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OPTOGALVANIC SPECTROSCOPY IN NEON
T. Caesar, J. Heully
To cite this version:
T. Caesar, J. Heully. OPTOGALVANIC SPECTROSCOPY IN NEON. Journal de Physique Collo-
ques, 1983, 44 (C7), pp.C7-261-C7-266. �10.1051/jphyscol:1983722�. �jpa-00223279�
JOURNAL DE PHYSIQUE
Colloque C7, suppl6ment au nO1l, Tome 44, novembre 1983 page C7-261
OPTOGALVANIC SPECTROSCOPY IN NEON
T. Caesar and J . L . Heully
Department of Physics, Chalmers University of Technology, 5-412 96 Giiteborg, Sweden
Resume
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La s p e c t r o s c o p i e o p t o g a l v a n i q u e de l ' a t o m e de neon a e t 6 f a i t e . Deux s e r i e s de Rydberg d i s c r P t e s e t deux s e r i e s de Rydberg a u t o i o n f s e e s o n t@t@ observees. Les r e s u l t a t s obtenus s o n t en t r e s bon a c c o r d avec l e s
?re-
d i c t i o n s t h @ o r i q u e s . L ' i n f l u e n c e du champ e l e c t r i q u e s u r l e s e t a t s a u t o i o n i - sgs t r e s e x c i t e s e s t d i s c u t e e .
A b s t r a c t
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O p t o g a l v a n i c s p e c t r o s c o p y has been done i n Neon. Two o p t i c a l Rydberg s e r i e s and two a u t o i o n i z i n g Rydberg s e r i e s have been observed. The r e s u l t s a r e i n good agreement w i t h t h e o r e t i c a l c a l c u l a t i o n s . The i n f l u e n c e o f t h e e l e c t r i c a l f i e l d on h i g h a u t o i o n i z i n g s t a t e s i s discussed.I n e x c i t i n g one o f t h e 2p e l e c t r o n s f r o m t h e ground s t a t e o f Neon two d i f f e r e n t k i n d o f s t a t e s can be o b t a i n e d : one w i t h t h e c o r e h a v i n g j c = 3 / 2 and t h e second one w i t h jc=1/2. These two s t a t e s have d i f f e r e n t i o n i z a t i o n l i m i t s . High s t a t e s w i t h j c = 1 / 2 l i e above t h e 2p3/2 l i m i t and t h u s a r e degenerate w i t h an i o n i z a t i o n continuum which l e a d s t o a u t o i o n i z a t i o n . Some o f t h e s e a u t o i o n i z i n g s t a t e s have been observed by C o d l i n g e t a l . /l/ and R a d l e r /2/. The energy d i f f e r e n c e between t h e ground s t a t e and t h e f i r s t i o n i z a t i o n l i m i t 2pgb2 i s 21.5 eV which i m p l i e s t h e use a v e r y h i g h energy r a d i a t i o n sources such as t e e l e c t r o n s y n c h r o t r o n . I n u s i n g so s h o r t wave- l e n g t h s ( a b o u t 57 nm) i t i s d i f f i c u l t t o o b t a i n a good s p e c t r a l r e s o l u t i o n . I t i s p o s s i b l e t o e x c i t e s t a t e s (17 eV) by u s i n g a dc discharge. From t h e s e s t a t e s f u r t h e r e x c i t a t i o n i s o b t a i n e d by t w o l a s e r s . The d e t e c t i o n method a r e u s u a l l y photoabsorp- t i o n o r i o n d e t e c t i o n . W i t h t h e o p t o g a l v a n i c method t h e d i s c h a r g e i t s e l f can a l s o be used as an i o n o r e l e c t r o n d e t e c t o r and no f u r t h e r d e t e c t o r equipment i s needed.
Using t h i s method we h ve been a b l e t o observe f o u r Rydberg s e r i e s : 2p5ns and 2p5nd c o n v e r g i n g t o t h e i o n qP31z and t h e 2p5ns1 and 2p5nd' c o n v e r g i n g upon 2~
.
Ther e s u l t s o b t a i n e d a r e compared w i t h R a d l e r and B e r k o w i t z ' s r e s u l t s and w i l A 2 t h e t h e o r e t i c a l r e s u l t s o f Johnson /3/
Experimental arrangement
The e x p e r i m e n t a l set-up i s shown i n f i g . 1. The h o l l o w cathode lamp was a commercial l i t h i u m t u b e w i t h a neon p r e s s u r e o f about 5 T o r r . The f i e l d between t h e e l e c t r o d e s was a p p r o x i m a t e l y 300 V/cm (inhomogeneous) g i v i n g r i s e t o a plasma c u r r e n t o f 5 mA.
The l a s e r l i g h t , produced b y two excimer pumped dye l a s e r s , was d i r e c t e d c o l l i n e a r i n t o t h e d i s c h a r g e between t h e e l e c t r o d e s p e r p e n d i c u l a r t o t h e anode-cathode a x i s , t o a v o i d p h o t o e l e c t r o n s f r o m t h e cathode. The d u r a t i o n o f t h e l a s e r p u l s e s was 10 ns and t h e l i n e w i d t h was 40 GHz. The f i r s t l a s e r was used t o p o p u l a t e f o u r 2p53p i n t e r m e d i a t e l e v e l s ( 2 ~ 2 , 2p3, 2p4, 2p5 i n Paschen n o t a t i o n ) . ~ h e second l a s e r e x c i t e s atoms f r o m t h e i n t e r m e d i a t e s t a t e s t o t h e Rydberg s t a t e s and t h e i o n i z a t i o n continuum.
Experimental r e s u l t s
The p r i n c i p a l f a c t o r w h i c h governs t h e energy v a l u e s o f t h e e x c i t e d s t a t e s i n neon
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983722
JOURNAL DE PHYSIQUE
hollow
cathode
F i g . 1. Experimental set-up.
i s t h e s p i n - o r b i t i n t e r a c t i o n i n t h e a l m o s t c l o s e d 2p s h e l l o f t h e core. T h i s means t h a t a good quantum number t o d e s c r i b e t h se s t a t e s i s t h e J - v a l u e o f t h e c o r e j c = 3 / 2 o r j c = 1 / 2 (denoted w i t h a p r i m e 2p n l ' ) b u t n o t t h e J - v a l u e o f t h e o u t e r -
5
e l e c t r o n . Thus jl c o u p l i n g i s t h e b e s t way t o d e s c r i b e neon. I n g o i n g t o l o w e r n- v a l u e s o f t h e o u t e r - e l e c t r o n a t r a n s i t i o n t o LS c o u p l i n g i s observed. F o r t h e
~ ~ 5 3 ~ ' s t a t e s (2p 2p2, 2pq and 2p5 i n Paschen n o t a t i o n ) t h e r e i s some m i x i n g w i t h t h e 2p53p s t a t e s 1;c=3/2).
The second l a s e r induces a d i p o l e t r a n s i t i o n f r o m t o h i g h e r e x c i t e d s t a t e s : I t s h o u l d be n o t e d t h a t a d i p o l e t r a n s i t i o n c a n n o t cause c o r e - s w i t c h i n g .
a ) ns-nd s e r i e s
As mentioned above t h e ~ ~s t a t e s a r e n o t p u r e I j s t a t e s . Thus ~ 3 ~ ' i t i s p o s s i b l e t o observe t r a n s i t i o n s f r o m t h o s e s t a t e s t o t h e ns and nd Rydberg s e r i e s . The f o l l o w i n g t r a n s i t i o n s have been r e c o r d e d
The energy l e v e l s and t h e quantum d e f e c t s a r e l i s t e d i n t a b l e 1.
I n o r d e r t o determine t h e a b s o l u t e energy l e v e l s and quantum d e f e c t s we used t h e f o l l o w i n g energy l e v e l - v a l u e s g i v e n by Kaufman and Minnhagen /4/
2p2 3p1 [1/2] 151038.4524 cm-' 2p5 3 p 1 [3/2] 150772.1 118 cm-' i o n i z a t i o n l i m i t t o 2p:12 173929.75 cm-'
We have been a b l e t o r e s o l v e t h e ns s t a t e s f r o m t h e ( n - l ) d u p t o n=21. The quantum d e f e c t 6 o b t a i n e d f r o m o u r energy l e v e l s a r e much more r e g u l a r t h a n t h o s e o b t a i n e d
t h i s t h i s
work Bashkin
'
nd work'
Bashkin14 173248 1.313 173253 1.266 13 173279 0.014 173277
w i t h l e v e l s g i v e n i n Bashkin t a b l e / 5 / . A l t h o u g h o u r u n c e r t a i n t i e s on 6 a r e l a r g e , t h e ns and nd s t a t e s seem t o be p e r t u r b e d b y t h e n s ' and t h e nd: 161. Because t h e s e two Rydberg s e r i e s were n o t t h e main i n t e r e s t o f t h i s work, no p a r t i c u l a r e f f o r t was done i n o r d e r t o e x t r a c t t h e energy l e v e l s . Improvement c o u l d be o b t a i n e d by u s i n g o t h e r i n t e r m e d i a t e s t a t e s ( j =3/2), more monochromatic l i g h t sources and b y t a k i n g c a r e o f p r e s s u r e and f i e l d e f f e c t s .
b ) n s ' and n d ' , a u t o i o n i z i n g s t a t e s .
The aim o f t h i s work was i n f a c t t o see i f t h e o p t o g a l v a n i c method i s u s e f u l i n o r d e r t o i n v e s t i g a t e a u t o i o n i z i n g s t a t e s . The r e s u l t s o b t a i n e d show t h a t t h i s method i s q u i t e s u c c e s s f u l f o r t h i s purpose. Three i n t e r m e d i a t e l e v e l s have been e x c i t e d i n o r d e r t o r e a c h t h e n s ' and n d ' s t a t e s , 2p2, 2p4 and 2p5. F i g . 2 shows a p o r t i o n o f t h g spectrum f o r t h e t r a n s i t i o n s 2p2+ns',nd' which l i e above t h e i o n i z a t i o n l i m i t 2p3 2. The energy l e v e l s a r e l i s t e d i n t a b l e 2. Our measured energy l e v e l s a r e cornbared w i t h t h o s e o f R a d l e r and B e r k o w i t z . They a r e a l s o compared w i t h c a l c u l a t e d energy l e v e l s o b t a i n e d by u s i n g t h e quantum d e f e c t g i v e n by Johnson and Le Dourneuf /3/. The agreement between o u r r e s u l t s and t h e t h e o r e t i c a l r e s u l t s o f Johnson a r e s t r i k i n g . The l a r g e d i s c r e p a n c i e s between R a d l e r and B e r k o w i t z ' s r e s u l t s and o u r i s due t o t h e i r poor r e s o l u t i o n nE=13 cm around 5 7 nm.
F i g . 2. P a r t o f t h e spectrum f o r t h e t r a n s i t i o n 3 p ' ~ 1 / 2 ] , + n s ' ,nd'.
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this
work J o h n s o n Radler nd' J o h n s o n Radler
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TABLE 2 8 REF (4)
For n<22 n o t y p i c a l F a n o - p r o f i k i s o b t a i n e d . T h i s f a c t i s p r e d i c t e d b y Johnson and Le Dourneuf /3/. They a l s o c a l c u l a t e d t h e l i n e w i d t h o f t h e resonances:
n, 3 r ( n d ' ) = 0.001 au n? r ( n s ' ) = 0.0033 au
F o r example r ( 1 2 d ' )=3.8 GHz. T h i s i s i n disagreement w i t h e x p e r i m e n t a l r e s u l t s o f R a d l e r and B e r k o w i t z / 2 / : r ( 1 2 d 1 )=245 GHz. Our photonbandwidth was 40 GHz and t h e r e c o r d e d l i n e w i d t h s have t h e same values. O f t h a t we can conclude t h a t r ( 1 2 d 1 ) i s c e r t a i n l y l e s s t h a n 40 GHz which c o n f i r m e d t h e c a l c u l a t i o n o f Johnson and Le Dourneuf.
R e c e n t l y Ganz e t a1 /7/, u s i n g a n o t h e r method, have measured t h i s resonancewidth t o be 2 GHz c o n f i r m i n g d e f i n i t e l y Johnson and Le D o u r n e u f ' s r e s u l t s .
As p o i n t e d o u t by Fano /8/ t h e numerical parameter q, which c h a r a c t e r i z e s t h e assymetry o f t h e l i n e s h a p e s h o u l d remain n e a r l y c o n s t a n t t h r o u g h o u t t h e upper p a r t
I
174502 174569 cm-'
F i g . 3. P a r t o f t h e spectrum f o r t h e t r a n s i t i o n 3p' [3/2] -+ n s ' ,nd'
.
o f t h e a u t o i o n i z e d Rydberg s e r i e s . As s a i d b e f o r e a l l l i n e s w i t h n<22 a r e v e r y symmetric and i t i s s u r p r i s i n g t o o b t a i n f o r h i g h e r n such assymmetric l i n e s as shown i n f i g . 3.
The Fano c o n c l u s i o n about r e g u l a r i t y o f t h e l i n e s h a p e was o b t a i n e d under t h e assump- t i o n t h a t no c o l l i s i o n a l broadening o r o t h e r e x t e r n a l p e r t u r b a t i o n s were p r e s e n t . But i n f a c t i n h o l l o w cathode lamp i t i s known t h a t t h e c o l l i s i o n a l broadening cannot b e n e g l i g e d . Furthermore t h e e l e c t r i c f i e l d between t h e e l e c t r o d e s (300 V/cm) must be a s t r o n g p e r t u r b a t o r . I n t h e case o f an e l e c t r o n i n a Coulomb f i e l d a c r i t i c a l energy can be d e f i n e d Ec=E ( 2 ~ 0 / 2 ) ; ~ ~ i (au). Above t h i s energy t h e atom i o n i z e s . I n o u r case EC l i e s a b o u t
Id!
cm-! below t h e P? i o n i z a t i o n l i m i t . S t a t e s l y i n g j u s t below Ec have two p o s s i b i l i t i e s f o r i o n i z a t i o n { ' f i r s t v i a t h e a u t o i o n i z i n g . . p r o c e r s t o2 ~ 9 / 2
continuum and second v i a t u n n e l l i n g t o 2 ~ ~ / 2 continuum. I t has been observed /9/ t h a t f o r bound s t a t e s t h e second process can g i v e f i s e t o t y p i c a l F a n o - p r o f i l e .
I n o u r case t h e d e f o r m a t i o n o f t h e l i n e s h a p e occurs ( f i g . 3 ) i n t h e r e g i o n around Ec.
I t s h o u l d a l s o be n o t e d t h a t i n t h e same r e g i o n t h e c o n t i n u o u s background i n c r e a s e s r a p i d l y .
Some o t h e r f a c t o r s such as t h e bandwidth and t h e e f f e c t o f t h e l a s e r s c o u l d p l a y an i m p o r t a n t r o l e .
Conclusion
I t has been shown i n t h i s work t h a t t h e o p t o g a l v a n i c method can be v e r y p o w e r f u l i n o r d e r t o do spectroscopy o f Rydberg s t a t e s and h i g h a u t o i o n i z i n g s t a t e s . The main advangates a r e :
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a v e r y s i m p l e e x p e r i m e n t a l set-up i s needed- f o r atoms w i t h v e r y h i g h i o n i z a t i o n l i m i t t h e discharge i s an e f f i c i e n t way t o produce e x c i t e d s t a t e s
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i n t h e same t i m e t h e d i s c h a r g e i s a v e r y good d e t e c t o r f o r e l e c t r o n o r i o n s . F o r h i g h a u t o i o n i z i n g s t a t e s f i e l d p e r t u r b a t i o n s have been observed.C7-266 JOURNAL D€ PHYSIQUE
Acknowledgement
We thank Dr A. S i e g e l f o r communication of h i s work b e f o r e p u b l i c a t i o n , and Dr L.
Pendri l 1 f o r i n t e r e s t i n g d i s c u s s i o n s .
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