STATE-SELECTIVE PHOTOIONIZATION AND PHOTODISSOCIATION SPECTROSCOPY OF THE H2 MOLECULE FROM EXCITED STATES

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STATE-SELECTIVE PHOTOIONIZATION AND PHOTODISSOCIATION SPECTROSCOPY OF THE

H2 MOLECULE FROM EXCITED STATES

H. Rottke, K. Welge

To cite this version:

H. Rottke, K. Welge. STATE-SELECTIVE PHOTOIONIZATION AND PHOTODISSOCIATION

SPECTROSCOPY OF THE H2 MOLECULE FROM EXCITED STATES. Journal de Physique Col-

loques, 1985, 46 (C1), pp.C1-127-C1-134. �10.1051/jphyscol:1985112�. �jpa-00224483�

JOURNAL DE PHYSIQUE

Colloque CI, supplCment au nO1, T o m e 46, janvier 1985 page CI-127

STATE-SELECTIVE PHOTOIONIZATION AND PHOTODISSOCIATION SPECTROSCOPY OF THE

H2

MOLECULE FROM EXCITED STATES

+

^{+ * A}

H. Rottke and K. Welge

' ~ a k u l t a e t f u e r P h y s i k , U n i v e r s i t a e t B i e l e f e l d , F.R. G .

* ~ o i n t I n s t i t u t e f o r Laboratory A s t r o p h y s i c s , U n i v e r s i t y o f Colorado and National Bureau o f Standards, Boulder, Colorado 8 0 3 0 9 , U.S.A.

RQsum6

-

De premieres experiences ont Bt6 rCalisCes sur la photoionisation et photodissociation

B

deux photons de la mol'cule H

'i B

partir de niveaux de rotation-vibration individuels de 1 '&tat B CG, 2

f

'aide de rayonnement laser vuv et uv accordable pulse et polarise lineairement.

Grzce 5 la photoionisation par l'impulsion uv des atomes H excitds,

H(n=2)+uv

->

~ + + e , il a 6t6 possible d'observer simultan6ment les proces- sus de dissociation et d'ionisation. En accordant le rayonnement vuv 5 des transitions X-B donnees et en depla~ant la longueur d'onde uv, des spectres d ' i o n i s a t i o n - d i s s o c i a t i o n ont &t6 obtenus par mesure du taux de formation d'ions H; et H+ en fonction de l'Qnergie d'excitation. Des &tats de Rydberg QlevCs ont &t6 detectes par ionisation par champ produite apres l'impulsion laser d'excitation.

Des exemples de spectres d ' i o n i s a t i o n - d i s s o c i a t i o n obtenus 5 partir des niveaux v'=O, J r = O et 1 de l'dtat B sont presentCs. Les spectres couvrent des energies d'excitation voisines des limites d'ionisation les plus basses, i.e. H?(X~C+, v=O,N)

+

e avec N = 0,l et 2. Des rgsultats obtenus dans la region de d~ssociation g sont Ggalement pr&sentes.

Abstract - First experiments have been carried out on the two-step photo- ionization and photodissociation of the Hz molecule from individual rota- tional-vibrational levels in the B~Z; state, employing tunable, pulsed, li- nearly polarized vuv and uv laser radiation :

1

+

H2(x11;, vl1=O, ,It1)

+

vuv + H2(B Iu, v l , J ' )

+

uv ⁺ H;

H; 1 H ~ ( X

1 I,

v, N ) + e

+ H ( n = l )

+

H(n=2).

Via the photoionization of the excited H atoms in the uv laser pulse, H(n=Z) uv -' H+ + e, it was possible to observe the dissociation and the ionization processes in the same measurement. Tuning the vuv to given X - B transition lines and scanning the uv wavelength, ionization-dissociation spectra have been obtained by monitoring the yield of H; and H+ formation, as a function of the excitation energy. High Rydberg states have been detected by field ionization, applied after the laser excitation pulse.

Examples of ioniza~ion-dissociation spectra taken from v' = 0 ; J' = 0 and 1 levels in the B state are given. The spectra cover excitation energies around the lowest ionization limits, i.e. H~!x~c$, v = 0,N) + e with N = 0, 1 and 2. Also, some results obtained ln dissociation region are reported.

A JILA 1983-84 Visiting Fellow. Permanent address : Univ. Bielefeld, F.R.G.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985112

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INTRODUCTION

Elementary p h o t o p h y s i c a l and c o l l i s i o n a l processes o f atomic and m o l e c u l a r hydrogen are, by t h e v e r y n a t u r e o f these species, o f b a s i c i n t e r e s t i n many areas o f p h y s i c s and c h e m i s t r y . But, t h e atom and, p a r t i c u l a r l y , t h e molecule have n o t y e t been e x t e n s i v e l y i n v e s t i g a t e d by l a s e r techniques, which would, i n p r i n c i p l e , make d e t a i l e d and v a r i e d s t u d i e s p o s s i b l e . The reason i s t h a t t h e e x c i t e d s t a t e s , and t h e i o n i z a t i o n and d i s s o c i a t i o n t h r e s h o l d s l i e i n t h e f a r vacuum u l t r a v i o l e t , where o n l y d u r i n g t h e l a s t few y e a r s b r o a d l y t u n a b l e p u l s e d l a s e r r a d i a t i o n has become a v a i l a b l e . The vuv l a s e r t e c h n i q u e has now reached a s t a t e , where i n t h e whole wave- l e n g t h range down t o , and i n some r e g i o n s below 1000

8

l a s e r l i g h t can be generated.

I t has been a p p l i e d i n q u i t e a number o f s t a t e - s e l e c t i v e works on t h e atom and i n some i n s t a n c e s a l s o on t h e molecule.

We have c a r r i e d o u t some new experiments on t h e s t e p w i s e two-photon p h o t o i o n i z a t i o n and p h o t o d i s s o c i a t i o n o f t h e Hz molecule w i t h t u n a b l e vuv and uv l a s e r r a d i a t i o n . References o f p r e v i o u s s t a t e - s e l e c t i v e m u l t i p h o t o n work on Hz u n t i l about 1983 may be t a k e n from a paper by R o t t k e and Welge ( 1 ) . Other r e c e n t experiments have been concerned w i t h : (3+1) MPI-PES v i a t h e I;B' three-photon r e s o n a n t i n t e r m e d i a t e s t a t e (Z), (2+1) MPI-PES v i a t h e E,F '1' two-photon r e s o n a n t i n t e r m e d i a t e s t a t e ( 3 ) , one- photon e x c i t a t i o n o f t h e B'Z: s t a t e a t 1058-1068 9

8,

m o n i t o r e d by L I F ( 4 ) , and f i r s t i n t e r n a l s t a t e d i s t r i b u t i o n measurement o f t h e molecule i n t h e i m p o r t a n t H-H

2

r e a c - t i o n system, m o n i t o r e d w i t h t h e E,F s t a t e (2+1) MPI t e c h n i q u e ( 5 ) .

I n t h i s work t h e H2 molecule has been i o n i z e d and d i s s o c i a t e d f r o m i n d i v i d u a l r o t a - 1

+

t i o n a l - v i b r a t i o n a l l e v e l s ( v ' , 3 ' ) i n t h e B Cu s t a t e as a f u n c t i o n o f t h e e x c i t a - t i o n energy:

1

+

H~(X~I;, vl'=O, J " )

+

vuv

-

^H2(B

^ru,

v ' , J ' ) + uv + HZ*.

I n c o n t r a s t t o t h e p r e v i o u s m u l t i p h o t o n s t u d i e s w i t h Hz, t h e vuv was tuned t o a g i v e n l i n e o f X-B t r a n s i t i o n , w h i l e t h e uv l a s e r was scanned, i n d e p e n d e n t l y f r o m t h e vuv. The e x c i t a t i o n energy covered ranges f r o m t h e r e g i o n o f t h e Hz -t H ( n = l )

+

H(n=2) d i s s o c i a t i o n process ( 1 im i t a t 118376.4 cm-l) i n t o t h e i o n i z a t i o n r e g i o n , w i t h t h e a d i a b a t i c l i m i t f o r H ~ + ( x ~ z + v=O, N=O)

+

e- a t 124417.2 cm-'

g ' ( 6 ) .

W i t h r e s p e c t t o t h e s u b j e c t o f t h i s conference i t may be mentioned t h a t t h e e x c i t a - t i o n has been performed i n t h i s work a t r e l a t i v e l y l o w i n t e n s i t i e s , a l s o i n t h e second step, which means t h a t t h e a b s o r p t i o n and m o l e c u l a r decay were e s s e n t i a l l y separated i n t i m e . I n t h e second s t e p one can, o f course, go t o s u b s t a n t i a l l y h i g h e r i n t e n s i t i e s where l a s e r induced i n t e r a c t i o n on t h e decaying m o l e c u l a r system

m i g h t be observed. The s i m p l i c i t y o f t h e Hz m o l e c u l e w i t h i t s open s p e c t r a l s t r u c - t u r e and t h e v a r i e t y o f l i n e shapes i n t h e d i s s o c i a t i o n and i o n i z a t i o n r e g i o n , i n - d i c a t i n g d i f f e r e n t k i n d s o f " l i n e a r " i n t e r a c t i o n s among d i s c r e t e and continuum s t a t e s , makes i t p o t e n t i a l l y v e r y i n t e r e s t i n g and a l s o t r a c t a b l e c a n d i d a t e f o r ex- p e r i m e n t s o f l a s e r - i n d u c e d e f f e c t s on t h e decay processes.

I n t h i s paper we can p r e s e n t o n l y a p o r t i o n o f t h e s p e c t r a l r e s u l t s o b t a i n e d . They have been s e l e c t e d t o i n d i c a t e t h e d e t a i l e d i n f o r m a t i o n o b t a i n a b l e on t h e s p e c t r o - scopy and t h e dynamics o f t h e d i s s o c i a t i o n and i o n i z a t i o n o f Hz f r o m e x c i t e d s t a t e s . EXPERIMENTAL

The e x p e r i m e n t a l s e t u p used i n t h i s work i s shown s c h e m a t i c a l l y i n F i g . 1. Hydrogen gas was i n t r o d u c e d i n an e f f u s i v e beam t h r o u g h a c a p i l l a r y i n t o t h e e x c i t a t i o n r e g i o n between two p a r a l l e l e l e c t r i c f i e l d p l a t e s , P1 and

P 2 .

The p u l s e d vuv and uv l a s e r beams (- 5 ns p u l s e d u r a t i o n , 10 Hz r e p e t i t i o n r a t e ) were d i r e c t e d a n t i c o l - l i n e a r l y , a t r i g h t a n g l e t o t h e m o l e c u l a r beam a x i s , t h r o u g h t h e system. B o t h l a s e r p u l s e s a r r i v e d a t t h e i n t e r s e c t i o n s i m u l t a n e o u s l y so t h a t t h e t o t a l e x c i t a t i o n t i m e was

-

⁵ ns (spontaneous r a d i a t i o n l i f e t i m e o f t h e B s t a t e

-

0.6 n s ) . The i o n i z a t i o n was m o n i t o r e d by t h e e l e c t r o n s , e x t r a c t e d t h r o u g h t h e g r i d i n one o f t h e f i e l d

p l a t e s and a c c e l e r a t e d towards t h e e l e c t r o n m u l t i p l i e r . W h i l e scanning t h e uv l a s e r , t h e e l e c t r o n s i g n a l , and t h u s t h e y i e l d o f i o n i z a t i o n , i. e. t h e f o r m a t i o n o f H ~ + and H', was measured as a f u n c t i o n o f t h e uv wavelength, r e s u l t i n g i n an i o n i z a - t i o n spectrum f r o m t h e s e l e c t e d (B, v ' , J ' ) l e v e l . The s p e c t r a were r e c o r d e d i n terms of an a b s o l u t e wavelength s c a l e c a l i b r a t e d r e l a t i v e l y by means o f a Fabry- P e r o t i n t e r f e r o m e t e r t r a n s m i s s i o n c u r v e (FSR: 1.6931

+

0.0009 cm-' and a b s o l u t e l y by neon h o l l o w cathode l i n e s , t a k e n s i m u l t a n e o u s l y w i t h t h e s p e c t r a .

F i g . 1. Experimental arrangement.

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I t i s i m p o r t a n t t o n o t e t h a t , i n o r d e r t o a v o i d t h e S t a r k e f f e c t , no e l e c t r i c f i e l d e x i s t e d d u r i n g t h e l a s e r e x c i t a t i o n . As was expected, and observed i n p r e l i m i n a r y experiments, small f i e l d s p r o f o u n d l y a f f e c t e d t h e e x c i t a t i o n , p a r t i c u l a r l y o f h i g h s t a t e s . An e l e c t r i c f i e l d , Fion, was, however, a p p l i e d i n p u l s e d f a s h i o n a f t e r t h e l a s e r e x c i t a t i o n . T h i s f i e l d served two purposes: ( 1 ) f i e l d i o n i z a t i o n o f e x c i t e d H2 molecules, s p e c i f i c a l l y i n h i g h Rydberg s t a t e s , which had f i e l d - f r e e i o n i z a t i o n l i f e t i m e s t o o l o n g t o be observable, and ( 2 ) e x t r a c t i o n o f a l l e l e c t r o n s f r o m t h e i o n i z a t i o n zone i n c l u d i n g , o f course, t h e ones t h a t were formed d i r e c t l y i n f i e l d - f r e e i o n i z a t i o n mechanisms. More d e t a i l s o f t h e e x p e r i m e n t a l set-up and procedure w i l l be g i v e n elsewhere ( 7 ) .

RESULTS

A t room temperature, where t h e experiments have been c a r r i e d o u t , most o f t h e H2 p o p u l a t i o n i s c o n t a i n e d i n t h e JH=O, 1, and 2 r o t a t i o n a l l e v e l s o f t h e v"=O s t a t e . From t h e s e J " l e v e l s , t h e J1=O, 1, 2, and 3 l e v e l s i n t h e B s t a t e can be reached, a c c o r d i n g t o AJ =

+

1 s e l e c t i o n r u l e .

F i g u r e 2 shows t h e i o n i z a t i o n spectrum f r o m t h e B, vl=O, J t = O l e v e l , 1 t

(X' I;

, vl'=O, JIt=1)

+

vuv ⁺ ( B Iu, vl=O, J 1 = O )

+

uv ⁺ H2*, ( 2 ) i n t h e s p e c t r a l range 124660-123720 cm-l. The e x c i t a t i o n wavelength i n t h e f i r s t s t e p was k e p t a t t h e (X, J " = 1 ⁺ B, J1=O) t r a n s i t i o n w h i l e t h e uv l a s e r was scanned t h r o u g h t h e c o r r e s p o n d i n g wavelength range, i. e.

-

^{298 t o}

-

290 nm. The spectrum has been t a k e n w i t h a f i e l d i o n i z a t i o n p u l s e o f Fion = 178.6 V/cm a f t e r t h e l a s e r e x c i t a t i o n . Note t h e d i f f e r e n t energy s c a l e s i n t h e v a r i o u s s p e c t r a l s e c t i o n s . Also, n o t e t h a t t h e i n t e n s i t y has been normalized, as i n d i c a t e d by t h e a m p l i f i c a - t i o n o f f s e t values f o r t h e r e s p e c t i v e energy i n t e r v a l s . The spectrum t h u s r e p r e s e n t s t h e wavelength dependence o f t h e i o n i z a t i o n y i e l d , though, a t t h e i o n i z i n g f i e l d s t r e n g t h o f Fion = 178.6 V/cm, which determined t h e i o n i z a t i o n o f h i g h l y i n g Rydberg s t a t e s .

The spectrum spans an energy r e g i o n around t h e l o w e s t i o n i z a t i o n l i m i t s o f H2 (X, t

v=O, N) + e- f o r N=O, 1 and 2, as i n d i c a t e d i n t h e f i g u r e . Corresponding t o t h e e x t e r n a l f i e l d , Fion, t h e i o n i z a t i o n t h r e s h o l d s a r e lowered, c l a s s i c a l l y , by a v a l u e determined by t h e f i e l d i o n i z a t i o n saddle p o i n t energy, E = -2 Fion a. u.. F o r Fion = 178.6 V/cm one o b t a i n s E = -24 cm-l, c o r r e s p o n d i n g t o an energy IPo- IE SP I

s P s P

= 124393 cm-l, w i t h I P o b e i n g t h e a d i a b a t i c i o n i z a t i o n l i m i t .

Somewhat unexpectedly, i o n i z a t i o n i s observed n o t o n l y a t e n e r g i e s above t h e ( H ~ + ^f e ) l i m i t . I P o

-

I E I b u t a l s o below t h i s l i m i t , t h a t i s i n t h e r e g i o n o f t h e

s P

H ( n = l )

+

H(n=2) d i s s o c i a t i o n . As was c o n f i r m e d i n separate, p r e l i m i n a r y experiments w i t h mass s p e c t r o s c o p i c i o n d e t e c t i o n , a l l s p e c t r a l f e a t u r e s below IPo

-

I E I a r e due t o t h e f o r m a t i o n o f H' i o n s . T h i s i s e x p l a i n e d by t h e p h o t o d i s s o c i a t i o n H2 SP* ^-t H ( n = l ) ^t H(n=2) w i t h c o n s e c u t i v e p h o t o i o n i z a t i o n o f t h e e x c i t e d atoms, H(n=2)

+

uv

I I I I I I I I I I I I I I

124250 124200 124150

ENERGY (cm-I)

I , , , , , , ,

124100 124000

ENERGY (cm-I)

124450 124400

ENERGY (cm-I)

124350 124300

ENERGY (cm-1) - -

F i g . 2. S e c t i o n o f i o n i z a t i o n - d i s s o c i a -

I I I I I I I I ,

123900 123800 t i o n spectrum o f Hz f r o m t h e BIZ:, v=O, J=O s t a t e . F i o p . = 178.6 V/CT. N = 0, 1, E N E R G Y ( c m - l ) and 2 a r e t h e i m i t s f o r H2 (X'C;,

v=0, N )

+

e.

-.

H+

+

e, by t h e uv l a s e r p u l s e . T h i s e x p l a n a t i o n i s c o n s i s t e n t w i t h t h e observa- t i o n t h a t t h e spectrum below I P o

-

I E I was independent o f t h e Fion f i e l d . We t h u s

s P

have an e x p e r i m e n t a l t e c h n i q u e t h a t a l l o w s t o observe t h e m o l e c u l a r i o n i z a t i o n and t h e d i s s o c i a t i o n processes a t t h e same t i m e .

I n t h e s p e c t r a l range between 124457 cm-' ( t h e p o s i t i o n o f t h e pronounced a u t o i o - n i z i n g l i n e f e a t u r e ) and 124402 cm-I a w e l l developed ~ ; ( n ) Rydberg s e r i e s i s ob- served. T h i s i s shown e n l a r g e d a g a i n i n t h e upper p a r t o f F i g . 3. The l o w e r p a r t o f t h e f i g u r e r e p r e s e n t s t h e same s p e c t r a l s e c t i o n , however, observed w i t h a weaker e l e c t r i c f i e l d o f Fion = 35.7 V/cm. The Rydberg s t a t e s o f t h i s s e r i e s w i t h e n e r g i e s s u f f i c i e n t l y above t h e s a d d l e p o i n t E a r e e v i d e n t l y f i e l d i o n i z e d , w i t h r a t e s

s P

depending on t h e f i e l d s t r e n g t h . The s t a t e n=37, n e a r e s t t o t h e saddle p o i n t , i s t o o weakly i o n i z e d t o be o b s e r v a b l e . The h i g h e s t Rydberg s t a t e observed, j u s t below t h e a u t o i o n i z a t i o n f e a t u r e a t 124459 cm-l, has a quantum number n = 77. Members o f t h i s s e r i e s a t l o w e r e n e r g i e s appeared t h r o u g h p r e d i s s o c i a t i o n , ~ ; ( n ) ⁺ H(n-1) t H(n=2), o b s e r v a b l e v i a t h e p h o t o i o n i z a t i o n o f t h e H(n=2) atoms, as d i s c u s s e d above.

T a k i n g t h e h i g h as w e l l as t h e l o w observed members i n t o account, a n a l y s i s o f t h e s e r i e s showed t h a t i t converges t o t h e N = 1 l i m i t and t h a t t h e e x c i t e d e l e c t r o n i s

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o f ( n d l ) t y p e , u s i n g Hund's case ( d ) n o t a t i o n . Comparison o f t h e two s p e c t r a i n F i g . 3 shows t h a t t h e i s o l a t e d l i n e i n t h e l o w e r spectrum a t 124422 cm-I apparent- l y i s n o t a f f e c t e d n o t i c e a b l y by t h e e x t e r n a l f i e l d . I n t h e upper spectrum t h i s l i n e i s c l o s e t o t h e 4 6 d l member o f t h e n d l s e r i e s . As d e r i v e d f r o m a n a n a l y s i s of s p e c t r a l m a t e r i a l n o t shown here, t h i s l i n e i s t h e 18d3 member o f t h e nd3 s e r i e s , c o n v e r g i n g t o t h e N = 3 l i m i t . The 18d3 s t a t e decays v i a p r e d i s s o c i a t i o n H 2 ( l s 6 , 18d3)

-.

H ( n = l )

+

H(n=2), i n t e r a c t i n g v e r y l i t t l e w i t h t h e n d l s e r i e s , f o r o t h e r - w i s e i t s h o u l d have been a f f e c t e d more s t r o n g l y by t h e e x t e r n a l f i e l d .

BINDING ENERGY (cm-')

F i g u r e 4 shows two s p e c t r a t a k e n f r o m t h e J ' = 1 l e v e l , t h a t i s

1

+

1

+

H2(X Cg, v"=O, J U = 0 )

+

vuv +

H2(B

Cu, v ' = 0, J ' = 1 )

+

uv ⁺

Hz*,

( 3 ) i n a wavelength r e g i o n

-

430 cm-' below t h e a d i a b a t i c i o n i z a t i o n 1 i m i t . I n t h i s energy r e g i o n i o n i z a t i o n i n t o

H ~ + +

e, spontaneously as we1 1 as by t h e f i e l d , i s

F i g . 3. S e c t i o n o f i o n i z a t i o n spec- t r u m o f

H p

f r o m t h e BIZ:, v=O, J=O s t a t e i n t h e n d l Rydberg s e r i e s r e - g i o n . Upper spectrum t a k e n w i t h Fion

= 178.6 V/cm, l o w e r spectrum w i t h F j o n = 35.7 V/cm. E5 denotes c l a s - s i c a l f i e l d i o n i z a t i g n s a d d l e p o i n t energy.

J

z d

2

ENERGY (cm-')

-10 -30 -50 -70 -90

I I I I I I I I I

F=178.6 Vlcm

F ~ 3 5 . 7 V~cm

jLVA,

^(ESP

124450 124400

F i g . 4. S e c t i o n o f p h o t o d i s s o c i a - t i o n spectrum o f H, f r o m t h e

B ~ c ' ,

v=O, J=1 s t a t e . P o l a r i z a t i o n o f

VUV

and UV l i g h t i n upper spec- t r u m (

I I

) i n l o w e r spectrum ( 1 ) w i t h r e s p e c t t o each o t h e r .

ENERGY (cm-')

excluded, so t h a t t h e s p e c t r a can be a t t r i b u t e d t o t h e d i s s o c i a t i o n process, H;(n)

-+ H ( n = l ) + H(n=2). The upper spectrum has been t a k e n w i t h t h e vuv and u v l a s e r p o l a r i z e d p a r a l l e l (

I I

⁾ w i t h r e s p e c t t o each o t h e r , w h i l e t h e l o w e r one has been t a k e n w i t h p e r p e n d i c u l a r ( I ) p o l a r i z a t i o n .

A pronounced p o l a r i z a t i o n e f f e c t i s observed which can be accounted f o r as f o l l o w s . The e x c i t a t i o n i n t h e f i r s t s t e p causes an a l i g n m e n t of H2 molecules i n t h e i n t e r - mediate B s t a t e . Choosing t h e p o l a r i z a t i o n d i r e c t i o n a s t h e q u a n t i z a t i o n a x i s , t h e f i r s t a b s o r p t i o n (nMJ = 0 ) prepares a J ' = 1, MJ = 0 l e v e l i n t h e B s t a t e . With t h e p o l a r i z a t i o n i n t h e second s t e p b e i n g p a r a l l e l ( 1

1 )

t o t h e f i r s t one, one ob- t a i n s

M;

= 0, l e v e l s i n t h e f i n a l H; s t a t e s w h i l e w i t h p e r p e n d i c u l a r ( I ) p o l a r i z a - t i o n (aMJ =

+

1) M? =

+

1 l e v e l s a r e e x c i t e d . The complete m a n i f o l d o f f i n a l s t a t e s p o s s i b l e i s g i v e n by: InsO, J=0 >, lnd2, J=0 >, lnd2, J-1 >, lns2, J=2 >, IndO, J=2>,

Ind2, J = 2 >, and lnd4, J=2 >, u s i n g Hund's case d n o t a t i o n f o r t h e h i g h l y e x c i t e d e l e c t r o n . W i t h

( I 1 )

p o l a r i z a t i o n i n b o t h s t e p s a l l f i n a l s t a t e s can be e x c i t e d , w h i l e w i t h (1) p o l a r i z a t i o n o n l y t h e l a s t f i v e ones w i t h J = 1 and 2 can be obtained.

C1-134 J O U R N A L DE PHYSIQUE

Comparison o f t h e ( 1 1 ) and (I) s p e c t r a shows t h a t t h e asymmetric and r a t h e r broad s p e c t r a l f e a t u r e w i t h i t s peak a t 123935 cm-I i s e v i d e n t l y due t o a f i n a l s t a t e w i t h J = 0 and an e l e c t r o n o f nsO o r nd2 t y p e .

CONCLUSIONS

The p h o t o i o n i z a t i o n and p h o t o d i s s o c i a t i o n o f t h e H2 m o l e c u l e f r o m s e l e c t e d r o t a - t i o n a l - v i b r a t i o n a l l e v e l s i n an e l e c t r o n i c a l l y e x c i t e d s t a t e , BIZ:, has been i n - v e s t i g a t e d as a f u n c t i o n o f t h e e x c i t a t i o n energy f o r t h e f i r s t t i m e . Examples o f s p e c t r a have been shown i n t h e v i c i n i t y o f t h e H ~ +

+

e i o n i z a t i o n l i m i t s w i t h t h e H ~ ' i o n i n t h e v=O, N=O, 1 and 2 l e v e l s . W i t h 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 H2(n) Rydberg s e r i e s have been observed. The t e c h n i q u e employed i s found t o a l l o w t h e o b s e r v a t i o n o f t h e m o l e c u l a r i o n i r a t i o n , Hz -+ H ~ +

+

e, and t h e d i s s o c i a t i o n , Hz + H ( n = l )

+

H(n=2), i n t h e same measurement. The work i n d i c a t e s t h e f e a s i b i l t y o f f u r t h e r s t u d i e s a t h i g h l a s e r i n t e n s i t i e s on i n t e r a c t i o n s o f t h e l a s e r f i e l d on t h e decay processes.

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