A SURVEY ON THE EXPERIMENTAL DEVELOPMENTS IN ATOMIC PHOTOIONIZATION

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A SURVEY ON THE EXPERIMENTAL

DEVELOPMENTS IN ATOMIC PHOTOIONIZATION

B. Sonntag

To cite this version:

B. Sonntag. A SURVEY ON THE EXPERIMENTAL DEVELOPMENTS IN ATOMIC PHOTOIONIZATION. Journal de Physique Colloques, 1987, 48 (C9), pp.C9-439-C9-451.

�10.1051/jphyscol:1987973�. �jpa-00227394�

JOURNAL D E PHYSIQUE

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

A SURVEY ON THE EXPERIMENTAL DEVELOPMENTS IN ATOMIC PHOTOIONIZATION

B . SONNTAG

11. Institut fiir ~xperimentalphysik, Universitat Hamburg, Luruper Chaussee 149, 0-2000 Hamburg 50, F.R.G.

A b s t r a c t

Progress i n atomic p h o t o i o n i z a t i o n i s e x e m p l i f i e d by recent h i g h r e s o l u t i o n s t u d i e s , l a s e r plasma s t u d i e s , p h o t o e l e c t r o n s a t e l l i t e spectroscopy, e l e c t r o n - e l e c t r o n c o i n - cidence experiments and e x c i t e d s t a t e spectroscopy o f a l i g n e d atoms.

I n t r a d u c t i o n

toioni ionization s t u d i e s form an e x c e l l e n t t o o l t o probe t h e s t a t e s and t h e dynamics

n f stoms

/1-5,'.

The improvements o f t h e experimental techniques have enabled us t o

~ r n r ~ v e l more and more d e t a i l s and thereby promoted our understanding o f these many s l e c t r o n systems. The scoDe o f t h e s t u d i e s i s o u t l i n e d i n F i g . 1.

incoming outgoing measured

photon beom portlcles quantities

ouger- photoelectrons intense

monochromotic ground-

polarized intensity

tunable

scattered angular Representation o f p h o t o i o n i z a t i o n stla- distribution dies.

orientation time

gas discharge g dependence

spark discharge atomic

coincidence

laser atoms experiments

x-ray tube

synchrotron-radiation

ions

monochromotor ground-

excited state

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987973

C9-440 JOURNAL

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Hiqh r e s o l u t i o n VUV a b s o r p t i o n s p e c t r a have been determined f o r r a r e qas and m e t a l . - - -

atoms /6,7,8/. As an example F i g . 2 shows t h e p r i n c i p a l 1s2

'so

- 1s np 'pl s e r i e s o f He up t o n = 50. T h i s spectrum was o b t a i n e d i n 1 0 t h o r d e r o f a 6.65 m s p e c t r o - graph. A r e s o l v i n g power g r e a t e r than 2.5 x 10 has been achieved w i t h t h i s e x p e r i - 5 mental set-up / 9 / . From t h e data t h e I s s e r i e s l i m i t (198310.75 2 0.04 cm-'1, t h e term values and t h e e f f e c t i v e quantum numbers c o u l d be deduced w i t h h i g h accuracy.

F i g . 2 He 1s'

'so

- I s np a b s o r p t i o n s e r i e s ( f r o m Ref. /6/).

A t w i n laser-produced plasma technique has been proven t o be i d e a l l y s u i t e d f o r photoabsorption s t u d i e s on i o n s and atomic r e f r a c t o r y elements /10,11/. A schematic diagram o f t h e system i s presented i n F i g . 3. Two Q switched l a s e r s were used i n t h e experiment. A ruby l a s e r generated the absorbing plasma w h i l e a neodymium-glass os- c i l l a t o r - a m p l i f i e r system produced t h e VUV e m i t t i n g plasma from a tungsten t a r g e t . The pumping p u l s e s i n the two l a s e r systems were synchronized. A c o n t i n o u s l y v a r i - a b l e delay from 250 nsec t o 10 msec c o u l d be i n t r o d u c e d between the o u t p u t p u l s e s o f t h e two l a s e r s . A t s h o r t delay times t h e a b s o r p t i o n o f i o n i c species p r e v a i l s where- as f o r l o n g e r d e l a y s t h e absorbing plasma has r e v e r t e d t o n e u t r a l c o n d i t i o n . Both source and absorbing plasma were viewed by a 2 m g r a z i n g i n c i d e n c e Rowland spectro- graph. A comparison between 5d-5,E f Uraniumm vapour a b s o r p t i o n /11/ and s o l i d Ura- nium a b s o r p t i o n /12/, UF a b s o r p t i o n /13/ and t h e o r y i s given i n F i g . 4. I n agree-

4

ment w i t h t h e o r y a l l exoerimental s p e c t r a d i s p l a y a g i a n t 5d + 5 , ~ f resonance /15/.

Fiq.

3

Schematic diagram of dual- laser absorption physics.

MP, master pulse; G, de- layed pulse penerator; T1, triqger pulse to ruby-

P

1

laser control circuit; T2,

1

I

trigger pulse to neodymium 0 2

I oscillator-amplifier sys- tem; C1, C2, charging and control circuits; Fl,F2,

T 2

N, neodymium oscillator- amplifier rods; Pl,P2, Po- ckels cells; PU1, PU2, Po- ckels-cell control units;

8,

beam splitter; L, light sensitive diode; A, ampli- fier; D, delay circuit;

01,02, optical output pul- ses.

(CI

JLik ^{. . ..}

^{. I}

^{. . . .... ...}

^{( .}

^-.

^),

(a) solid uranium absorption,

(b)

UF4 absorption data and

( c )

the- ory (from Ref. /11/).

90 100 120 140

Photon energy ( e V )

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L a s e r s p e c t r o s c o p y o f c o r e - e x c i t e d l e v e l s a l l o w s v e r y a c c u r a t e m e a s u r e m e n t s o f a u t o - i o n i z i n g t i m e s , l i n e w i d t h s , r e l a t i v e l e v e l p o s i t i o n s a n d t r a n s i t i o n o s c i l l a t o r s t r e n g t h s o f c o r e - e x c i t e d l e v e l s /16/. The new t e c h n i q u e i s b a s e d on t h e r a d i a t i v e d e c a y o f q u a s i m e t a s t a b l e l e v e l s /17/. F i g u r e 5 shows a s c h e m a t i c o f t h e e x p e r i m e n t a l a r r a n g e m e n t . The q u a s i m e t a s t a b l e l e v e l s a r e p o p u l a t e d by a l a s e r p r o d u c e d plasma.

The XUV r a d i a t i o n e m i t t e d upon d e c a y i s m o n i t o r e d . T h i s XUV f l u o r e s c e n c e is d e p l e c - t e d when t h e p o p u l a t i o n o f t h e m e t a s t a b l e l e v e l i s t r a n s f e r r e d t o a u t o i o n i z i n g

F i g . 5 S c h e m a t i c o f t h e e x p e r i m e n t a l s e t - u p ( f r o m Ref. / 1 6 / ) .

I

82.4

nmd-

--_2_

x

l e v e l s by a t u n a b l e d y e l a s e r ( s e e F i g . 6 ) . The s h a p e and p o s i t i o n o f t h e d i p i n t h e s p e c t r u m XUV f l u o r e s c e n c e v e r s u s d y e l a s e r w a v e l e n g t h , d e t e r m i n e t h e p o s i t i o n a n d t h e l i n e w i d t h o f t h e a u t o i o n i z i n g l e v e l .

--

With t h i s method t h e a c c u r a c y o f l a s e r s p e c t r o s c o p y c a n b e t r a n s f e r r e d t o t h e XUV r e g i m e .

E n e r g y , a n g l e a n d s p i n r e s o l v e d p h o t o e l e c t r o n s t u d i e s p l a y a n e m i n e n t r o l e i n t h e e l u c i d a t i o n o f a t o m i c p r o p e r t i e s . I n r e c e n t p u b l i c a t i o n s new r e s u l t s on t o t a l and p a r t i a l c r o s s s e c t i o n s , a n g u l a r asymmetry p a r a m e t e r s / l a - 2 1 / , d e c a y c h a n n e l s o f c o r e e x c i t a t i o n r e s o n a n c e s /22-25/, s a t e l l i t e e m i s s i o n /26-28/, t h r e s h o l d e f f e c t s /29/

and s p i n p o l a r i z a t i o n /30/ h a v e b e e n r e p o r t e d . F i g . 7 shows t h e p h o t o e l e c t r o n s p e c -

5 2 6 2

t r u m o f a t o m i c Ca o b t a i n e d by e x c i t i n g t h e ~ ~ ~ 3 + ~2p 3 s 3p 4 s 3d r e s o n a n c e ~ 3 ~ ~ 4 s ~

Ta TARGET/ : 1

. . ^-:. R b H E A T SPECTROMETER

ngnc

,/

^LEVELS

DYE

LASEY -

DYE LASER

F i g . 6 P a r t i a l e n e r g y - l e v e l d i a g r a m o f Rb s h o w i n g t h e d e p l e t i o n s p e c t r o s c o p y t e c h - n i q u e ( f r o m Ref. /16/.

h

w

~352.8eV

,-.

03 -0 .d

c

3

d

L

V

4J X -4

03 s

(Y L,

s

Y

0 100 200 300

Kinetic Energy (eV)

F i g . 7 P h o t o e l e c t r o n s p e c t r a o f a t o m i c Ca.

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6 2 5 2 a t f i w = 3 5 2 . 8 eV. The a u t o i o n i z a t i o n o f t h i s r e s o n a n c e i n t o t h e ( 2 p 3 s 3p 4 s

,

6 6 2 6 2 4 2 6 5 2

2 p 3 s 3 p 4 s

,

2p 3 s 3p 4 s 3 d , 2p 3 s 3 p 4 s 3 s ) e l c o n t i n u a g i v e s r i s e t o t h e l i n e s f o r k i n e t i c e n e r g i e s a b o v e 250 eV. Between 3 0 eV a n d 250 eV t h e r e is a weak c o n t i n u u m , a l m o s t s y m m e t r i c i n r e s p e c t t o t h e minimum a t

-

^{1 5 0} eV. Shake o f f p r o c e s s e s ( e . q . ,

5 2 6 2 6 2 4 2

2p 3 s 3p 4 s 3d + 2p 3 s 3p 4 s ) p r o b a b l y a c c o u n t f o r most o f t h i s c o n t i n u u m . Weak

5 2 6 2 6 6 2

2p 3 s 3p 4 s 3d + 2p 3p 4 s 3 d e l a u t o i o n i z a t i o n l i n e s and s a t e l l i t e l i n e s may b e

6 4 2 5

b u r i e d i n t h e c o n t i n u u m . Auger l i n e s , e . g . 3p54s2 + 3p €1, 3p 4 s 3d ⁺ 3p 3dCl a r e d i s c e r n i b l e f o r k i n e t i c e n e r g i e s below 3 0 eV. The i m p o r t a n c e o f s h a k e o f f p r o c e s s e s i s c o n s i s t e n t w i t h t h e s t r o n g ca3+ y i e l d o b s e r v e d by N a g a t a e t a l . /31/ ( s e e F i g . 8 ) .

m

340 350 360 370 388

Photon

Eenrgy

( e V >

F i g . 8

~ a + , Ca 2+

,

a n d ca3+ y i e l d s p e c - t r a ( f r o m R e f . / 3 1 / ) .

T h r e e s t e p d e c a y p r o c e s s e s may a l s o c o n t r i b u t e t o t h e ca3+ y i e l d . For more d e t a i l s on t h e Ca 2p p h o t o e l e c t r o n s p e c t r a t h e r e a d e r i s r e f e r r e d t o Ref. /32/.

C o r e - l e v e l s a t e l l i t e s p e c t r a y i e l d d e t a l l e d i n f o r m a t i o n o n e l e c t r o n c o r r e l a t ~ o n . Re- c e n t l y S v e n s s o n e t a l . /33/ have shown t h a t f o r shake-up s a t e l l i t e s l y i n g a b o v e t h e s h a k e - o f f t h r e s h o l d , t h e s t r u c t u r e s o b s e r v e d c o r r e s p o n d t o a u t o l o n i z i n g r e s o n a n c e s .

The shake-up c h a n n e l , i . e . p h o t o i o n i z a t i o n p l u s e x c i t a t i o n o f a n e l e c t r o n t o a d i s - c r e t e s t a t e , i n t e r a c t s w i t h t h e s h a k e - o f f c h a n n e l , i n which b o t h e l e c t r o n s a r e

3 6

s i m u l t a n e o u s l y e x c i t e d i n t o c o n t i n u u m s t a t e s . The Ne 1 ~ ~ '5 2⁺ l s 2 s ( S ) 2 p 3 s ~ ~ 2 ~ ~ ( ' 5 ) d shake-up s a t e l l i t e , d e t e r m i n e d w i t h t h e u s e o f monochromatized X-rays is p r e -

2 2 6 l S s e n t e d i n F i g . 9. The i n t e r a c t i o n w i t h t h e Ne 1s 2 s 2p

6 4 6 3 6 2 61 6 6 59 58 57 56 55

SHAKE-UP ENERGY CEV)

3 6 2

'S -+ l s 2 s ( S ) 2 p 3 s ( S ) E ~ s h a k e - u p s a t e l l i t e ( f r o m R e f . F i q . 9 The Ne 1s 2 s 2p

/ 3 3 / ) .

s h a k e - o f f c o n t i n u a g i v e s r i s e t o t h e a s y m m e t r i c Fano t y p e p r o f i l e . The d a s h e d l i n e i n d i c a t e s t h e u n p e r t u r b e d c o n t i n u u m i n t e n s i t y . The s o l i d l i n e r e p r e s e n t s a f i t o f a Fano l i n e p r o f i l e /34/ t o t h e e x p e r i m e n t a l p o i n t s .

The f i t p a r a m e t e r s q , p 2 and

r

a r e g i v e n i n F i g . 9.

I n a p i o n e e r i n g e l e c t r o n - e l e c t r o n , e l e c t r o n - i o n c o i n c i d e n c e e x p e r i m e n t L a b l a n q u i e e t 5 3 a l . /35/ s t u d i e d t h e d y n a m i c s o f d o u b l e p h o t o i o n i z a t i o n c l o s e t o t h e Ar 2 p ( P, 'D, '5) t h r e s h o l d s . The s p e c t r a o f c o i n c i d e n c e s a t 5 6 eV p h o t o n e n e r g y b e t w e e n e l e c t r o n o f d i f f e r e n t e n e r g i e s s e l e c t e d by a n e l e c t r o s t a t i c a n a l y z e r and ( a ) low e n e r g y e l e c -

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42 4 4 46 48 50

E N E R C Y T R A N S F E R /eY

8 0 I

( c )

I

0

-

8 0 ' ^'

.

^'

vl k- ( b )

5

^{6 0 -}

Q U

Crl 20 ^' U 0 0 .

u

4 1 4 1 2 10 8 8

t r o n s , ( b ) Ar2+ i o n s and ( c ) Arf i o n s a r e g i v e n i n F i g . 1 0 . These c o i n c i d e n c e s p e c - t r a c l e a r l y d e m o n s t r a t e t h a t t h e ~ r ' + ( 3 ~ ) i o n is formed a l m o s t e x c l u s i v e l y . T h i s is

F i g . 1 0

E l e c t r o n - e l e c t r o n a n d e l e c t r o n i o n c o i n c i d e n c e s p e c t r a ( f r o m Ref. / 3 5 / ) .

i n a c c o r d a n c e w i t h t h e p r e d i c t e d w a v e f u n c t i o n symmetry e f f e c t s f o r d i r e c t d o u b l e

2

ELECTRON E S E H G Y

/

^{e V}

0 40

+

( a )

3 0

20 10

0 1 3

P 'D S

A=..

I

i o n i z a t i o n /36/. F i g . l o b shows t h a t t h e e l e c t r o n d i s t r i b u t i o n is e s s e n t i a l l y f l a t . The y i e l d o f z e r o - e n e r g y e l e c t r o n s ( E

<

0 . 5 eV) m e a s u r e d i n c o i n c i d e n c e w i t h A r 2 + i o n s s a t i s f a c t o r a l l y o b e y s t h e e x p e c t e d ( h v

-

I ) " - ~ t h r e s h o l d law /37/ ( s e e F i g . 1 1 ) . I n d i r e c t d o u b l e i o n i z a t i o n p r o b a b l y a c c o u n t s f o r t h e m o d u l a t i o n o f t h e y i e l d .

G r e a t p r o g r e s s h a s b e e n made i n XUV-studies o f e x c i t e d a t o m s and i o n s . A b s o r p t i o n s p e c t r a h a v e b e e n r e p o r t e d f o r Ca*, Ca+ / 3 8 / , Ba* /39/ a n d Mn*, ~ n + /40/. A b s o l u t 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 f o r c a C , ~ r + were m e a s u r e d by m e r g i n g beams o f VUV s y n c h r o t r o n r a d i a t i o n and Ca+, ~ r + i o n s / 4 1 , 4 2 / . The d e v e l o p m e n t o f p h o t o e l e c t r o n s t u d i e s on l a s e r - e x c i t e d a t o m s h a s been r e v i e w e d a t t h i s c o n f e r e n c e /43/.

PHOTON ENERGY / e V

F i g . 11 Spectra o f coincidence between low-energy ( E

<

^0.5 eV) e l e c t r o n s and ( a ) A r 2 + i o n s w i t h a s o l i d l i n e showing t h e t h e o r e t i c a l n-1

=

0.056 power-law behaviour, ( b ) Ar+ i o n s , b o t h as f u n c t i o n s o f photon energy ( f r o m Ref. / 3 6 / ) .

VUV p h o t o e l e c t r o n spectroscopy on l a s e r - a l i g n e d o r o r i e n t e d atoms i s t h e most r e c e n t s t e p i n t h i s development. The i n t e n s i t y and angular d i s t r i b u t i o n o f t h e photoelec- t r o n s c r i t i c a l l y depend on t h e alignment o r o r i e n t a t i o n o f t h e e x c i t e d atom and on t h e r e l a t i v e o r i e n t a t i o n o f t h e p o l a r i z a t i o n v e c t o r s o f t h e l a s e r and the VUV r a d i a - t i o n . Therefore t h e new method can be used as a very s e n s i t i v e probe t o study c o r e e x c i t e d s t a t e s o f atoms. The power o f t h e method has been demonstrated f o r atomic C i /44/. R e s u l t s on Na and d e t a i l s o f t h e experimental set-up have been r e p o r t e d a t

2 2 t h i s meeting /45/. L i atoms were e x c i t e d and a l i g n e d by pumping t h e L i I s 2s

2 2 s1/2 -

L i 1s 2p P1,,2,3,2 t r a n s i t i o n s u s i n g a l i n e a r l y p o l a r i z e d dye-laser beam. Subse- q u e n t l y the e x c i t e d atoms were r a i s e d t o c o r e e x c i t e d l s n l n ' l ' 2 S, 'P, 'D s t a t e s by l i n e a r l y p o l a r i z e d s y n c h r o t r o n r a d i a t i o n . The energy and i n t e n s i t y o f t h e e l e c t r o n s e j e c t e d upon t h e L i l s n l n ' l ' '5, 'D 4 L i l s 2 'SE~ a u t o i o n i z a t i o n were monitored w i t h a c y l i n d r i c a l m i r r o r analyzer. The e x c i t a t i o n - and decay channels a r e d i s p l a y e d i n F i g . 12.

JOURNAL DE PHYSIQUE

. -

F i g . 12 Schematic energy l e v e l diagram f o r t h e L i e v e n - p a r i t y c o r e e x c i t e d s t a t e s .

F i g . 13 shows a s e c t i o n o f t h r e e constant i o n i c f i n a l s t a t e spectra. The uppermost spectrum r e p r e s e n t s t h e a b s o r p t i o n l i n e s /46/ convoluted by Gaussians having 0.12 eV h a l f w i d t h . The p a r t i a l a u t o i o n i z a t i o n c r o s s s e c t i o n f o r t h e e x c i t a t i o n v i a 2

P1/2 is r e p r e s e n t e d by t h e second spectrum. For t h e two remaining s p e c t r a t h e a u t o i o n i z i n g c o r e e x c i t e d s t a t e s have been reached v i a t h e 1 s 2p 2 2~3,2 s t a t e . The p o l a r i z a t i o n

v e c t o r s o f t h e l a s e r l i g h t and t h e s y n c h r o t r o n r a d i a t i o n were o r i e n t e d p a r a l l e l t o one another i n t h e case o f spectrum 3, and p e r p e n d i c u l a r t o one o t h e r i n case o f spectrum 4. The s o l i d l i n e r e p r e s e n t s a s u p e r p o s i t i o n o f Guassians (FWHM

=

0.12 eV) f i t t e d t o the e x p e r i m e n t a l data. W i t h i n t h e accuracy o f our measurements, t h e ampli- t u d e o f l i n e 5 i s t h e same i n s p e c t r a 2-4. I n c o n t r a s t , t h e amplitudes o f t h e l i n e s 6, 7 and 9 vary d r a m a t i c a l l y upon changing t h e i n t e r m e d i a t e s t a t e and t h e r e l a t i v e p o l a r i z a t i o n o f t h e two l i g h t beams. For maximum aligment o f t h e 2~312 i n t e r m e d i a t e

2 2

s t a t e , t h e i n t e n s i t y o f 1s 2p 2 ~ 3 / Z + 1s n l n ' l t 'D and 2s 2p 2~3,,2 ⁺ 1s n l n ' l T '5 t r a n s i t i o n s a r e expected t o i n c r e a s e by a f a c t o r o f 1.1 and 2, r e s p e c t i v e l y , com- pared t o t h e i n t e n s i t y o f t h e t r a n s i t i o n s t o t h e same f i n a l s t a t e from t h e 1s 2p 2

Photon energy (eV)

F i g . 1 3 Li* 1 s ' ~ ~ 'P + Li l s n l n ' l ' a b s o r p t i o n ( u p p e r m o s t c u r v e ) a n d p a r t i a l a u t o -

,.

i o n i z a t i o n c r o s s s e c t i o n f o r e x c i t a t i o n v i a t h e L ~ 1 / 2 ( s p e c t r u m 2 ) a n d 2 ~ 3 , 2 ( s p e c t r a 3 and 4 ) i n t e r m e d i a t e s t a t e . The p o l a r i z a t i o n v e c t o r o f t h e l a s e r was o r i e n t e d p a r a l l e l t o t h e p o l a r i z a t i o n v e c t o r o f t h e s y n c h r o t r o n r a d i a t i o n f o r s p e c t r a 2 a n d 3 a n d p e r p e n d i c u l a r f o r s p e c t r u m 4.

L ~ 1 / 2 i n t e r m e d i a t e s t a t e /47/. T h i s is c o r r o b o r a t e d by t h e i n t e n s i t y c h a n g e s o f l i n e s 5, 6 and 7, w h e r e a s l i n e 9 , which is a s s i g n e d t o a t r a n s i t i o n t o a ' 0 s t a t e /46/ b e h a v e s l i k e a t r a n s i t i o n t o a 'S s t a t e . O r i e n t i n g t h e l a s e r p o l a r i z a t i o n vec- t o r p e r p e n d i c u l a r t o t h e p o l a r i z a t i o n v e c t o r o f t h e s y n c h r o t r o n r a d i a t i o n r e s u l t s i n a d i f f e r e n t p o p u l a t i o n o f t h e m s u b l e v e l s o f t h e i n t e r m e d i a t e 2 ~ 3 / 2 s t a t e s e e n by t h e s y n c h r o t r o n r a d i a t i o n . C o n s i s t e n t w i t h t h e e x p e r i m e n t a l r e s u l t we e x p e c t a v e r y s m a l l c h a n g e f o r t h e 2 ~ 3 / 2 + 2~1,,2,5/2 t r a n s i t i o n s , b u t a r e d u c t i o n by a f a c t o r o f 4 f o r t h e 2 ~ 3 / 2 + 2 ~ 1 / 2 t r a n s i t i o n s . L i n e 9 c l e a r l y is d u e t o a 'P

->

2 5 t r a n s i t i o n .

R e f e r e n c e s

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