A BEAM-FOIL STUDY OF LITHIUM

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A BEAM-FOIL STUDY OF LITHIUM

J. Bromander, S. Hultberg, B. Jelenkovik, L. Liljeby, S. Mannervik

To cite this version:

J. Bromander, S. Hultberg, B. Jelenkovik, L. Liljeby, S. Mannervik. A BEAM-FOIL STUDY OF LITHIUM. Journal de Physique Colloques, 1979, 40 (C1), pp.C1-10-C1-13.

�10.1051/jphyscol:1979103�. �jpa-00218382�

JOURNAL DE PHYSIQUE

Colloque

C1,

supplt?ment au n o 2, Tome 40, fe'vrier 1979, page

C1-10

A BEAM-FOIL STUDY OF LITHIUM

J . Bromander, S. Hultberg, B. JelenkoviE, L. L i l j e b y and S. Mannervik Research I n s t i t u t e of P h y s i c s , 104 05 Stockholm, Sweden

RGsum6. Bien q u ' i l e x i s t e p l u s i e u r s e x p l o r a t i o n s des niveaux doublement e x c i t e s d u L i e t s a sequence i s o d l e c t r o n i ye,il y a encore des i n c e r t i t u d e s concernant l e s schZmas dffitermes. Rgcemment,

9

d i f f e ' r - e n t s a u t e u r s o n t prBsent6 des c a l c u l s nouveaux s u r l e s Q n e r g i e s des niveaux e n L i I**, impliquant des r e - c l a s s i f i c a t i o n s e t a u s s i l ' e x i s t e n c e de t r a n s i t i o n s non d d j 8 o b s e r v s e s . Dans l e diagramme des niveaux quelques-unes de c e s t r a n s i t i o n s f o m e n t d e s b o u c l e s ferm6es

,

dont on p e u t c o n t r 5 l e r prgcisement l a b a l a n c e d r B n e r g i e pourvu que l e s longueurs d'ondes s o i e n t b i e n connues. En consgquence, nous avons f a i t un nouvel examen d S t a i l l 6 expgrimental d e s s p e c t r e s L i 1-111 dans l a r e g i o n 2000-5500 A e n employ- a n t l a technique beam-foil. Environ 125 r a i e s onte'tb r e g i s t r e e s e t soigneusement a n a l y s s e s e n u t i l i s a n t un programme d ' a j u s t e m e n t p a r desgaussiennes. Des r d s u l t a t s s ~ l e s l o n g u e u r s d'ondes,%intensit$s r e l a t i v e s e t l e s d d v i a t i o n s s t a n d a r d s s o n t donnds.

A b s t r a c t . Although s e v e r a l i n v e s t i g a t i o n s of doubly e x c i t e d s t a t e s i n t h e L i i s o e l e c t r o n i c sequence have been made, many u n c e r t a i n t i e s remain with t h e term schemes p r e s e n t e d s o f a r . Recently, many a u t h o r s have p r e s e n t e d new c a l c u l a t i o n s of energy l e v e l s i n L i I**, s u g g e s t i n g r e c l a s s i f i c a t i o n s and a l s o t h e e x i s t e n - c e of t r a n s i t i o n s n o t p r e v i o u s l y observed. I n t h e l e v e l diagram, some of t h e l a t t e r a r e members of c l o s - ed loops which c a n be checked t o a high degree of accuracy i f t h e wavelengths a r e w e l l known. We have, t h e r e f o r e , undertaken a thorough s t u d y of L i 1-111 s p e c t r a i n t h e wavelength r e g i o n 2000-5500 A, u s i n g t h e beam-foil method. A t o t a l of about 125 l i n e s have been recorded and c a r e f u l l y analyzed, u s i n g a Gaussian f i t t i n g r o u t i n e . Wavelengths, r e l a t i v e i n t e n s i t i e s and s t a n d a r d d e v i a t i o n s a r e deduced.

INTRODUCTION

It i s w e l l known t h a t doubly e x c i t e d q u a r t e t l e v e l s i n t h e L i I sequences a r e e a s i l y p o p u l a t e d by beam-foil e x c i t a t i o n . The a n a l y s i s , however, is hampered by t h e l i m i t e d r e s o l u t i o n and t h e moderate wavelength accuracy s o f a r o b t a i n - ed i n beam-foil s t u d i e s of t h e s e systems. The nar- row f i n e s t r u c t u r e of t h e q u a r t e t m u l t i p l e t s i s impossible t o r e s o l v e u s i n g t h e beam-foil s o u r c e , thus making i t impossible t o make assignments w i t h t h e h e l p of r e l a t i v e i n t e n s i t i e s and s p l i t t i n g s w i t h i n a m u l t i p l e t . The only o t h e r way l e f t t o make c l a s s i f i c a t i o n s independently of t h e o r e t i c a l c a l c u l a t i o n s i s t o make use of c l o s e d loops i n t h e l e v e l diagram, u s i n g t h e Rydberg-Ritz combination p r i n c i p l e . The accuracy of t h i s method i s h i g h l y dependent dn t h e wavelength accuracy t h a t can b e achieved i n t h e e v a l u a t i o n of t h e beam-foil s p e c t r a .

Previous experimental i n v e s t i g a t i o n s of t h e L i I** system (1) have been based mostly on t h e o r e t i c a l c a l c u l a t i o n s by Holbien and Geltman ( 2 ) . However, l a t e r c a l c u l a t i o n s ( 3 , 4 ) have shown t h a t t h e e x c i t a t i o n e n e r g i e s of l s 2 s 3 p 4~ and l s 2 s 4 p 4~ were t o o low i n Holbien and Geltman's paper. Thus s e v e r a l of t h e t r a n s i t i o n s i d e n t i f i e d

by Berry e t a l . (1) have t o be r e c l a s s i f i e d i n view of t h e new c a l c u l a t i o n s .

The scope of t h e p r e s e n t i n v e s t i g a t i o n h a s been t o provide an experimental b a s i s f o r fur- t h e r work on t h e l e v e l diagram of doubly e x c i t e d n e u t r a l l i t h i u m . Thus g r e a t c a r e h a s been t a k e n t o o b t a i n good wavelength v a l u e s f o r t h e L i I**

t r a n s i t i o n s by u s i n g a c c u r a t e l y known L i I , L i I1 and L i I11 l i n e s a s s t a n d a r d s . Care was a l s o tak- e n t o improve t h e r e s o l u t i o n i n o r d e r t o s e p a r a t e L i I** l i n e s from L i I1 l i n e s , which i n some c a s e s was e s s e n t i a l f o r t h e c l a s s i f i c a t i o n work. E f f o r t s have a l s o been made t o o b t a i n an i n t e n s i t y s c a l e u s e f u l i n t h e e n t i r e wavelength r e g i o n i n v e s t i g a t e d .

EXPERIMENT

The 400 kV heavy i o n a c c e l e r a t o r of t h e Research I n s t i t u t e of P h y s i c s (AFI) was used t o a c c e l e r a t e 7 ~ i + i o n s t o e n e r g i e s between 120 keV and 300 keV. A f t e r e x c i t a t i o n i n a 12 pg/cm2 car- bon f o i l spectrum was observed u s i n g a Heath EUE 700 monochromator equipped w i t h a cooled EM19789 QB p h o t o m u l t i p l i e r . To o b t a i n a s good r e s o l u t i o n a s p o s s i b l e t h e s p e c t r o m e t e r was modified t o make

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979103

use of t h e r e f o c u s i n g technique ( 5 ) . S p e c t r a were a u t o m a t i c a l l y s t o r e d i n t h e memory of a BM96 I n t e r technique multichannel a n a l y z e r i n s t e p s of 0 . 2 5 - -O.SA/ch. The spectrum was punched on paper tape f o r f u r t h e r a n a l y s i s i n an o f f - l i n e computer. The s i g n a l p u l s e s of each channel were accumulated un- t i l a monitoring p h o t o m u l t i p l i e r viewing t h e t o t a l l i g h t a t a f i x e d p o s i t i o n on t h e downstream s i d e of t h e f o i l reached a p r e s e t number of c o u n t s . I n t h i s way t h e beam and f o i l c o n d i t i o n s a r e monitor- e d t o make i t p o s s i b l e t o determine t h e r e l a t i v e amplitudes of t h e l i n e s more a c c u r a t e l y . To g e t t h e s e one a l s o has t o t a k e i n t o account t h e wave- l e n g t h dependence of t h e s e n s i t i v i t y of t h e spec- t r o m e t e r . Since t h e Heath monochromator has been c a l i b r a t e d f o r quantum e f f i c i e n c y on a r e l a t i v e s c a l e , i t was p o s s i b l e t o normalize t h e spectrum t o t h i s c u r v e and t o g e t t h e r e l a t i v e amplitudes of t h e l i n e s . The wavelength r e g i o n covered was 2000-5500 A. To avoid second o r d e r l i n e s a g l a s s p l a t e was i n s e r t e d d u r i n g t h e measurement of t h e upper p a r t of t h e r e g i o n .

DATA ANALYSIS

To o b t a i n p r e c i s e wavelengths and e s t i - mates of peak i n t e n s i t i e s we used a computer pro- gram which f i t s Gaussian and L o r e n t z i a n shapes.

S u i t a b l e r e f e r e n c e l i n e s were picked and f i t t e d t o y i e l d a wavelength c a l i b r a t i o n . Gaussian shapes were t h e n f i t t e d t o t h e s p e c t r a l l i n e s , and i n two c a s e s a l s o L o r e n t z i a n s . Usually, backgrounds were f i t t e d s e p a r a t e l y b e f o r e t h e Gaussians t o o b t a i n t h e b e s t p o s s i b l e t r a c i n g of t h e former, u s i n g cu- b i c s p l i n e s . The f i t t i n g program allows a r a t h e r e x t e n s i v e c o n t r o l of parameters, such a s s t r i n g i n g peaks t o g e t h e r i n groups w i t h r e s p e c t t o fwhm:s, r e l a t i v e d i s t a n c e s o r r e l a t i v e h e i g h t s , and some o t h e r u s e f u l f e a t u r e s .

t h a t we could n o t s e e a r e 2801 A, 2846 A, 2968 A and 2895 A i n t h e l i s t of t r a n s i t i o n s i n doubly e x c i t e d l i t h i u m r e p o r t e d by Berry e t a l . ( 1 ) . These l i n e s a r e r e p o r t e d a s being very weak and c l a s s i - f i e d i n r e f . 1 e i t h e r a s two e l e c t r o n jumps o r tran- s i t i o n s i n t h e doubly e x c i t e d d o u b l e t system. Also we could only s e e one of t h e u n c l a s s i f i e d l i t h i u m l i n e s r e p o r t e d i n r e f . 1,namely t h e s t r o n g e s t one a t 2640 A.

COMMZNTS ON THE LEVEL DIAGRAM OF L i I**

A f t e r t h e r e v i s i o n of t h e l s 2 s 3 p 4~ and l s 2 s 4 p 4~ term v a l u e s , f i v e t r a n s i t i o n s c l a s s i f i e d a s combinations w i t h e i t h e r of t h e s e l e v e l s had t o be r e c o n s i d e r e d , namely 8517 A (never observed i n beam-foil s p e c t r o s c o p y ) , 5315 A, 4760 A, 3488 A and 2801 A. I n t h e p r e s e n t experiment we cannot observe 8517 because i t i s o u t s i d e t h e s p e c t r a l range covered. The 2801 l i n e was observed by Berry e t a l . (1) a s a very weak l i n e and i t i s not p r e s e n t i n t h e new s p e c t r a . The 5315 A and t h e 4760 l i n e s have new t e n t a t i v e assignments a s l s 2 s 4 p 4~

-

ls2p4p "P and l s 2 p 2 P'

-

l s 2 s 4 p 4 ~ , r e s p e c t i v e l y , w h i l e t h e r a t h e r s t r o n g 3489 l i n e remains u n c l a s s i f i e d .

S e v e r a l c l o s e d loops have been i n v e s t i - g a t e d , and t h e new improved wavelength v a l u e s to- g e t h e r w i t h some new o b s e r v a t i o n s seem t o v e r i f y some p r e v i o u s c l a s s i f i c a t i o n s .

A f t e r a s u g g e s t i o n by Wittmann and Andrl (6) a Li I** l i n e was found a t 4885.1 j u s t r e - solved from t h e L i I1 l i n e a t 4881 8.

With a few e x c e p t i o n s , a l l p r e v i o u s l y observed l i n e s i n L i I , L i I**, L i I1 and Li I11 i n t h e i n v e s t i g a t e d wavelength r e g i o n were seen.

I n a d d i t i o n t o t h e p r e v i o u s l y observed l i n e s we d i d observe some new l i n e s t h a t could be f i t t e d i n t h e Li** l e v e l diagram and a l s o around e i g h t l i n e s t h a t could n o t be c l a s s i f i e d , b u t p o s s i b l y belong t o t h e same system. The p r e v i o u s l y r e p o r t e d l i n e s

This loop was suggested by Berry e t a l . (1) and f i t s w e l l w i t h t h e observed wavelength v a l u e s .

JOURNAL DE PHYSIQUE

I n a r e c e n t p a p e r Bunge and Bunge (7) have e x t e n d e d t h e t h e o r e t i c a l c a l c u l a t i o n s t o t h e l s 2 s 2 p 4~ l s 2 p 4 d 4~ l s 2 s n f 4~ and l s 2 p n f 4~ l e v e l s and s u g g e s t e d t h a t 2039.6 A \

/

^{5395.6 A} a s t r o n g t r a n s i t i o n , l s 2 s 4 f 4~

-

l s 2 p 4 f 4 ~ , s h o u l d

l s 2 s 4 d 4~ o c c u r a t 5486 A. This i s v e r y c l o s e t o t h e s t r o n g L i I1 resonance t r a n s i t i o n a t 5485 A which probab- T h i s l o o p was a l s o s u g g e s t e d b y B e r r y e t a l . l y c o m p l e t e l y masks t h e L i I** t r a n s i t i o n . However,

( 1 ) and f i t s w e l l w i t h t h e o b s e r v a t i o n s . we do s e e a p r e v i o u s l y u n r e p o r t e d l i n e a t 5471.9

8

which c o u l d b e t h e s u g g e s t e d t r a n s i t i o n b u t s i n c e t h e c a l c u l a t e d wavelengths f o r o t h e r t r a n s i t i o n s i n r e f . 7 a r e i n much b e t t e r agreement w i t h expe- l s 2 s 2 p 4~ l s 2 p 4 d 4~ r i m e n t t h e 5471.9 l i n e might h a v e a n o t h e r e x p l a -

2 3 3 6 . 9 * \

/

4 0 3 6 . 9 A n a t i o n .

l s 2 s 4 d 4~

T h i s l o o p i s a combination o f b) and c ) i n - v o l v i n g t h e v e r y weak b u t c l e a r l y o b s e r v a b l e l i n e a t 4036.9

8.

A f i f t h l o o p h a s b e e n s u g g e s t e d b y Ahle- n i u s e t a l . (4) i n c l u d i n g t h e l s 2 s 3 p 4 ~ , l s 2 s 4 p 4 ~ , l s 2 p 3 p 4~ and l s 2 p 4 p 4~ terms. T h i s l o o p was p a r t - l y b a s e d on a L i I** t r a n s i t i o n masked by t h e L i I1 l i n e a t 5037.9 A. A c l o s e r i n s p e c t i o n o f t h i s L i I1 t r a n s i t i o n r e v e a l e d a n o t h e r component a t 5033.4 which c o u l d b e l o n g t o t h e L i I** s y s - tem. However, t h i s wavelength does n o t f i t i n t o t h e l o o p proposed by Ahlenius e t a l . ( 4 ) , and t h u s we h a v e n o t b e e n a b l e t o v e r i f y t h e i r sugges- t i o n . Another way t o check t h e p o s i t i o n s of l s 2 p 3 p 4~ and l s 2 p 4 p 4~ would b e t o l o o k f o r t h e t r a n s i t i o n s from t h e s e l e v e l s down t o l s 2 s 2 p 4 ~ . These l i n e s s h o u l d f a l l i n t h e vacuum u l t r a v i o l e t r e g i o n a r o u n d 1675 and 1500 8. We have t r i e d t o o b s e r v e t h e s e l i n e s b u t n o t h i n g was s e e n pro- b a b l y b e c a u s e of t h e s t r o n g L i I1 t r a n s i t i o n s a t 1681 A and 1493 A which c o u l d c o m p l e t e l y mask t h e L i I** l i n e s .

CONCLUSIONS

The g e n e r a l s t r u c t u r e o f t h e Li** l e v e l d i a g r a m seems t o b e w e l l u n d e r s t o o d and t h e a g r e e - ment 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 i s s a t i s f a c t o r y . However, much new i n f o r m a t i o n s h o u l d p o s s i b l y b e o b t a i n e d i f t h e s p e c t r a l r a n g e c o u l d b e e x t e n d e d above 5500 A i n t o t h e r e d and n e a r i n f r a r e d r e g i o n . Also a s e a r c h f o r i m p o r t a n t t r a n s i t i o n s i n t h e vacuum u l t r a v i o l e t r e g i o n c o u l d b e f r u i t f u l .

BIBLIOGRAPHY

(1) H.G. B e r r y , E.H. P i n n i n g t o n and J.L. S u b t i l , (5) J . O . S t o n e r and J.A. L e a v i t t , Appl. Phys.

J. Opt. Soc. Am.

62,

767 (1972). L e t t .

18,

477 (1971)

(2) E. H o l b i e n and S. Geltman, Phys. Rev.

153,

(6) W. Wittmann and J. AndrS, p r i v . comm. (1978).

8 1 (-1967).

(7) C.F. Bunge and A.V. Bunge, Phys. Rev.

E ,

(3) S. L u n e l l , P h y s i c a S c r i p t a

16,

1 3 (1977). 822 (1978).

(4) T. A h l e n i u s ,

B.

C r o s s l e y and S. L a r s s o n , (8) G. H e r z b e r g and H.R. Moore, Can. J. Phys.

Phys. L e t t e r s

e,

270 (1977).

-

37, 1293 (1959).

TABLE 1

P r e l i m i n a r y l i s t o f t r a n s i t i o n s i n t h e doubly e x c i t e d s y s t e m o f n e u t r a l l i t h i u m and u n c l a s s i f i e d t r a n s i t i o n s between 2000 .& and 5500 A

I n t e n - s i t y

1110 560 180 170 8770 87 52 180 29 80 5980 1 4 3940

380 11

The i n t e n s i t y f i g u r e s a r e a r e a s u n d e r t h e p e a k s e v a l u a t e d on a l i n e a r s c a l e c o r r e c t e d f o r t h e s e n s i t i v i t y of t h e s p e c t r o m e t e r and t h e d e t e c t o r . The wavelength v a l u e s a r e i n t e r p o l a t e d v a l u e s u s i n g k n o n t r a n s i - t i o n s i n L i I , L i I** (from r e f . 8 ) , L i I1 (from r e f . 8 ) , and L i 111.

1 )

T h i s wavelength v a l u e i s a weighted meanvalue of t h e d i f f e r e n t components measured by Herzberg and Moore ( 8 ) .

F i g . 1

3618.9 2 0 . 1 1 ~4~ 2- l s 2 p 3 s ~ ~ 4~

l s 2 s 2 p 4p

-

l s 2 p 2 4P Wavelength

(1) 2039.6 f 0 . 1 2173.4 f 0 . 1 2308.5 f 3 . 3 2313.5 f 2.1 2336.92l) 2351.9 f 1.6 2359.0 f 2.4 2460.4 f 0 . 2 2515.4 f 0.9 2 6 4 0 . 1 f l . O 2868.8 f 0 . 1 2883.4 f 3.6 2934. lo1) 2 9 5 4 . 1 f 0 . 2 2994.1 f 0.3 3144.5

+

0.2

C l a s s i f i c a t i o n

?

l s 2 s 3 d 4~

-

l s 2 p 4 d 4~

?

?

l s 2 p 2 4~

-

l s 2 s 6 p 4~ ?

?

l s 2 p 2 4~

-

l s 2 s 5 p 4~ ?

?

?

?

1 ~4~ 2

-

l s 2 s 4 p ~ ~ 4~

l s 2 s 3 s 4~

-

l s 2 p 3 s 4~

?

l s 2 s 3 p 4~

-

l s 2 p 3 p 4~ ? l s 2 s 3 d 4~

-

l s 2 p 3 d 4~

l s 2 s 4 p 4~

-

l s 2 p 4 p 4~

I n t e n - s i t y

27 6 20 1 8 18 29 38 1 3 78 52 140 1450 23 1090 540

P a r t s of t h e Li-spectrum.

C l a s s i f i c a t i o n

l s 2 s 2 p 4~

-

1s2s4d 4~

1 ~4~ 2

-

l s 2 s 4 s ~ ~ 4~

?

l s 2 p 2 4~

-

ls2p5d 4~

l s 2 s 2 p 4~

-

l s 2 s 3 d 4~

?

?

l s z p 2 4~ - l s 2 p 4 d 4~

?

?

l s 2 p 2 4~

-

ls2p3d 4~

?

l s 2 s 2 p 4~

-

l s 2 s 3 s 4~

?

?

l s 2 s 2 p 2~

-

l s 2 p 3 p 2~

5395.6 f 0 . 8 l s 2 s 4 d 4~

-

l s 2 p 4 d 4~

1 1

^5440.9 5471.9 f 1.4 ^{2 1.0}

^I

l s 2 s 4 f *F l s 2 s 5 d 4~

- -

l s 2 p 4 f l s 2 p 5 d 4~ .D ?

1

Wavelength (A) 3901.3 i 0 . 5 4036.8 f 0 . 8 4061.4 f 0.4 4069.9 f 0.4 4194.8 f 0.7 4 2 0 0 . 4 f 0.5 4390.5 f 0 . 3 4432.5 f 0 . 4 4535.2 2 0.2 4541.0 f 0.4 4759.7 f 0.5 4885.2 f 0 . 3 4 9 0 3 . 2 2 0 . 3 5033.4 f 0 . 1 5266.7 f 0.5 5316.4 f 0.6

a ) A h i g h r e s o l u t i o n s c a n o f t h e p a r t where t h e two new l i n e s a t 4885.2 A and 5033.4 d were found. The i n s e r t shows t h e l i n e s a t 5033.4 and 5037.9 r e c o r d - e d w i t h h i g h e r i n t e n s i t y . b ) The f i g u r e shows t h e L i I**

l i n e a t 5395.6 f4 complete- l y r e s o l v e d f r o m t h e Li I1 l i n e s .

F i g . 1 a F i g 1 b