavior at small orbital radius. There are many phenomena which support this notion. Probably the most familiar is photoabsorption from the ground state, a state of small orbital radius, in which a constant value of oscillator strength per unit energy is observed in the vicinity of the ionization limit. A less well known example is the configuration or channel interaction of states converging to different limits. For example, consider two series of states converging to each of two limits. Above the first limit the interaction of higher members of the series converging to the higher limit with the continuum above the first limit leads to autoionization. Below the first limit the same interaction of lower members of the same series converging to the higher limit leads to a perturbation of the regularity of the series converging to the lower limit. Since the configuration interaction is dominated by small orbital radii, the autoionization rates observed above the first limit can be used to predict the series perturbations below the limit and vice versa. This is hardly surprising as 2 both are direct measures of the strength of the configuration interaction between the two types of states. Thus, in the case of configuration interaction the insignificance of the ionization is also apparent.

It is interesting to consider the possibility of observing the interaction of the same state converging to a higher limit with both a continuum and a series of Rydberg states of the first limit. Although this sounds impossible, it may be done by using an external field to artificially depress the ionization limit by producing

3

a Stark continuum. This was first discovered by Garton et al. who found that in the absorption spectrum of shock-heated Ba the high lying member of the principal

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982214

C2- 178 JOURNAL DE PHYSIQUE

s e r i e s was not r e s o l v e d , down t o n

-

12, and two 5d8p l e v e l s , members of a s e r i e s , converging t o t h e &+5d l i m i t appeared a s an asymmetric Beutler-Fano p r o f i l e s . 4 They suggested t h a t t h e e l e c t r i c f i e l d s p r e s e n t due t o shock h e a t i n g t h e Ba had c o n v e r t e d t h e 6snp series i n t o a S t a r k continuum e x t e n d i n g down t o n=12 and t h e valence s t a t e appeared a s a n a u t o i o n i z a t i o n resonance i n it. They c a l l e d t h i s p r o c e s s "Forced Autoionization."

Although t h e i r work d e m o n s t r a t e s t h e c e n t r a l p o i n t , i t l e a v e s s e v e r a l q u e s t i o n s t o be answered by more c o n t r o l l e d experiments. A s examples, what i s t h e e l e c t r i c f i e l d dependence of f o r c e d a u t o i o n i z a t i o n , and may m l t i c h a n n e l quantum d e f e c t t h e o r y o r c o n f i g u r a t i o n i n t e r a c t i o n t h e o r y b e used t o d i r e c t l y e x t r a p o l a t e from t h e z e r o f i e l d p e r t u r b a t i o n of t h e Rydberg s e r i e s t o p r e d i c t t h e f o r c e d a u t o i o n i z a t i o n p r o f i l e i n an e l e c t r i c f i e l d o r v i c e v e r s a ? With t h e s e q u e s t i o n s i n mind we s e t o u t t o s t u d y t h e & 5d7d ID2 s t a t e which i n t e r a c t s w i t h t h e 6snd s e r i e s a s shown i n F i g u r e 1. It has a term energy of 41841.660 It l i e s

-

^{200 cm-' below}

t h e f i r s t i o n i z a t i o n l i m i t o f 42034.9 cm-1,6 and n e a r t h e 6s25d s t a t e . The a p p l i c a t i o n o f a f a i r l y modest e l e c t r i c 5d7d f i e l d

-

³ kV/cm produces a S t a r k continuum f

-

e x t e n d i n g below t h e 5d7d s t a t e . As we s h a l l s e e , t h i s s t u d y i s q u i t e i n t e r e s t i n g i n i t s own r i g h t and c l e a r l y i l l u s t r a t e s b o t h t h e q u a l i t a t i v e u s e f u l n e s s and q u a n t i - t a t i v e l i m i t a t i o n s of f o r c e d a u t o i o n - i z a t i o n .

2. Experimental Study.

-

l h e g e n e r a l approach used i n t h e s e e x p e r i m e n t s i s two- s t e p e x c i t a t i o n of atoms i n a t h e r m a l atomic beam. The r e l e v a n t energy l e v e l s a r e shown i n F i g u r e 1, which shows t h e two PIGIlllE THE RELEVANT LEVELS a l t e r n a t i v e e x c i t a t i o n methods. P a t h A,

SHOWING 5d7d STATE IN THE MIDDLE t h r o u g h t h e 6s6p s t a t e , f a v o r s e x c i t a t i o n OF THE 6snd SERIES AND THE TWO EXCITA- t o t h e 6snd s e r i e s o r t h e S t a r k induced

TION PATHS A AND B. continuum. P a t h B, on t h e o t h e r hand,

f a v o r s e x c i t a t i o n t o t h e 5d7d s t a t e . In e i t h e r c a s e t h e wavelength of t h e f i r s t l a s e r i s f i x e d and t h a t of t h e second l a s e r scanned t o o b t a i n t h e spectrum. In almost a l l c a s e s e a c h atom which h a s a b s o r b e d two photons i s c o n v e r t e d t o a n i o n s o t h a t we observe an i o n s i g n a l which i s p r o p o r t i o n a l t o t h e t r a n s i t i o n p r o b a b i l i t y a t t h e second l a s e r

'

s wavelength. The experiments a r e , of c o u r s e , c a r r i e d o u t i n e l e c t r i c f i e l d s from 0-5 kV/cm t o a l l o w s y s t e m a t i c s t u d i e s of t h e e l e c t r i c f i e l d dependence. In a d d i t i o n , by a p p r o p r i a t e c h o i c e of l a s e r p o l a r i z a t i o n we may s e p a r a t e l y e x c i t e e i t h e r t h e 1m]=0 o r 1 s t a t e s . Here m i s t h e p r o j e c t i o n of o r b i t a l a n g u l a r momentum on t h e f i e l d d i r e c t i o n .

The a p p a r a t u s c o n s i s t s of two dye l a s e r s and an atomic beam a p p a r a t u s . The Ba atomic beam e f f u s e s from a r e s i s t i v e l y h e a t e d oven and i s c o l l i m a t e d t o a d i a m e t e r of 4 mm b e f o r e e n t e r i n g t h e i n t e r a c t i o n r e g i o n where i t h a s a d e n s i t y of

-

~ O ~ C I I I - ~ . A f t e r t h e c o l l i m a t o r t h e atomic beam p a s s e s midway between a p l a t e and g r i d 1.12 cm a p a r t where i t i s c r o s s e d by t h e two l a s e r beams a t 90'. The upper

t h e atoms f i e l d i o n i z e d u r i n g t h e l a s e r p u l s e . I£ t h e d c f i e l d i s l o e r t h a n 2 kV/cm, a p o s i t i v e v o l t a g e p u l s e i s a p p l i e d t o t h e lower p l a t e a f t e r t h e l a s e r p u l s e s t o f i e l d i o n i z e t h e atoms. In e i t h e r c a s e e a c h e x c i t e d atom i s c o n v e r t e d t o a n i o n which i s a c c e l e r a t e d through t h e g r i d i n t h e upper p l a t e t o a p a r t i c l e m u l t i p l i e r . Since t y p i c a l l y more t h a n one i o n i s formed on e a c h l a s e r p u l s e and a l l t h e i o n s s t r i k e t h e m u l t i p l i e r s i m u l t a n e o u s l y , t h e s i g n a l i s a n i n h e r e n t l y a n a l o g s i g n a l which we d e t e c t w i t h a boxcar a v e r a g e r and r e c o r d w i t h an x-y p l o t t e r . The dye l a s e r s used i n t h e s e experiments a r e modest: 5ns p u l s e l e n g t h s ,

-

^{100 pj} p u l s e e n e r g i e s , and 1 cm-I l i n e w i d t h s .

If we c o n s i d e r f i r s t t h e r e s u l t s f o r t h e I m l = l s t a t e s , we f i n d t h a t our q u a l i t a t i v e e x p e c t a t i o n s a r e r e a l i z e d .

-

^In F i g u r e 2a we show t h e

.- E

e x c i t a t i o n spectrum i n z e r o

c 3 f i e l d , o b t a i n e d u s i n g t h e

t E

^{p a t h B} e x c i t a t i o n of

.-

.-

F i g u r e l S 7 R e c a l l t h a t u s i n g

- g

t h i s approach we e x c i t e p r e f -

-I

0

e r e n t i a l l y t h e 5d7d s t a t e and

a

Z n o t t h e 6snd s e r i e s . Thus,

52

V) below t h e i o n i z a t i o n l i m i t we

Z o n l y o b s e r v e s t r o n g s i g n a l s

0

from s t a t e s which c o n t a i n

k

K s u b s t a n t i a l f r a c t i o n s of 5d7d

0

V) s t a t e . Above t h e l i m i t we

m

Q o n l y o b s e r v e t h e r e s o n a n c e

E'

c o r r e s p o n d i n g t o o t h e r s t a t e s

0 I converging t o t h e '5d~a

n l i m i t . The z e r o f i e l d spec-

trum of F i g u r e 2a i s t h u s

0

^{composed o f} a u t o i o n i z a t i o n

r e s o n a n c e s above t h e l i m t t and t h o s e s t a t e s i n t h e 41 700 42000 42300 v i c i n i t y of t h e 5d7d s t a t e

5d7d('

Dz)

' 0 which have s i g n i f i c a n t

admixtures of 5d7d charac-

crn-1 _{t e r .} A c t u a l l y t h e r e l a t i v e s t r e n g t h s of t h e l i n e s a r e FIGURE 2 EXCITATION SPECTRA V I A PATH B I N ( a ) not completely reliable in ZERO FIELD (b) 7.5 kV/cm.

t h i s c a s e , because t h o s e s t a t e s w i t h a p p r e c i a b l e 5d7d admixtures decay r a p i d l y , -200 n s , s o t h a t some o f t h e atoms i n t h o s e s t a t e s decay b e f o r e t h e f i e l d i o n i z a t i o n pulse.8 When a f i e l d o f 7.5 kV/cm i s a p p l i e d we observe t h e spectrum of F i g u r e 2b. The a u t o i o n i z a t i o n resonance above t h e i o n i z a t i o n l i m i t i s e s s e n t i a l l y u n a f f e c t e d , but t h e s t a t e s i n t h e v i c i n i t y of t h e M7d a r e c o a l e s c e d i n t o a f o r c e d a u t o i o n i z a t i o n resonance.

F i t t i n g t h i s resonance t o t h e f a m i l i a r Beutler-Fano resonance p r o f i l e l e a d s t o t h e v a l u e s of 15 cm-I f o r t h e width

r

( s e e Eq. 8) and term energy of 41846(5) cm-l, b o t h

C2-180 JOURNAL DE PHYSIQUE

of which a r e i n r e a s o n a b l e agreement w i t h t h e v a l u e s e x p e c t e d on t h e b a s i s of a z e r o f i e l d quantum d e f e c t a n a l y s i s , which i m p l i e s a width of 20 cm-I and a t e r m energy of 41841 cm1. T h i s

comparison is shown i n

F i g u r e 3. I n c i d e n t a l l y , we n o t e t h e r a t i o of t h e e x c i - t a t i o n of t h e 5d7d resonance t o t h e u n d e r l y i n g continuum i s g i v e n by t h e parameter q ( s e e Eq. 6) = -25.

The a p p a r e n t s i m i l a r i t y

of t h e z e r o f i e l d quantum

z _o _- -

^{I I I 1 1 1 1 1 1 1 1 -}

(b)

-

d e f e c t result w i t h f o r c e d

- -

a u t o i o n i z a t i o n d i s a p p e a r s Fit to Experiment (7.5 kV/cm)

-

when we e x c i t e t h e Im(=0

-

s t a t e s . As shown i n F i g u r e

-

4 , s h a r p s t r u c t u r e s a r e s e e n

- -

on t o p of t h e 20 cm wide

-

resonance which are a t t r i -

-

buted t o s t r u c t u r e i n t h e

S t a r k induced lm

1

^=O c o n t i n -

_- - _-

uum. Such s t r u c t u r e i n t h e

- -

continuum has been observed

- ^-

e x p e r i m e n t a l l y a s s t r o n g v a r i a t i o n s i n t h e o s c i l l a t o r s t r e n g t h from t h e ground

s t a t e l o , ' ' and low l y i n g 4 1 8 4 1 * 2 4 1 8 4 6 k 2

e x c i t e d s t a t e s , l 2 even above t h e z e r o f i e l d i o n i z a t i o n

l i m i t , and has been t r e a t e d FIGURE 3 EXCITATION SPECTKA OF Iml =I STATES t h e o r e t i c a l l y .I3 The e x i s t - VIA PATH B I N 7.5 kV/cm(a) EXPERIMENTAL (b) ence of such s t r u c t u r e and COMPARISON W I T H MQDT.

i t s almost c e r t a i n l y d i f f e r e n t i n t e r a c t i o n w i t h a v a l e n c e s t a t e such a s 5d7d a r e c o n t r a r y t o one of t h e b a s i c assumptions of quantum d e f e c t t h e o r y , t h e smooth and slow v a r i a t i o n of p r o p e r t i e s a c r o s s t h e i o n i z a t i o n l i m i t . l%us i t i s u n r e a l i s t i c t o expect t o be a b l e t o p r e d i c t such s p e c t r a from a z e r o f i e l d quantum d e f e c t t h e o r y a n a l y s i s , c o n t r a r y t o r e c e n t suggestions.14 To v e r i f y t h a t t h e observed s t r u c t u r e s a r e i n f a c t due t o S t a r k induced r e s o n a n c e s , s p e c t r a were r e c o r d e d s y s t e m a t i c a l l y a s a f u n c t i o n of a p p l i e d e l e c t r i c f i e l d t o determine t h e f i e l d s h i f t of e a c h of t h e S t a r k resonances.

F i n a l l y , a s F i g u r e 1 s u g g e s t s , i f we u s e p a t h A t o e x c i t e t h e atoms, we s h o u l d p r e f e r e n t i a l l y e x c i t e t h e 6snd S t a r k continuum but n o t t h e 5d7d s t a t e . A s shown by F i g u r e 5 , t h i s i s i n f a c t t h e c a s e ; a d i p r a t h e r t h a n a peak i n t h e spectrum i s observed, i n d i c a t i n g q

-

0 , o r more s p e c i f i c a l l y qg = l / q A where t h e s u b s c r i p t s A and B r e f e r t o t h e e x c i t a t i o n p a t h s of F i g u r e 2.

a n a l y s i s i t i s worth i n t r o d u c i n g a

-

₂

p h y s i c a l p i c t u r e of t h e atom i n an '5

0

³

e l e c t r i c f i e l d . As shown by

t

Figure 6 , t h e coulomb p o t e n t i a l i s

a

0 .%

p u l l e d down on t h e anode s i d e of

$

t h e atom s o t h a t above t h e s a d d l e

5

p o i n t a continuum of s t a t e s is

6 5

p o s s i b l e . This continuum i s evi-

2

⁽³

d e n t l y q u i t e smooth f o r t h e I m ( = l V)

s t a t e s . For t h e Iml=0 s t a t e s 0 t h e r e i s a l s o a smooth continuum,

but t h e r e e x i s t Stark resonances

FIG- 4 EXCITATION SPECTRUM OF lml=0 a s w e l l which correspond t o t h e STATES AT 4 . 8 kv,cm VIA PATH B.

e l e c t r o n ' s being i n a q u a s i s t a b l e

o r b i t around t h e i o n i c c o r e b e f o r e f i e l d i o n i z a t i o n , a s shown i n F i g u r e 6. Also shown i s t h e s p e c t r a l d e n s i - F = 4.80 kV/cm t y t o r e p r e s e n t t h e lml=0 continuum, a

smooth continuum

above t h e s a d d l e p o i n t of t h e poten- t i a l w i t h super- imposed peaks.

The 5d7d s t a t e i s one i n which b o t h of t h e two valence e l e c t r o n s have small o r b i t a l r a d i i ; t h u s , t h e f a c t t h a t f o r t h e S t a r k resonances t h e Rydberg e l e c - t r o n i s s p a t i a l l y l o c a l i z e d n e a r t h e o r i g i n should l e a d t o a s t r o n g e r con- f i g u r a t i o n i n t e r a c -

IJ t i o n . In a d d i t i o n ,

s i n c e t h e e x c i t a - FIGURE 5 EXCITATION SPECTRA V I A PATH A AT

(a) 4.8 kV/cm AND ( b ) ZERO FIELD.

t i o n f o r p a t h A o r p a t h B o c c u r s when b o t h e l e c t r o n s a r e

-

n e a r t h e o r i g i n , we

C2-182 JOURNAL DE PHYSIQUE

would expect t h e t r a n s i t i o n moment t o t h e S t a r k resonances t o be s t r o n g e r t h a n t h a t t o t h e underlying smooth continuum.

FIGURE 6 CROSS SECTION OF THE ATOMIC POTENTIAL I N A FIELD SHOWING THE SPATIAL

LOCALIZATIONS OF THE STARK STATES AND THEIR CORRESPONDINGLY VARYING SPECTRAL

DENSITIES ON THE RIGHTHAND SIDE.

The formal t r e a t m e n t of t h e d a t a i s based on t h e c o n f i g u r a t i o n i n t e r a c t i o n f o r - mulation of ~ a n o . ~ S p e c i f i c a l l y we c o n s i d e r t h e i n t e r a c t i o n of a d i s c r e t e s t a t e w i t h s t r u c t u r e d continuum, a s shown by Figure 6. Zhe d i s c r e t e s t a t e 4 and t h e con- tinuum XE a r e assumed t o d i a g o n a l i z e t h e Hamiltonian i n t h e absence of t h e configu- r a t i o n i n t e r a c t i o n VE and a r e normalized i n t h e u s u a l fashion. Thus we t a k e a s our s t a r t i n g p o i n t :

i s o b t a i n e d by d i a g n o n a l i z i n g t h e above equation and i s given formally by

That i s , a t any energy t h e wave f u n c t i o n w i l l have some continuum and some d i s c r e t e ( p e r t u r b e r ) c h a r a c t e r . Although we would w r i t e o u t Eq. 4 i n a more e x p l i c i t f a s h - i o n , i t i s worth r e c a l l i n g t h a t we a r e i n f a c t i n t e r e s t e d i n c h a r a c t e r i z i n g a photo- e x c i t a t i o n spectrum. Thus we i n t r o d u c e t h e u s u a l p a r a m e t e r i z a t i o n of t h e photoab- s o r p t i o n s i g n a l

Here t h e T matrix elements ar,e t h e t r a n s i t i o n matrix elements from t h e i n i t a l s t a t e i t o t h e a c t u a l continuum YE and t h e unperturbed continuum XE. Roughly speaking, q i s t h e r a t i o of t h e e x c i t a t i o n s t r e n g t h t o t h e p e r t u r b e r and t o t h e amount of continuum over which i t i s s p r e a d , and is d e f i n e d by

It i s i n t e r e s t i n g t o n o t e t h a t f o r a smooth continuum f o r which VE i s a c o n s t a n t t h e p r i n c i p a l p a r t i n t e g r a l P i n Eq. (6) vanishes. The parameter E i s a reduced energy parameter, which i s e s s e n t i a l l y t h e r a t i o of t h e energy from t h e c e n t e r of t h e resonance t o t h e width of t h e resonance. E x p l i c i t y ,

We a l s o d e f i n e t h e width

r

of t h e a u t o i o n i z a t i o n resonance

Using t h e approach o u t l i n e d above it i s s t r a i g h t f o r w a r d t o t r e a t t h e smooth I m l = 1 continuum, which, a s noted p r e v i o u s l y , l e a d s t o

r

= 15 cm-' and q = -25, independent of t h e e l e c t r i c f i e l d . To t r e a t t h e h i g h l y s t r u c t u r e d i m l = 0 continuum we m s t i n t r o d u c e an e x p l i c i t model f o r t h e s t r u c t u r e d Iml = 0 S t a r k continuum. We have chosen t o f o l l o w t h e approach of Luc-Koenig and Bachelier13, and we u s e t h e f o l l o w i n g a n s a t z f o r t h e t r a n s i t i o n m a t r i x element T and c o n f i g u r a t i o n i n t e r a c t i o n m a t r i x element V:

JOURNAL DE PHYSIQUE

and

Here t h e sum i s o v e r v, t h e i n d e x of t h e S t a r k s t a t e r e s o n a n c e s , and

\,

^{A,,, and Bv}

a r e t h e w i d t h , C I enhancement, and a b s o r p t i o n enhancement of e a c h of t h e S t a r k r e s o - nances. Since each of t h e S t a r k resonances can have a d i f f e r e n t v a l u e of

rv,

Bv and Av, t h e r e i s a huge number of f r e e p a r a m e t e r s , and i f a l l a r e l e f t f r e e i t i s p o s s i - b l e t o f i t any c o n c e i v a b l e spectrum. Thus we make t h e r e s t r i c t i v e assumption t h a t Bv and Av a r e c o n s t a n t f o r a l l t h e S t a r k s t a t e s of i n t e r e s t . 'Ihe e n e r g i e s of Ev of t h e S t a r k s t a t e resonances a r e , of c o u r s e , c o n s t r a i n e d t o move a c c o r d i n g t o t h e i r observed S t a r k s h i f t s , and t h e widths

r

a r e a l l e q u a t e d t o 10 cm-l, t h e observed w i d t h away from t h e 5d7d s t a t e . With t h e s e r e s t r i c t i o n s we f i n d t h a t we a r e a b l e t o v o b t a i n a n a l y t i c e x p r e s s i o n f o r q ,

which i s complex, and E. In a d d i t i o n we f i n d t h a t between t h e narrow S t a r k s t a t e s

E + E 0 ( 1 1 )

Y

a z

(-7

V)

t h e v a l u e s which r e l a t e t o t h e under-

l y i n g smooth continuum. Thus we c a n

$ ⁰

o b t a i n

v;,

t h e i n t e r a c t i o n s t r e n g t h of

,,,

t h e 5d7d s t a t e w i t h t h e u n d e r l y i n g smooth Iml = O continuum by f i t t i n g t h e smooth p a r t s of t h e observed p h o t o e x c i t a t i o n spectrum t o a Beutler- Fano p r o f i l e f o r a smooth continuum.

Having done t h i s we s u b t r a c t t h e

Beutler-Fano p r o f i l e a r i s i n g from t h e 0 smooth continuum from t h e observed

s i g n a l and f i t t h e d i f f e r e n c e t o o b t a i n t h e i n t e r a c t i o n between t h e

5d7d s t a t e and t h e S t a r k resonance. 41700 41 800 41 900

In F i g u r e 7 we show a s a n example t h e d i f f e r e n c e s p e c t r a o b t a i n e d f o r t h e

f i e l d s of 3.3 kV/cm, 4.0 kV/cm, and 'IGURE DIFFERENCE SPECTRA l m l = ~

FOR DIFFERENT ELECTRIC FIELDS. THE X

4.8 kV/cm, which i l l u s t r a t e s one S t a r k MARKS PROGRESSION OF A SINGLE STARK resonance p a s s i n g through t h e 5d7d STATE ACROSS THE 5d7d RESONANCE.

resonance. We a l s o show t h e

f e r e n c e spectrum. This c h o i c e of parameter i m p l i e s t h a t t h e S t a r k r e s o n a n c e s i n t e r a c t w i t h t h e 5d7d s t a t e w i t h a s t r e n g t h t o g i v e

r

⁼ 3 1 cm-l, t h a t i s , t w i c e a s s t r o n g a s t h e f l a t continuum. This i s c e r t a i n l y i n a c c o r d w i t h o u r i n t u i t i v e expec- t a t i o n s , s i n c e f o r t h e S t a r k r e s o n a n c e s t h e e l e c t r o n i s l o c a l i z e d n e a r t h e o r i g i n . It i s a l s o i n t e r e s t i n g t o r e c a l l t h a t t h e MQDT t r e a t m e n t i m p l i e s t h a t

r

⁼ 20 cm-I.

Thus t h e s t r o n g l y i n t e r a c t i n g S t a r k r e s o n a n c e s p u l l t h e f o r c e d a u t o i o n i z a t i o n i n t e r - a c t i o n s t r e n g t h toward t h e z e r o f i e l d MQDT v a l u e .

4. Conclusion.

-

Although t h e s e experiments were begun i n a n a t t e m p t t o s t u d y f o r c e d a u t o i o n i z a t i o n s y s t e m a t i c a l l y , we have a p p a r e n t l y found m c h more t h a n we expected. S p e c i f i c a l l y t h i s s t u d y i s , t o o u r knowledge, t h e f i r s t d e t a i l e d s t u d y o f t h e c o n f i g u r a t i o n i n t e r a c t i o n between a d i s c r e t e s t a t e and a h i g h l y s t r u c t u r e d continuum.

With r e g a r d t o f o r c e d a u t o i o n i z a t i o n i t s e l f we f i n d t h a t i t i s a good way t o o b t a i n q u a l i t a t i v e i n f o r m a t i o n r e g a r d i n g p e r t u r b i n g s t a t e s . However, t h e s e and o t h e r more r e c e n t s t u d i e s of t h e p e r t u r b e d Ba 6sng s t a t e s i n d i c a t e t h a t t h e p a r a m e t e r s deduced from f o r c e d a u t o i o n i z a t i o n experiments do n o t a g r e e w i t h t h o s e o b t a i n e d from t h e a n a l y s i s of p e r t u r b e d z e r o f i e l d s p e c t r a , a r e s u l t which i s n o t e n t i r e l y s u r p r i s i n g .

5. Acknowledgments. - W e would l i k e t o acknowledge h e l p f u l d i s c u s s i o n s w i t h D. L.

H u e s t i s and F. Gounand. This work was s u p p o r t e d by t h e U.S. Department of Energy, Off i c e of Basic Energy Sciences.

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