PHOTOIONIZATION IN THE K-SHELL OF ATOMIC BERYLLIUM

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PHOTOIONIZATION IN THE K-SHELL OF ATOMIC BERYLLIUM

M. Krause, C. Caldwell

To cite this version:

M. Krause, C. Caldwell. PHOTOIONIZATION IN THE K-SHELL OF ATOMIC BERYLLIUM.

Journal de Physique Colloques, 1987, 48 (C9), pp.C9-473-C9-478. �10.1051/jphyscol:1987975�. �jpa-

00227396�

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JOURNAL DE PHYSIQUE

Colloque C9, suppl6ment au n012, Tome 48, decembre 1987

PHOTOIONIZATION IN THE K-SHELL OF ATOMIC BERYLLIUM

M.O. K R A U S E and C.D. CALDWELL'

Oak Ridge National Laboratory, Oak Ridge, TN 37831-6201, U.S.A.

'university of Central Florida, Orlando, F L 32816, U.S.A.

Resum6

-

P h o t o i o n i s a t i o n dans l e Be atomique a 6 t 6 e t u d i 6 en couche K 2 l ' a i d e de rayonnement s y n c h r o t r o n . Des s p e c t r e s Auger e t p h o t o e l e c t r o n o n t 6 t c e n r e g i s t r g s e n t r e 121 e t 190 eV; nous avons obtenus des r e l a t i v e s s e c t i o n s e f f i c a c e s p a r t i e l l e s pour l e s t r a n s i t i o n s 'a s i m p l e e t d o u b l e e l e c t r o n s .

A b s t r a c t .

-

K - s h e l l p h o t o i o n i z a t i o n o f a t o m i c Be was s t u d i e d w i t h t h e use o f s y n c h r o t r o n r a d i a t i o n . Both p h o t o e l e c t r o n and Auger s p e c t r a were measured, and r e l a t i v e p a r t i a l c r o s s s e c t i o n s were o b t a i n e d f o r s i n g l e and t w o - e l e c t r o n t r a n s i t i o n s f r o m 121 t o 190 eV.

The Be atom i s an a t t r a c t i v e c a n d i d a t e f o r t h e s t u d y o f p h o t o i o n i z a t i o n processes and e l e c t r o n c o r r e l a t i o n s i n t h e i s o l a t i o n t h a t i s i n h e r e n t i n a s i m p l e system.

I t i s a s u p r a - h e l i u m atom w i t h an e l e c t r o n p a i r i n each o f t h e f i r s t p r i n c i p a l s h e l l s . Thus, i t s h o u l d a l l o w us t o t e s t and expand t o an atom o f ascending c o m p l e x i t y t h e knowledge t h a t we have g a i n e d i n t h e r e c e n t p a s t f r o m many e x p e r i m e n t a l and t h e o r e t i c a l s t u d i e s on He. U n t i l now, no work has been r e p o r t e d on t h e p h o t o e f f e c t i n t h e 1s s h e l l o f t h e Be atom. I n f a c t , o n l y few s t u d i e s have been made on t h e metal [I], and t h e s e s t u d i e s gave d i v e r g e n t r e s u l t s even i n a l i m i t e d energy range. An a b s o r p t i o n measurement i n t h e atom has been performed [ 2 ] f o r t h e L s h e l l . Work w i t h Be p r o j e c t i l e s has y i e l d e d a w e a l t h o f s p e c t r o s c o p i c i n f o r m a t i o n f o r K s h e l l i o n i z a t i o n f r o m b e a m f o i l e x p e r i m e n t s [3,4], and a number o f r a d i a t i v e t r a n s i t i o n s have been i d e n t i f i e d i n a plasma source [5]. Several t h e o r e t i c a l c a l c u l a t i o n s have been made f o r c r o s s s e c t i o n s [6,7], r a d i a t i v e and r a d i a t i o n l e s s t r a n s i t i o n r a t e s [ 8 ] and energy l e v e l s [9]. No c a l c u l a t i o n t o d a t e c o v e r s t h e c o r r e l a t i o n e f f e c t s i n c o n j u n c t i o n w i t h 1s i o n i z a t i o n . However, i t i s p r i m a r i l y e l e c t r o n c o r r e l a t i o n t h a t can be d e l i n e a t e d by s t u d y i n g Be. W i t h o u t e l e c t r o n c o r r e l a t i o n , p h o t o i o n i z a t i o n o f Be would be t r i v i a l h a v i n g a s i n g l e e x c i t a t i o n channel l s 2 2 s 2 ⁺1s2s2

+

e ( ~ p ) and a s i n g l e decay channel 1 ~ 2 s ' ⁺l s 2

+

e (Auger).

The work p r e s e n t e d h e r e can be b e s t i l l u s t r a t e d by t h e schematic g i v e n i n F i g . 1.

I t shows t h a t a photon w i t h energy h v somewhat above t h e I s t h r e s h o l d can produce a v a r i e t y o f p h o t o e l e c t r o n s l e a v i n g t h e i o n i n d i f f e r e n t s t a t e s (some o f which, as l s 2 s 3 s , a r e o m i t t e d ) . The schematic a l s o shows t h e corres+ponding Auger decay channels i n s e r t e d i n t o t h e t w o - e l e c t r o n continuum above E(Be2 ) = 27.535 eV. The Be atom i s t h e f i r s t atom i n which Auger decay occurs which can t h e n be used as a probe f o r t h e i n i t i a l i o n i z a t i o n event. I t i s i m p o r t a n t t o r e a l i z e t h a t t h e n p h o t o e f f e c t can be i n v e s t i g a t e d by way o f e i t h e r t h e p h o t o e l e c t r o n o r t h e Auger e l e c t r o n spectrum because each has a common s t a r t i n g p o i n t and end p o i n t , r e s p e c t i v e l y . As a r e s u l t , one spectrum r e f l e c t s t h e o t h e r .

I n t h i s experiment, b o t h p h o t o e l e c t r o n and Auger s p e c t r a were measured w i t h o u r e l e c t r o n s p e c t r o m e t e r [10,11] a t t h e A l a d d i n 800 MeV s t o r a g e r i n g i n Wisconsin.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987975

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JOURNAL DE PHYSIQUE

IONIZATION LIMITS AN0 ELECTRON CONTINUA 6 e t ( 4 s 2 ) O e t ( t s ) 0 e 2 +

F i g . 1 Schematic o f m a j o r e x c i t a t i o n and d e e x c i t a t i o n processes i n Be.

A g r a z i n g i n c i d e n c e monochromator o f t h e grasshopper t y p e d e l i v e r e d narrow band (0.55(5)8) r a d i a t i o n w i t h a h i g h p o l a r i z a t i o n (88%) i n t h e r e g i o n o f i n t e r e s t , f r o m 120 t o 190 eV. Two o f t h r e e a v a i l a b l e e l e c t r o n energy a n a l y z e r s were used i n tandem, a t O0 and 90" f o r $ d e t e r m i n a t i o n s , a t 55' and 55' + T f o r u measurements, and most f r e q u e n t l y a t 55' and 55P + n/2 f o r a combined u and $ measurement. The energy r e s o l u t i o n was A E I E = 1%, and, i n a t e s t case, 0.3%.

E l e c t r o n s p e c t r a were measured a t f i x e d photon energies, b u t t h e near t h r e s h o l d r e g i o n was scanned i n 0.2 eV s t e p s b y l o c k i n g i n on t h e Auger .peak o f i n t e r e s t . Most o f t h e u d a t a were o b t a i n e d f r o m Auger s p e c t r a because o f t h e l o w background i n t h e s p e c t r a l r e g i o n around 100 eV. P o s s i b l e c o n t r i b u t i o n s f r o m second-order r a d i a t i o n were determined t o be l e s s t h a n 3% and were n e g l e c t e d i n t h e a n a l y s i s . B e r y l l i u m was e v a p o r a t e d f r o m a Ta oven a t 1160°C and t h e vapor p r e s s u r e i n t h e e l e c t r o n source c e l l was e s t i m a t e d t o be l e s s t h a n Pa. T h i s 1ow.pressure

Enli, L E V E L ENERGY (eV)

420 430 4 4 0 4 5 0

2 5 0 0 I I I I I I

I

m 2 0 0 0 -

I

Y

a a

e

A 4 5 0 0

-

4s 2 p 2 p

f z

8 0

4 0 0 0 -

a

"7 I-

= 8 ^{5 0 0}

-

AES I

9 0 1 0 0 440 420

EA, AUGER ELECTRON ENERGY (eV1

F i g . 2 P h o t o e l e c t r o n spectrum and Auger spectrum f o r K s h e l l i o n i z a t i o n i n Be a t 4 = 55". F o r PES, h u = 170 eV; f o r AES, h u = 160 eV.

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combined w i t h a t o t a l c r o s s s e c t i o n o f about 2 Mb r e s u l t e d i n o n l y moderate s i g n a l s t r e n g t h s f o r t h e v a r i o u s s p e c t r a l l i n e s . More d e t a i l s about t h e apparatus and procedures can be f o u n d i n r e f e r e n c e s

[ l o ]

and [ll].

A p h o t o e l e c t r o n spectrum r e c o r d e d a t h v = 170 eV and an Auger spectrum r e c o r d e d a t h v = 160 eV a r e p l o t t e d i n F i g . 2. The correspondence of t h e p h o t o e l e c t r o n s and Auger s p e c t r a t h a t f o l l o w s f r o m t h e schematic o f F i g . 1 can c l e a r l y be seen.

The s p e c t r a show t h e main l i n e c o r r e s p o n d i n g t o i o n i z a t i o n o f a s i n g l e I s e l e c t r o n and s a t e l l i t e l i n e s c o r r e s p o n d i n g t o i o n i z a t i o n of a I s e l e c t r o n and simultaneous e x c i t a t i o n o f a 2s e l e c t r o n . The m a j o r t w o - e l e c t r o n t r a n s i t i o n s i n v o l v e t h e e x c i t e d s t a t e s l s ( 2 s 2 p 1'3P)2P and ( l s 2 s l Y 3 S ) 3 s 2S. On t h e b a s i s o f t h e s i n g l e - p a r t i c l e model d e s c r i p t i o n , i n which t h e ground s t a t e i s l s 2 2 s 2 IS, t h e 2s2p s a t e l l i t e s i n d i c a t e s o - c a l l e d c o n j u g a t e shakeup t r a n s i t i o n s i n which t h e e x c i t e d e l e c t r o n changes t h e a n g u l a r momentum quantum number, a' = E + 1.

S i m i l a r l y , t h e 2s3s s a t e 1 1 i t e s correspond t o shakeup t r a n s i t i o n s i n which t h e e x c i t e d e l e c t r o n r e t a i n s i t s 2 . Conjugate shakeup i s seen t o be pronounced i n Be and i s , i n f a c t , much s t r o n g e r t h a n p r e v i o u s l y observed i n o t h e r atoms, e.g. Hg [12]. So f a r , o n l y f o r t h e n e i g h b o r s L i (13,141 and He [15-181 have s u b s t a n t i a l i n t e n s i t i e s been r e p o r t e d . The m a j o r reason f o r t h e s t r e n g t h o f t h i s t y p e o f c o r r e l a t i o n s a t e l l i t e must be sought i n t h e a d m i x t u r e o f t h e 2p2 c o n f i g u r a t i o n t o t h e s i n g l e p a r t i c l e ground s t a t e [19-211 and t h e presence o f p a i r c o r r e l a t i o n s l s 2 p and 2s2p, t h u s p r o v i d i n g e a s i e r access t o t h e l s 2 s 2 p s t a t e s i n p h o t o e f f e c t . The energy dependence o f t h e l s ( 2 s 2 p l y 3 P ) e x c i t a t i o n s has been r e p o r t e d elsewhere [22] f o r t h e n e a r t h r e s h o l d r e g i o n . We demonstrated t h a t a sharp i n t e n s i t y decrease o c c u r s f o r t h e 3P component w i t h energy, and a n e a r l y c o n s t a n t i n t e n s i t y p e r s i s t s f o r t h e 'P component r e l a t i v e t o s i n g l e 1s e x c i t a t i o n (main l i n e ) up t o 140 eV. W i t h i n c r e a s i n g e n e r g i e s , t h e r e l a t i v e i n t e n s i t i e s appear t o r e a c h a c o n s t a n t value, 1.2% f o r 3P, and 9% f o r 'P. F o r L i , no measurements were made c l o s e t o t h r e s h o l d , S 5 eV [13,14]. A t h i g h e r energies, a decrease i n t h e 'P i n t e n s i t y o f L i was observed w i t h a l e v e l i n g o u t a t about 3.5%, w h i l e t h e 3P i n t e n s i t y , though somewhat u n c e r t a i n because o f i n t e r f e r e n c e w i t h t h e 'S t e r m o f t h e main l i n e , appears t o have a r a t h e r c o n s t a n t v a l u e o f 8%. R e t u r n i n g t o Be, we d i s p l a y t h e r e l a t i v e i n t e n s i t i e s of a l l c o r r e l a t i o n s a t e l l i t e s i n F i g . 3 and

OTHERS

130 440 150 460 170 180 190 200

hv, PHOTON ENERGY (eV)

F i g . 3 R e l a t i v e i n t e n s i t i e s o f c o r r e l a t i o n s a t e l l i t e s a r i s i n g f r o m I s i o n i - z a t i o n i n Be. The S components c o n t a i n some 3p and 3d c o n t r i b u t i o n s .

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C9-476 JOURNAL DE PHYSIQUE

Table 1. Intensities of satellite lines*3' arising from photoionization in the Is shell of Be. Values were obtained from photoelectron and Auger spectra and are normalized to I{ls2s2) = 100

hv (eV) 140 142 150 160 170 175 180 190ld>

ls(2s2p)

3P 5.4(5) 5.0(8) 2.0(2) 1.9(5) 1 1.5(5) 1.3(3) 1

h

9.4(4) 11.0(9) 10.1(3) 9.8(8) 9.5(8) 9.7(1.0) 9.3(5) 8.5

l s 2 p2

's

0.5 —

0.5 0.6 1

— -"

(ls2s)3s 3s(b)

7.9(9) 5.7(4) —

7.8(7) 8.4(5) 9.5(1.0) 9.1(5) 10 (2)

1.5(4) 3.3(3) —

6.9(8) 9.4(8) 8.0(1.0) 9.2(5) 9 (2)

ls2sn£

nS4

..

— —

4 2 3.5

4 3

SUM

24.7 21.6 —

30.9 30.9 33.2 32.9 31.5

^'Identification according to excited state.

' 'Contains a 2s3p contribution.

*c'Conta1ns a 2s3d contribution.

^'Values measured at * = 0 ° .

in Table 1. We note that the Is2s2p excitations amount to about 10.5% of single Is ionization and the Is2s3s excitations to about 19% in the high energy limit, which we believe has been reached at 190 eV. The probability for the ionization with excitation processes is 24.5%. This value is slightly higher on a per- electron basis than that reported for Li. A sudden approximation calculation [23] for creating a vacancy in the Be 2s shell in the case of e" decay gives a value of 20 to 25%, if we extrapolate the predictions for Z s 10 and apply a correction for the change in the effective nuclear charge when going from e decay to Is photoionization. We note that this calculation would include transitions of the 2s electron to the continuum.

The threshold behavior for Is2s3s excitation was not examined in detail. It seems, however, that the (ls2s 3S)3s intensity has a high threshold value, while (ls2s iS)3s has a rather low value. At higher energies both components exhibit a similar strength, if we ignore contributions from interfering Is2s3p and Is2s3d excitations. Angular distribution measurements show, however, that the e parame- ter for the lines labelled in Fig. 3 as 3S and is deviate little from the expected values, g = 2, for the photoelectrons and g = 0 for the Auger electrons.

We find, however, that the deviation is larger for the peak 3S than for 1S, suggesting that Is2s3p has some strength while Is2s3d has negligible strength.

We measured the Is level energy in reference to the Xe 4 d5 / 2 level energy (E = 67.54 eV) and obtained E(Bels) = 123.56(6)eV in good agreement with an earlier value [4] of 123.6(1) eV. The corresponding Is - 2s2s Auger energy is then 96.03

Table 2. Energies (in eV) of major energy levels in Be

No.

1 2 3 4 5 6 7 8 9 10

Assignment

Is2s2p2 1 , 3P l s 2 s2 2S ls(2s2p3P) 2P ls(2s2pXP) 2P

l s 2 p^{2 2}0 l s 2 p^{2 2}S ( l s 2 s3S ) 3 s 2S (ls2s3S)3p 2P (ls2s³S)3d²D ( l s 2 s1S ) 3 s 2S

E^a

122.3 123.64 128.12 130.33 131.80 135.11 138.15 138.73 139.90 140.55

4E"

-1.34 O 4.48 6.69 8.16 11.47 14.51 15.09 16.26 16.91

4E"

-1.91(5) 0 4.4 (1) 6.68(5)

—

11.6(1)

! 14.9(1)C

I

I 17.1(2)c

i

u' F r o m Ref. 3, except level 1 which is from Ref. 9.

"''our values; Note that our value for level 2 is E = 123.66(6) eV.

(c)Peak center.

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0 1 ' ' " a I

I20 126 132 138 144

PHOTON ENERGY (eV)

Fig. 4 R e l a t i v e p a r t i a l c r o s s s e c t i o n f o r I s s h e l l o f Be, ~ ( l s ) , and t h e sum ~ ( l s ) + ~ ( 2 s 2 p ) .

eV, u s i n g 27.53 eV f o r E ( B ~ z + ) . Relevant l e v e l e n e r g i e s a r e summarized i n Table 2; o u r values a r e d e r i v e d m o s t l y f r o m Auger s p e c t r a as a r e t h o s e of Rbdbro e t a l . [3]. Agreement among t h e experimental and t h e o r e t i c a l d a t a s e t s i s seen t o be g e n e r a l l y good. The peak p o s i t i o n o f 3S which we p l a c e a t E = 110.9 eV i n d i c a t e s , a l o n g w i t h t h e a n g u l a r d i s t r i b u t i o n r e s u l t s , t h a t t h e l s 2 s 3 p s t a t e i s a1 s o populated.

The near t h r e s h o l d r e s u l t s f o r t h e p h o t o i o n i z a t i o n c r o s s s e c t i o n s o a r e shown i n F i g . 4, where ~ ( 1 s ) and tot) = ~ ( 1 s )

+

a [ l s ( 2 s 2 p l Y 3 ~ ) ] a r e p l o t t e d . The tot) i s c l o s e t o t h e t o t a l c r o s s s e c t i o n tot), because o u t e r s h e l l i o n i z a t i o n makes a s m a l l c o n t r i b u t i o n and t h e onset f o r l s 2 s 3 s occurs o n l y a t 138 eV. The r e g i o n covered i n Fig. 4 i s dominated by resonances, some o f which have been i d e n t i f i e d [22]. A b s o r p t i o n measurements i n Be m e t a l a l s o e x h i b i t pronounced s t r u c t u r e near t h r e s h o l d [1,24]. A comparison o f t h e metal d a t a w i t h t h e p r e s e n t d a t a suggests t h a t i n a d d i t i o n t o d e n s i t y - o f - s t a t e e f f e c t s , atomic e f f e c t s s h o u l d be c o n s i d e r e d f o r an i n t e r p r e t a t i o n o f t h e s t r u c t u r e o c c u r r i n g i n t h e metal.

A p a r t i t i o n o f a ( t o t ) can be made, i f we use a ( 2 s ) / a ( l s )

-

3% [13] and e s t i m a t e double-to-sing1 e p h o t o i o n i z a t i o n t o be 10%. Then we o b t a i n t h e f o l 1 owing r e l a t i v e p a r t i a l c r o s s s e c t i o n s a t hv=190 eV: ~ ( 1 s ) = 0.69, o ( 2 p sat.) = 0.08, u ( n 2 3 s a t . ) = 0.14, a f o u t e r s h e l l ) = 0.02, and a(doub1e i o n i z a t i o n ) = 0.07.

Under t h e s e c o n d i t i o n s a f l u o r e s c e n c e y i e l d o f 0.9 x s h o u l d be observed, u s i n g K e l l y ' s t r a n s i t i o n r a t e s [8] and n e g l e c t i n g n 2 3 c o n t r i b u t i o n s .

We measured t h e a n g u l a r d i s t r i b u t i o n parameters f o r t h e I s p h o t o e l e c t r o n between 160 and 180 eV and f o r t h e l s 2

-

2s2s Auger e l e c t r o n a t 170 eV. We o b t a i n e d values o f B(1s) = 1.98(6) and B ( l s 2

-

2 ~ 2 s ) = -0.03(6), which a r e values t h a t a r e v e r y c l o s e t o t h e expected B ( 1 s ) = 2.0 and @ ( A ) = 0. The Auger 1 in e group between 111 and 114 eV gave a v a l u e o f $(A) = 0.3(2), i n d i c a t i n g as noted above t h a t t h e l s 2

-

^{2 ~ 3 s}s a t e l l i t e s (which s h o u l d have B(A) = 0) c o n t a i n some l s 2

-

2s3p and perhaps l s 2

-

2s3d c o n t r i b u t i o n s . We n o t e t h a t B(A) C 0 would a r i s e f r o m t h e a l i g n m e n t o f i n t e r m e d i a t e s t a t e s w i t h j > 1/2. Almost maximum alignment near t h r e s h o l d was determined f o r t h e l s 2 s 2 p s t a t e s f r o m t h e 6 measurement f o r t h e l s 2

-

2s2p s a t e l l i t e s [22]. T h i s a l i g n m e n t i m p l i e s t h a t i n t h e e x c i t a t i o n process hv + l s 2 2 s 2 ⁺ls2s2p, E R , t h e p h o t o e l e c t r o n goes p r e f e r e n t i a l l y (60% of t h e t i m e s ) i n t o t h e E S continuum. I n an e a r l i e r experiment on t h e r e l a t e d case o f H e ( l s 2 ⁺2p, ER), a f l u o r e s c e n c e measurement showed an even s t r o n g e r popula- t i o n o f t h e E S channel [25].

These f i r s t , and a t t h e same time, d e t a i l e d measurements o f p h o t o i o n i z a t i o n and decay processes i n atomic Be have borne o u t t h e e x p e c t a t i o n s t h a t t h i s atom may s e r v e w e l l t o d e l i n e a t e e l e c t r o n c o r r e l a t i o n e f f e c t s and p h o t o e l e c t r o n dynamics i n a small system analogous t o t h e t w o - e l e c t r o n atom He. W i t h two e l e c t r o n s each

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C9-478 JOURNAL DE PHYSIQUE

i n t h e f i r s t two p r i n c i p a l s h e l l s , Be i s t h e n e x t s t e p p i n g s t o n e t o t h e many-electron atom. The c l o s e d she1 1 s t r u c t u r e o f f e r s t h e o p p o r t u n i t y t o p e r f o r m r a t h e r r i g o r o u s t h e o r e t i c a l c a l c u l a t i o n s , a f f o r d i n g one t o i s o l a t e t h e most i m p o r t a n t i n t e r a c t i o n s . As p r e l i m i n a r y d a t a on v a r i o u s aspects o f 1s p h o t o e x c i t a t i o n , e s p e c i a l l y below t h e t h r e s h o l d , i n d i c a t e , much experimental work remains t o be done t o f u l l y e x p l o r e t h e spectroscopy and dynamics o f Be i n t h e atomic s t a t e .

T h i s work was sponsored by t h e D i v i s i o n o f Chemical Sciences, O f f i c e o f Basic Energy Sciences, U.S. Department of Energy, under c o n t r a c t DE-AC05-840R21400 w i t h M a r t i n M a r i e t t a Energy Systems, I n c . C. D. C a l d w e l l was supported b y t h e N a t i o n a l Science Foundation under g r a n t NSF-PhY-8518598 and acknowledges t r a v e l s u p p o r t from t h e Oak Ridge A s s o c i a t e d U n i v e r s i t i e s . The U n i v e r s i t y o f t h e Wisconsin Synchrotron R a d i a t i o n Center i s operated under NSF g r a n t DMR-8601349.

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