OBSERVATION OF THE 5dnl EVEN DOUBLY-EXCITED STATES IN NEUTRAL BARIUM USING TWO-STEP OPTOGALVANIC SPECTROSCOPY

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OBSERVATION OF THE 5dnl EVEN

DOUBLY-EXCITED STATES IN NEUTRAL BARIUM USING TWO-STEP OPTOGALVANIC

SPECTROSCOPY

P. Camus, M. Dieulin, A. El Himdy

To cite this version:

P. Camus, M. Dieulin, A. El Himdy. OBSERVATION OF THE 5dnl EVEN DOUBLY-EXCITED

STATES IN NEUTRAL BARIUM USING TWO-STEP OPTOGALVANIC SPECTROSCOPY. Jour-

nal de Physique Colloques, 1983, 44 (C7), pp.C7-271-C7-274. �10.1051/jphyscol:1983724�. �jpa-

00223281�

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JOURNAL DE PHYSIQUE

Colloque C7, supp16ment au n O 1 l , Tome 44, novembre 1983 page C7-271

OBSERVATION OF THE

5dnR

EVEN DOUBLY-EXCITED S T A T E S IN NEUTRAL BARIUM USING TWO-STEP OPTOGALVANIC SPECTROSCOPY

P. Camus, M. D i e u l i n and A . E l Himdy

L a b o r a t o i r e Aimd ~ o t t o n * , C.N.R.S. II, et.505, 91405 Orsay, France

R6sum6

-

Les niveaux i m p a i r s du baryum n e u t r e s i t u k s au-dessus de l a l i m i t e d ' i o n i s a t i o n 6 s o n t Btk k t u d i b s B l ' a i d e d l u n e e x c i t a t i o n l a s e r B deux Btages combinbe avec une d e t e c t i o n optogalvanique des rksonances o p t i q u e s . A p a r t i r d e s niveaux m 6 t a s t a b l e s 5d6s peuples dans une vapeur du type caloduc e x c i t e e par une dkcharge c o n t i n u e , l e s s e r i e s de Rydberg doublement e x c i t e e s 5 d n j

,

pour des v a l e u r s de J comprises e n t r e 0 e t 5 e t l e niveau 6p2 IS0 ont Btk observes dans l e continuum e n t r e l a premiere l i m i t e d ' i o n i s a t i o n 6 s e t l a seconde l i m i t e d l i o n i s a t i o n 5d de Ba I

.

A b s t r a c t

-

Even p a r i t y l e v e l s of n e u t r a l barium above t h e 6 s i o n i z a t i o n l i m i t have been i n v e s t i g a t e d by u s i n g a two-step pulsed l a s e r e x c i t a t i o n a s s o c i a t e d w i t h optogalvanic d e t e c t i o n of t h e o p t i c a l resonances. From t h e m e t a s t a b l e s t a t e s , 5d6s

,

populated by a d , c . discharge i n a heat-pipe vapour, t h e doubly e x c i t e d 5dnJ Fpdberg s e r i e s w i t h J v a l u e s r a n g i n g from 0 t o 5 and t h e bp2

'so

l e v e l s have been observed i n t h e continuum between t h e f i r s t 6 s and t h e second 5d i o n i z a t i o n l i m i t s of Ba I

.

I

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WHY OPTOGALVANIC SPECTROSCOPY I N NEUTRAL BARIUM ?

Because u n t i l now o b s e r v a t i o n s of h i g h l y e x c i t e d even l e v e l s ( ~ ~ d b e r ~ l e v e l s ) by multiphoton o r m u l t i s t e p l a s e r spectroscopy were g e n e r a l l y l i m i t e d t o r a d i a t i v e p r o c e s s e s s t a r t i n g from t h e ground l e v e l 6s2 and consequently r e s t r i c t e d t o t h e J v a l u e s 0, 1 and 2 obeying t o t h e s e l e c t i o n r u l e s involved i n t h e two-photon o r two-step e x c i t a t i o n used.

I n o r d e r t o extend t h e J l e v e l knowledge of t h e even p a r i t y i n t h e continuum f o r t h e "doubly e x c i t e d " c o n f i g u r a t i o n s ( f o r example 5dnd c o n f i g u r a t i o n h a s 18 l e v e l s w i t h J v a l u e s r a n g i n g from 0 t o 5 ) i n t h e b e s t c o n d i t i o n s of o s c i l l a t o r s t r e n g t h and s e l e c t i v i t y , O.G. t e c h n i c s provides a t t h e same time two f u n c t i o n s :

-

f i r s t , e l e c t r i c c o l l i s i o n a l p o p u l a t i o n of t h e low 5d6s m e t a s t a b l e l e v e l s i n t h e f e e b l y d . c . e x c i t e d heat-pipe vapour ;

-

second, optogalvanic d e t e c t i o n of t h e o p t i c a l two-step resonances i n t h e plasma.

D e t a i l s on t h e experimental set-up have been p u b l i s h e d [1,2].

The accuracy of our r e s u l t s a r e l i m i t e d by t h e l i n e w i d t h of t h e dye l a s e r used (0.25 cm-l F W H M )

,

t h e accuracy of t h e measurement of t h e new 0. G. l i n e s r e f e r e n c e d t o one s t e p p r e v i o u s l y c l a s s i f i e d l i n e s u s i n g c a l i b r a t e d t r a n s m i s s i o n maxima of a vacuum-spaced Fabry-Perot i n t e r f e r o m e t e r (which g i v e s c o n s t a n t fre-.

quency i n t e r v a l between two p a k s of 1.31467(8) cm-l) and t h e measured energy of t h e i n t e r m e d i a t e l e v e l used.

t a b o r a t o i r e a s s o c i 6 h l f U n i v e r s i t 6 Paris-Sud

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983724

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C7-272 JOURNAL DE PHYSIQUE

A t l e a s t , t h e i n c e r t i t u d e i s i n t h e o r d e r of 0.1 cm-l f o r t h e new l e v e l s observed.

Optogalvanic s p e c t r a w i t h two-step l a s e r e x c i t a t i o n have been r e c o r d e d two times by t u n i n g t h e second s t e p l a s e r t o i n v e s t i g a t e h i g h l e v e l t r a n s i t i o n s : one time without t h e f i r s t s t e p s e l e c t i v e e x c i t a t i o n , a second time with t h e f i r s t s t e p e x c i t a t i o n . Comparison of i n t e n s i t i e s between t h e two r e c o r d s e n a b l e u s t o d i s t i n - g u i s h t h e one-step resonances from t h e two-step resonances a s shown on Figure 1.

F i g u r e 1

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Two-step optogalvanic spectrum of Ba I : Enhancement between one-step and two-step s p e c t r a . ( a ) Fabry-Perot f r i n g e s , Aa= 1.314678(5) cm-'. ( b ) Spectrum of Ba w i t h 5d6s

-

5d6p 'F; .:

7059.9

1

f o r t h e f i r s t s t e p t r a n s l - t i o n and second s t e p resonance between 5d6p 'F0

-

5d5 216d5/2 J=5.

( C ) S p c t r u m of 4b)

aiti

only t h e second s t e p l a s e r .

A s t r o n g enhancement (10 t o 50 t i m e s g r e a t e r ) of t h e O.G. s i g n a l a p p e a r s when a two-step p r o c e s s e s i s involved.

11. EXPERIMENTAL DATA BETWEEN THE 6s AND 5d LIMITS.

To f a c i l i t a t e i d e n t i f i c a t i o n of t h e d i f f e r e n t l e v e l s of t h e s e r i e s 5dns

,

5dnd and 5dng l e a d i n g t o t h e 5d l i m i t s , we have used t h e j l j 2 c o u p l i n g scheme w i t h the + o r

-

symbols f o r t h e maximum o r minimum j v a l u e s .

Five i n t e r m e d i a t e 5d6p l e v e l s have been used : '3'; : 22947.44 cm-'

'F; : 23757.@5 cm-' 'P(; : 25704.08 cm-'

'P; : 25956.49

cm1

I P ; ~ : 2 8 5 5 4 . 1 9 ~ ~ ~

t o e s t a b l i s h t h e energy of 676 i n t e r p r e t e d new l e v e l s

['l.

Figure 2a shows t h e O.G. spectrum r e c o r d e d i n bhe r e g i o n of t h e n = l l member of t h e doubly-excited 5d+nd s e r i e s o u t of t h e 5d6p 'F'' l e v e l : Four resonances a r e 'observed f o r t h e f i v e allowed t r a n s i t i o n s g i v i n g t h e g i n e s t r u c t u r e s p l i t t i n g s f o r

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t h e 5dl l d member : (++) ~ = 3

,

(++) J=5

, (+-l

J=4 and (++) J=4

.

^The ^(+-)^J=3 ^{i s}

n o t observed from t h e 5d6p

3 ~ ;

i n t e r m e d i a t e l e v e l . This f o u r l i n e s s e r i e s a s f a r a s n i s i n c r e a s i n g becomes u n r e s o l v e d around n=25 t a k i n g i n account t h e l a s e r l i n e w i d t h and l e a d t o t h e spectrum reproduced on Fig. 2b. I n t h e l e f t s i d e f o o t of t h e 5d /225d5/ J=5 resonance, i t c a n be s e e n (arrows pointed i t ) t h e unresolved J=4

(+-3

^and

L+)

resonances. Weak l i n e s between e a c h member 5d5/2nd 5=5 a r e 5d512ns J=3 r e s o n a n c e s , th@ only allowed t r a n s i t i o n s with 5d6p ?F;

.

F I N E S T R U C T U R E

F i g u r e 2a

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Two-step optogalvanic spectrum of Ba I : Fine s t r u c t u r e of t h e 5d5/211d term. ( a ) Fabry-Perot f r i n g e s . ( b ) Spectrum of Ba w i t h 5d6s 3 ~ 3

-

5d6p

3 ~ 4 0

^:^7059.9

1

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 range 4456

-

⁴⁴⁷⁶

1

f o r t h e second s t e p l a s e r .

F i g u r e 2b

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Two-step optogalvanic spectrum of Ba I : Rydberg s e r i e s

5d512ns J=3 and 5 d 5 ~ 2 n d . J = 3 , 4 , 5

.

( a ) Fabry-Perot f r i n e s . ( b ) Spectrum of

Ba

f i r s t s t e p t r a n s i t i o n .

9

below t h e 5d5/2 ionization l i m i t w i t h 5d6s 3~~

-

^{5d6p F}^: ^:^7059.^p

²

f o r t h e

More complex f e a t u r e s can be observed w i t h t h e medium J v a l u e s a s shown on F i g u r e 3 where t h e O.G. spectrum from t h e 5d6p ?F; l e v e l i s reproduced i n t h e v i c i n i t y of t h e 5d l i m i t .

3 / 2

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JOURNAL DE PHYSIQUE

Figure 3

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Two-step optogalvanic spectrum of Ba I

.

( a ) Fabry-Perot f r i n g e s . ( b ) Spectrum of B a i n t h e v i c i n i t y o f the 5d3i2 i o n i z a t i o n l i m i t w i t h 566s

-

5d6p

3 ~ 5

^:^7280.7

¹

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 .

On t h e r i g h t s i d e of t h e spectrum, members of t h e unresolved 5d3/2nd and 5d3j2ns s e r i e s l e a d i n g t o t h e

5d7j2 l i m i t a r e overcrossed by a broad l i n e corresponding t o t h e unresolved a u t o i o n l z e d (+-) 3=3,2 and 4 n=14 member of t h e 5d5i2nd s e r i e s l e a d i n g ts t h e upper 5d5l2 l i m i t . This broad p r o f i l e i s r e p e a t e d f o r n=15 and h i g h e r members. It i s due t o c o n f i g u r a t i o n m i x i r ~ g w i t h t h e (-+) group of l e v e l s a s shown by Lu-Fano g r a p h s g i v e n in t h e MQDT a n a l y s i s r e p o r t e d i n r e f . C41

A s a c o n t r a s t between doubly-exci-ced l e v e l s of t h e 5dnd s e r i e s showing q u i t e s h a r p p r o f i l e s , t h e fjp2 ISo l e v e l is e x h i b i t i n g v e r y s t r o n g a u t o i o n i z i n g p r o f i l e r e s u l - t i n g from i n t e r a c t i o n w i t h t h e 6sEs continuum [5]. The p r o f i l e i s expanded over more t h a n 300 cm-l showing mixing f e a t u r e s w i t h t h e 5dnd J=0 s e r i e s . A l l t h i s d a t a h a s been e x p l a i n e d u s i n g MQ3T c a l c u l a t i o n s [5].

I11

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CONCLUSION.

This e x p e r i m e n t a l s t u d y prooves t h e g r e a t e f f i c i e n c y and power of t h e two-step optogalvanic s p e c t r o s c o p y t o i n v e s t i g a t e t h e Rydberg l e v e l s t r u c t u r e a n a l y s i s i n t h e continuum above t h e f i r s t i o n i z a t i o n l i m i t of n e u t r a l barium.

REFERENCES.

[ l ] CAMUS P., DIEULIN M. and MORILLON C . , J. Physique L e t t . Q (1979) 5 13.

CAMUS P., DZULIN M. and EL HIMDY A . , Phys. Rev. A (1982) 379.

CAMUS P., DIEULIN M., EL HIMDY A. and AYMAR M., Physica S c r i p t a

a

(1983) 125.

[4] AYMAFt M., CAMUS P. and EL HIMDY A., Physica S c r i p t a

a

(1 983) 183.

[5] AY^ M., CAMUS P. and EL HIMDY A., J. Phys. B : Atom. Molec. PhYs.

E

(1982) L75 9.