THE MAIN PHASES OF DEVELOPMENT IN PHOTOEMISSION STUDIES ON LASER-EXCITED ATOMS USING SYNCHROTRON RADIATION

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THE MAIN PHASES OF DEVELOPMENT IN PHOTOEMISSION STUDIES ON LASER-EXCITED

ATOMS USING SYNCHROTRON RADIATION

D. Cubaynes, J. Bizau, F. Wuilleumier, D. Ederer, J. Picque, B. Carre, M.

Ferray, F. Gounand

To cite this version:

D. Cubaynes, J. Bizau, F. Wuilleumier, D. Ederer, J. Picque, et al.. THE MAIN PHASES OF DEVELOPMENT IN PHOTOEMISSION STUDIES ON LASER-EXCITED ATOMS USING SYNCHROTRON RADIATION. Journal de Physique Colloques, 1987, 48 (C9), pp.C9-453-C9-472.

�10.1051/jphyscol:1987974�. �jpa-00227395�

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JOURNAL DE PHYSIQUE

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

THE MAIN PHASES OF DEVELOPMENT IN PHOTOEMISSION STUDIES ON LASER-EXCITED ATOMS USING SYNCHROTRON RADIATION

D. CUBAYNES, J.M. B I Z A U , F.J. W U I L L E U M I E R , D.L. E D E R E R * , J.L. P I C Q U E * " , B. CARRE*'", M. F E R R A Y * * " and F. G O U N A N D " * * Laboratoire de Spectroscopic Atomique et Ionique and LURE

(Laboratoire commun au CNRS, CEA, MEN), Universite Paris-Sud, BSt. 350, F-91405 Orsay Cedex, France

* ~ a t i o n a l Bureau of Standards, Gaithersburg, MD 20889, U.S.A.

" " ~ a b o r a t o i r e Aime Cotton, Universite Paris-Sud and CNRS, Bdt. 505, F-91405 Orsay Cedex, France

"'"service de Physique des Atomes et des Surfaces, CEN Saclay, CEA, Bgt. 462, F-91191 Gif-sur-Yvette Cedex, France

L ' u t i l i s a t i o n combinee d ' u n l a s e r

a

c o l o r a n t c o n t i n u e t du rayonnement syn- c h r o t r o n emis par l'anneau de stockage

a

e l e c t r o n s ACO ii LURE a permis l e s p r e - miPres experiences de spectroscopie d ' e l e c t r o n s s u r des atomes p o r t s s dans un

@ t a t e x c i t e e t i o n i s e s en couche externe e t en couche i n t e r n e . Les premiers

@ l e -

ments e t u d i e s o n t e t 6 l e sodium e t l e baryum. Cette n o u v e l l e m6thode a permis de mesurer des f o r c e s d ' o s c i l l a t e u r e t des sections e f f i c a c e s de p h o t o i o n i s a t i o n dans des atomes e x c i t e s sur un domaine etendu d'energies de photons. Recemment, des e t a t s atomiques t r P s e x c i t e s o n t

e t e

prepargs par l ' a c t i o n simultanee de deux l a s e r s 5 c o l o r a n t continus e t du rayonnement synchrotron d'ACO, e t l e u r a u t o i o n i s a t i o n a St6 Gtudiee. Une revue de ces experiences e s t presentee. Leur extension p o s s i b l e grzce ?I l a mise en s e r v i c e d'onduleurs montes s u r des anneaux de stocl:age specialement c o n s t r u i t s pour l a production de rayonnement synchrotron, t e l que Super ACO, e s t discutee.

ABSTRACT

By combining a cw l a s e r beam w i t h synchrotron r a d i a t i o n e m i t t e d by t h e ACO storage r i n g a t LURE, i n Orsay, we have demonstrated t h e f e a s i b i l i t y o f such p h o t o i o n i z a t i o n experiments on e x c i t e d atoms using e l e c t r o n spectrometry and we have obtained t h e f i r s t p h o t o e l e c t r o n s p e c t r a o f l a s e r - e x c i t e d sodium and barium atoms. The o s c i l l a t o r s t r e n g t h s f o r t r a n s i t i o n s between core-electrons and o p t i c a l o r b i t a l s i n e x c i t e d atoms have been determined and p h o t o i o n i z a t i o n cross s e c t i o n s i n e x c i t e d atoms have been measured over a broad range o f photon energies. More r e c e n t l y , two-electron h i g h l y e x c i t e d a u t o i o n i z i n g s t a t e s have been produced and studied, u s i n g stepwise e x c i t a t i o n w i t h two cw dye l a s e r beams t o e x c i t e an o u t e r e l e c t r o n and synchrotron r a d i a t i o n t o e x c i t e an i n n e r e l e c t r o n i n sodium. A review of these experiments extending over a p e r i o d o f s e r v e r a l years i s given. The exten- s i o n o f these s t u d i e s u s i n g undulator r a d i a t i o n and new storage r i n g s i s discussed.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987974

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JOURNAL D E PHYSIQUE

I . INTRODUCTION

I t i s f a i r t o say t h a t the use of l a s e r s has transformed many aspects of mea- surement science. This transformation i s examplified by the many beautiful experi- ments t h a t have been performed since the l a s e r has been invented. For example, atoms can be laser-prepared with macroscopic dimensions and be used as ultra-sensitive detectors of infrared radiation.' Lasers have been used t o slow atoms and cool them t o mil l i kelvin temperatures. They have also been used t o prepare atoms and molecu- l e s i n a s p e c i f i c well characterized quantum s t a t e s 3 Experiments of t h i s d e l i c a t e nature would be d i f f i c u l t , i f not impossible, t o perform without t h e unique a t t r i - butes of l a s e r radiation. On the other hand, synchrotron radiation has a l s o made a similar impact on the s c i e n t i f i c community. Scientists have used the radiation for the development of small scale lithography ( l i n e p a i r resolution lower than one m i - crometer)

, 4

f o r softx-ray microscopy

,5

f o r determination of t h e s t r u c t u r e of virus and proteins6 and t o study t h e e l e c t r o n i c s t r u c t u r e of a very broad range of gases and s o l i d s . There a r e now about 25 synchrotron f a c i l i t i e s operational throughout the world. Yet, i n s p i t e of a l l the a c t i v i t y in both areas, the number of s c i e n t i s t s t h a t use synchrotron radiation and l a s e r radiation as tools t o do hybrid experiments has been very small up t o now. However, t h e experiments t h a t a r e possible have ex- c i t i n g s c i e n t i f i c potential .

In t h i s paper, we will give a brief outline of these laser-synchrotron radia- tion experiments i n the f i e l d of atomic photoionization. The object of t h i s paper i s t o stimulate the reader'sinterest t o these new a t t r a c t i v e applications of l a s e r and synchrotron radiation t o measurement science and not t o produce an exhaustive c r i t i c a l survey of t h i s research topic. In the f i r s t p a r t , we will describe how the- se radiation sources were combined t o observe the photoionization spectra of excited sodium and barium atoms, using electron spectrometry. Next , several examples of mea- surements of t h i s type will be given. Finally, possible future developments will be described.

11. PHOTOIONIZATION

OF

ATOMS IN EXCITED STATES

The study of photoionization of atoms from excited s t a t e s i s not a new sub- j e ~ t , ~ but new techniques make i t a more t r a c t a b l e research endeavor than the he- roic e f f o r t t h a t was required a decade ago t o obtain data on excited atoms.

⁸

Why i s the measurement of the subshell atomic and molecular photoioni zation

cross sections important? By studying the photoionization process, the geometrical

and dynamical properties of the e l e c t r o n i c motion can be explored.' One of the im-

portant topics i n atomic and molecular physics today i s the development of theore-

t i c a l models t h a t adequately explain electron correlations

.lo

Only a 1 imi ted class

of s t a t e s can be probed, when the atom is i n the ground s t a t e . Often, the ground

s t a t e i s an ensemble of nearly degenerate l e v e l s . While studies of ground s t a t e

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photoionization i s by no means complete,'' the development of synchrotron radiation and associated instrumentation t o u t i l i z e t h a t source e f f e c t i v e l y over the wide spectral range t h a t is available, and the development of l a s e r sources of suitable i n t e n s i t y t o maintain an adequate population of atoms i n an excited s t a t e has pro- vided an opportunity t o extend cross section measurements t o excited atoms and mole- cul es .

For the study of atoms i n the ground s t a t e , measurements. of the angular d i s t r i - bution'' and spin polarization of the ejected photoelectron^^^ have provided addi- tional information about the c h a r a c t e r i s t i c s of the wave function needed to describe the photoionization process.

Most of the measurements on excited atoms, up t o now, have been obtained by single- or multi-photon absorption using l a s e r s . With t h i s mode of ionization, t h e energy range exploited i s necessarily r e s t r i c t e d t o a few electron Volts above the f i r s t ionization threshold. By using a pulsed continuum source14 o r a laser-produced plasma,15 absorption measurements from excited atoms have been extended over a broad energy range.

These measurements have stimulated theoreticians t o c a l c u l a t e excited s t a t e cross sections. I t i s especially i n t e r e s t i n g t o t h e atomic physics community i f an atom i s prepared i n a s p e c i f i c well defined s t a t e . Photoionlzation of these atoms a r e among the most e a s i l y interpretable experiments t h a t may be performed. In par- t i c u l a r , the energy dependence of the .t +

( L - 7 ) cross section f o r some excited a-

toms has been oredicted16 t o have an unexpected minimum. This e f f e c t up t o now has not been demonstrated. The excitation of an outer electron has been predicted t o modify dramatically t h e e f f e c t i v e potential experienced by inner-shell electrons with large quantum numbers.17 Large changes in the o s c i l l a t o r strengths of some i n - ner-she1

1

t r a n s i t i o n s have been recently reported f o r excited atoms,

l8

Finally ,

photoionization from an excited s t a t e allows one to produce autoionizing s t a t e s with the same parity a s the ground s t a t e ; excited s t a t e s l i k e these have properties t h a t make them candidates f o r s o f t X-ray l a s e r s . Thus, there i s a great deal of mo- tivation t o study photoionization of atoms 'In exctted s t a t e s .

I1 I .

EXPERIMENTAL ARRANGEMENT

The simplest measurement t o determine photoionlzation cross sectlons i s t o de- t e c t the photoions produced by the interaction of the photons w i t h the atoms being studied. For most of these experiments have probed the conttnuum only a few eV a- bove the f i r s t ionization threshold, because tunable dye l a s e r s a r e generally m i - l a b l e only i n the v i s i b l e and the near U V . In t h a t case, the electrons of only the outermost s h e l l , s p e c i f i c a l l y of a l k a l i - and alkaline earth atoms, can be ejected i n t o the continuum.

One of the best example of experiments of t h i s type was the determination of

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JOURNAL DE PHYSIQUE

LINE A 6 1

---

MIRROR

TURUIDAL NOCHROMATISED

LIGHT SYNCHROTRON

A.C.O.

e- GRATING

POLARIZATION

CW RING

SYSTEM

Fig.1

-

Experimental set up for electron spectrometry studies of photoionization of excited atoms with synchrotron radiation. See detailed description in the text.

(from Ref. 19).

the 3p photoionization cross section i n laser-excited sodium atoms between the 3p ionization threshold (3.03 eV) and 4.3 eV, i . e . over a photon energy range of a l i t - t l e more than one electron v o l t . In one of the experiments associated with these mea- surements,20 the

MF=

3 magnetic s t a t e i s o p t i c a l l y pumped by the c i r c u l a r l y polarized l a s e r radiation and another c i r c u l a r l y polarized l a s e r beam ionized the excited elec- tron. This method allows the experimentalist t o study continuum s t a t e s t h a t a r e de- generate. This i s possible because the t r a n s i t i o n s obey dipole selection rules and c i r c u l a r l y polarized radiation can only produce t r a n s i t i o n s

w i t h AM =

*I. Thus, i n one polarization, only continuum s t a t e s w i t h d-symmetry can be produced, and with t h e other polarization the final s t a t e will be a l i n e a r combination of s-and d-symmetry.

Because of t h e lack of i n t e n s i t y , t h i s type of ultimate experiment cannot y e t be a- chieved using synchrotron radiation. The measurement of only the integrated photoio- nization cross section f o r Na was recently repeated i n the same photon energy range, using synchrotron radiation and ionic detection. 21

The only method available when electrons of several k i n e t i c energies can be pro-

duced by monochromatic photons (e.9. f o r h3

)

5.14 eV in the sodium case described

above) i s t o use electron spectrometry . This technique permits one t o s e l e c t e-

lectrons associated with the ionization from each subshell of i n t e r e s t . One can thus

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obtain the p a r t i a l subshell photoionization crosssections over a wide range of pho- ton energies. Photoelectron spectroscopy a l s o eliminates spurious r e s u l t s t h a t a r e possibly produced by radiation other than t h a t occuring a t the wavelength the mono- chromator i s s e t a t . This explains why photoelectron spectroscopy was the tool of choice t o achieve the success of the f i r s t f e a s i b i l i t y experiments combining the use of synchrotron and l a s e r radiation . ^22-25

Equipment f o r such an experiment consists of a high throughput monochromator, a l a s e r whose output i s taylored f o r the p a r t i c u l a r experiment, and a chamber con- taining apparatus t o detect the incoming photons and the ejected photoelectrons.

Figure 1 i s a representative example of a typical s e t up.'' The synchrotron radia- tion of the

ACO

storage ring i s monochromatized by a toroidal grating monochromator, delivering a flux of up t o 1012 photons/sec i n a 1% band pass f o r photon energies between

16

eV and 140 eV. The output of the monochromator is focussed

by

a second toroidal mirror onto the active region of the cylindrical mirror electron spectrome- t e r (CMA). Inside the electron spectrometer, an oven, mounted on t h e common axis of the CMA and of the VUV beam, emits a weakly collimated beam of metal atoms ( t h e ground s t a t e density i s maintained a t values lower than 1013 at/cm 3

).

The l a s e r beam i r r a d i a t e s the atomic beam i n a direction t h a t i s a t r i g h t angles t o the CMA a x i s . The beam waist of the l a s e r radiation i s adjusted t o f i t the s i z e

o f

the source vo- lume of the CMA. The l a s e r beam i s produced by an argon-ion l a s e r pumped ring dye l a s e r which has an i n t e n s i t y of up t o 10 watts/cm2, and i s l i n e a r l y polarized i n the o r b i t a l plane. The dye l a s e r i s s t a b i l i z e d to the t r a n s i t i o n between the ground s t a - t e and the f i r s t excited s t a t e of the atoms under investigation by observing the fluorescence emitted from the excited atoms i n an auxl'liary atomic beam; t h e VUV beam i s a l s o p a r t i a l l y polarized i n t h e horizontal plane a t r i g h t angles t o t h e l a - s e r beam polarization, which i s along the

CMA

a x i s .

A

similar s e t u p was used, l a t e r on, by the second group of s c i e n t i s t s who have succeeded t o combine synchrotron radiation and 1 a s e r f o r experiments of t h i s s o r t . 26

IV . PHOTOELECTRON SPECTRA OF LASER-EXCITED SODIUM AND BARIUM ATOMS

During the f i r s t s e t of experiments on photoionization processes In laser-exci- ted ~ o d i u m ~ ~ - ~ ~ - and barium

25y27

atoms, i n 1981-1982, the main features of i n t e r e s t ilave been observed. A typical photoelectron energy d i s t r i b u t i o n i n sodlum i s shown i n

Figure 2. The two photoelectron peaks i n the upper frame of the figure, obtained

with photons of 75 eV and without l a s e r , are due t o the ionization of one of the

2p electrons of sodium atoms i n the ground s t a t e (peak near 38 eV binding energy)

and t h e ionization of a 2p electron together w i t h the excitation of the 3s electron

t o a 3p o r b i t a l (so-called s a t e l l i t e peaks near 42 eV binding energy). The excita-

tion of t h i s group of ionic levels i s due t o f i n a l - s t a t e eTectron correlations. The

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JOURNAL DE PHYSIQUE

KINETIC ENERGY (eV)

,:

^, ^:4

^,

^3,6 ^3,E

^,

^;Ol

Na+ 2d 37 20000 G.r.p~??d.&&

ACO - hv -75.0 eV

-

^{BP .0.42}^eV

Excited state

+

Ground state . -. . . . . . . . - - . . . .. .. . . . . . . . -. . . . . - . . - . - - .

Fig. 2

-

P h o t o e l e c t r o n s p e c t r a of atomic sodium produced by 75 eV photons. Upper frame: t h e l a s e r i s o f f , a l l Na atoms a r e

-

i n t h e ground s t a t e ; t h e l a r g e peak, n e a r 38 eV binding energy, i s due t o photoio- n i z a t i o n of 2p e l e c t r o n s , w i t h t h e i o n l e f t i n i t s ground s t a t e ; t h e s m a l l e r peaks a r e due t o i o n i z a t i o n of a 2p e- l e c t r o n w i t h simultaneous e x c i t a t i o n o f t h e 3s e l e c t r o n onto a 3p o r b i t a l .

Lower

frame: t h e l a s e r i s on, about 10% of t h e

atoms

a r e i n t h e 2p63p 2 ~ 3 / 2 e x c i t e d s t a - t e ; i n a d d i t i o n t o t h e p h o t o e l e c t r o n spectrum from atoms i n t h e ground s t a t e , a new peak a p p e a r s (hatched a r e a ) , due t o i o n i z a t i o n of a 2p e l e c t r o n i n t h e e x c i t e d sodium atoms. YErom Ref.22 and 23; n o t e t h a t , i n r e f e r e n c e 23, t h i s f i - gure h a s been m i s t a k e l y i n t e r c h a n g e d by t h e e d i t o r s w i t h a f i g u r e s i m i l a r t o t h e f i g u r e 3 of t h e p r e s e n t paper, making t h e f i g u r e c a p t i o n u n i n t e l l i g i b l e ) .

r e l a t i v e energy position of these 2p 3p

5

excited levels i s noted on the figure

BINDING ENERGY (eV)

as v e r t i c a l 1 ines extending below t h e horizontal 1 ine identifying the confi gu- ration. The lower frame of Figure 2 corresponds t o a photoelectron spectrum taken with some of the sodium atoms laser-exci ted to t h e 3p level .

^A

new peak occurs, lo- cated a t about 40 eV binding energy, s h i f t e d by 2.11 eV ( t h e enerqy of the l a s e r ) from the 2p 3p electron s a t e l l i t e l i n e s

5 i n

the ground s t a t e . I t i s due t o phatoioni- zation of a 2p electron i n atoms w i t h the outer electron In a 3p-orbltal. By taking a spectrum similar t o t h i s one as a function of the photon energy,

tt

i s possible t o map out t h e p a r t i a l cross section t o photoionize the 2p-electrons i n t h e 2p 3p exci- 6 ted atom. The density of excited atoms must be determined i f the cross section i s to be obtained. This can be achieved by measuring the r e l a t i v e change i n i n t e n s i t y of the peak a t 38 eV binding energy i n t h e spectra w i t h t h e l a s e r o f f and with t h e l a s e r on. In t h i s f i r s t s e r i e s of experiments, r e l a t i v e populations of atoms i n t h e 3p excited s t a t e were measured t o be close t o 10%. 22,23

Figure 3 shows the second i n t e r e s t i n g feature observed i n these f i r s t experi- ments on sodium. 22-25 The spectrum i s taken a t 31.40 eV photon energy and displays the e f f e c t of resonant photoemission of the excited outer 3p-electron,

i

. e , t h e for- mation of a core-excited autoionizing s t a t e i n the 2p 3p excited atom, followed by

6

a non-radiative decay of t h i s s t a t e to, the ground s t a t e

of

~ a ' ion. This photon ener-

r n

gy corresponds t o the excitation energy of the 2 ~ P ' ~ atoms t o the 3 ~

5

2 3 2 3/2

2p

(

P)3s3p( P) D5,2 core-excited s t a t e via inner-shell excitation of a 2p electron

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KINETIC ENERGY ( e ~ )

20 22 24 26 28

I ' ' 1 ' 1 ' 1 ' 1 '

6000

- M

In

-

\ V)

5

4000-

8

G.S. E.S.

11 3 - l

2000 -

x 5

0 42 40 38 " 5 3

Fig.3

-

E l e c t r o n spectrum of Na ob- t a i n e d a t a photon energy of 31.40 eV w i t h t h e l a s e r on. P h o t o e l e c t r o n s due t o i o n i z a t i o n of a 2p e l e c t r o n by t h e r a d i a t i o n d i f f r a c t e d i n second o r d e r by t h e monochromator (62.80 eV) appear between 38 eV and 43 eV bin- d i n g e n e r g i e s , a s i n Fig.2. Photoe- l e c t r o n s from t h e d i r e c t i o n i z a t i o n of a 3 s e l e c t r o n appear a t a k i n e t i c energy of about 26.5 eV ( b i n d i n g energy of 5.14 eV). Note t h a t t h i s p a r t of t h e spectrum has been magni- f i e d by a f a c t o r 5. E l e c t r o n s emitted i n t h e decay of t h e 2 p 5 3 s 3 p ( 3 ~ ) 2 ~ 5 1 2 a u t o i o n i z i n g s t a t e formed by e x c i t a - t i o n of a 2p e l e c t r o n by 31.40 eV photons i n t h e e x c i t e d sodium atoms produce t h e i n t e n s e peak on t h e r i g h t p a r t of t h e f i g u r e , a t 3.03 eV bin- d i n g energy on t h e f i r s t o r d e r s c a l e : they correspond t o r e s o n a n t photoio- n i z a t i o n of t h e e x c i t e d 3p e l e c t r o n .

(From Ref. 24).

t i n 2nd order) (in 1st order BINDING ENERGY (eV)

t o the f i r s t empty 3s o r b i t a l

.28

The whole frame of the figure shows the spectrum obtained a t t h i s photon energy with the l a s e r on. In s p i t e of the f a c t t h a t the bin- ding energy of t h e 2p electrons i n sodium atoms i n the ground s t a t e i s about 38 eV and t h a t t h e monochromator i s s e t a t a 31.40 eV photon energy, one observes electron l i n e s due t o photoionization of sodium atoms i n the 2p-subshell. This occurs because the monochromator also transmits photons diffracted i n second order by the grating

(62.80 eV). Note t h a t the band pass of the monochromator i s high enough (0.08 eV) t o allow f o r the observation of the doublet due t o spin-orbit s p l i t t i n g in the

5 + 3

2p 3s ionic s t a t e configuration of Na

(

P and 'P f i n a l s t a t e s ) . The small peak a t about 26.5 eV k i n e t i c energy i s due t o the d i r e c t photoionization of the 3s electron

6 2

by 31.40

eV

photons. Since the l a s e r was tuned t o the 2p 3s

S

1/2 + 2p63p 2 ~ 3 / 2 t r a n s i t i o n , some of the atoms (about 30% i n t h i s case) are in the f i r s t excited 3p-state and one observes, a t 40 eV binding energy, t h e photoelectron peak due t o di- r e c t photoionization of the 2p-electrons i n the excited atom. The intense feature

5

2 3 2

near 28.5 eV kinetic energy a r i s e s from t h e decay of the 2p

(

P)3s3p( P)

D5,2

even-

p a r i t y autoionizino s t a t e resonantly excited by stepwise l a s e r (3p-excitation) plus

V U V (2p-excitation) photon absorption. On the binding energy scale corresponding to

f i r s t order photons, t h i s feature i s located a t the binding energy of a 3p-electron

in t h e excited atom (3.03 eV). When the synchrotron radiation i s detuned from the

resonance region, t h i s feature d i s a y e a r s , because the d i r e c t , non-resonant yho-

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C9-460 JOURNAL

DE

PHYSIQUE

BINDING ENERGY (eV) p t ,de,

LASER OFF

0

P ^Ea. hv4=l l9.8eV- B.P. = 0.64eV

I

16 18 20 22 24 26 28 KINETIC ENERGY (eV)

Fig.4

-

Photoelectron spectrum of atomic barium ionized in the 4d-subshell by 120 eV photons diffracted in 4th order by the monochromator and in the 6s- outer shell by 30 eV photons diffracted in 1st order by the monochromator (upper part of the figure). In the lower part of the figure, with the laser on, the various photoelectron lines originate from ionization of atoms in the ground state and in the 6s5d excited states, as indicated. (From Ref. 25)

.

toionization cross section of the 3p-electron i s very small a t this photon energy.

The present day i n t e n s i t y of synchrotron radiation i s too weak t o allow i t s obser- vation in photoemission studies.

I t is interesting t o note t h a t i f one wants t o study autoionization i n ground or excited s t a t e a l k a l i atoms, one must e x c i t e an inner-shell electron which has binding energy i n the

VUV.

No tunable l a s e r are available f o r t h i s photon energy region. The continuum radiation emitted by a BRV source28 or by a laser-produced plasma15 ( f o r photoabsorption experiments) o r by an electron (positron) storage ring ( f o r photoemission s t u d i e s ) must be used f o r these studies i n the a l k a l i s .

In barium, the l a s e r was tuned t o the 6s' 'so -+ ^6s6p 1

^PI

resonance l i n e a t 2.24 eV. Under our experimental conditions, t h i s excited s t a t e decays r a d i a t i v e l y t o the ground s t a t e and t o the 6s5d 1 y 3 ~ 2 metastable s t a t e r i n the source volume of the CMA. One of the f i r s t spectra t h a t has been recorded is shown i n Figure 4.25 In t h e upper p a r t of the f i g u r e , the photoelectron spectrum i s obtained without t h e l a s e r . The synchrotron radiation monochromator i s s e t a t a photon energy of about 30 eV i n f i r s t order. The radiation diffracted i n 4th order (near 120 eV) i s able to photoionize 4d-subshell electrons of Ba atoms i n the ground s t a t e (binding ener-

9 2 6 2 2 D gies of 98.4 eV and 101.0 e ~ , ~ ' corresponding t o the 4d 5s 5p 6s

5/23 3/2

(10)

f i n a l i o n i c s t a t e s o f F3af) One observes also, near 5 eV b i n d i n g energy (on t h e f i r s t order b i n d i n g energy s c a l e ) , t h e photoelectron l i n e due t o i o n i z a t i o n o f t h e o u t e r 6s e l e c t r o n s by photons o f 30 eV energy. When t h e l a s e r was t u r n e d on (lower p a r t o f t h e f i g u r e ) , t h e photoelectron spectrum was q u i t e changed. F i r s t , t h e number o f 4d- components i s increased and t h e e l e c t r o n l i n e s appear t o be broaden, p a r t l y because o f t h e overlapping between l i n e s corresponding t o d i f f e r e n t i n i t i a l s t a t e s o f Ba a- toms. The b i n d i n g energy o f these 4d-inner e l e c t r o n s i n the 6s5d 1 y 3 ~ 2 s t a t e s i s s h i f t e d by about 2.0 eV towards lower b i n d i n g energies, compared t o t h e i r values f o r

around s t a t e atoms .29 I n a d d i t i o n , t h e number o f p o s s i b l e f i n a l i o n i c s t a t e s i s i n - creased by m u l t i p l e t s p l i t t i n g , because o f the presence o f t h r e e open-shells i n the

9 2 6

4d 5s 5p 6s5d e l e c t r o n i c c o n f i g u r a t i o n o f t h e f i n a l i o n i c s t a t e s . Second, an i n t e n s e e l e c t r o n l i n e appears a t t h e b i n d i n g energy o f t h e 5d electrons,on t h e f i r s t order b i n d i n g energy scale, i n t h e e x c i t e d atoms ( 3.8 eV), i l l u s t r a t i n g t h e enhancement of t h e o u t e r - s h e l l p h o t o i o n i z a t i o n cross s e c t i o n a t these photon energies. A s i m i l a r s p l i t t i n g o f t h e photoelectron l i n e s was a l s o observed very c l e a r l y f o r t h e i o n i z a - t i o n o f t h e 5s and 5p e l e c t r o n s , i n an energy range which was f r e e o f l i n e overlap- p i n g a r i s i n g from higher order d i f f r a c t e d photons. A t y p i c a l example can be seen i n F i g u r e 5,19y30 corresponding t o a case where about 50% o f t h e atoms had been l a s e r - t r a n s f e r e d t o t h e metastable s t a t e s . The s h i f t o f t h e b i n d i n g energies occurs c l e a r - l y t o lower values i n t h e 6s5d 1 y 3 ~ 2 e x c i t e d atoms, w h i l e i t was t o h i g h e r values i n e x c i t e d sodium atoms, compared t o atoms i n the ground s t a t e . This d i f f e r e n t behavior was explained by t h e c a l c u l a t i o n s o f t h e r a d i a l dependence of t h e e l e c t r o n i c densi-

t i e s f o r each state.31 The Sd o r b i t a l s

s4 46 38 30 22 14 8 i n e x c i t e d barium atoms are more penetra-

%

t i n g than t h e 6s o r b i t a l . Consequently, the o u t e r screening f e l t by t h e i n n e r e- l e c t r o n s i s greater, which tends t o decrease t h e a t t r a c t i o n of t h e nucleus.

For barium atoms e x c i t e d i n t h e 6s6p 1

P1

s t a t e , t h e d i f f e r e n c e w i t h t h e ground s t a t e would be much s m a l l e r and o f op- p o s i t e sign. I n another experiment 26

,

i n n e r - s h e l l p h o t o i o n i z a t i o n i n t h e PI 1 s t a t e has been reported.

Fig.5

-

Photoelectron spectra ejected from Ba atoms in the ground state (upper pannel) and in the 6s5d 1 ,3D2 excited states (lower pannel) by 92 eV photons.

The electronic configurations of the va- t-ious final ionic states are marked on the figure. (From Ref.'l9 and Ref. 30).

KINETIC ENERGY (ev)

(11)

C9-462 JOURNAL DE PHYSIQUE

Fig.6

-

Electron spectra of Ba taken at 19.64 eV photon energy. In the upper part of the figure is shown the spectrum of Ba atoms in the 6s2

Iso

ground state. In the lower part Ba atoms are in both ground sta- te and 6s5d 1 9 3 ~ 2 excited states. About 70% of the atoms have been laser-transfered in . of these excited states. The elec- tronic configuration of the final ionic sta- tes arising from resonant ionization of the different barium atomic states present in the vapor are marked on the horizontal li- nes. (From Ref. 32).

Resonant photoemission was also obser- ved i n barium, following inner-shell exci-

250

t a t i o n of a 4d electron,29 and of a 5p e-

l ectron .

^32,33^A

resonant barium photoelec-

fi

tron spectrum i s more complicated due t o

10 8 6 4 2

the many open photoionization channels. We

show i n Figure 6 a p a r t i c u l a r l y interes- t i n g example, in the case of 5p-electron excitation. The upper panel i s an electron spectrum obtained a t a photon energy of 19.64 eV with the l a s e r o f f . A t t h i s photon energy, the inner-shell 5p electron can be excited t o o r b i t a l s t h a t form autoionizing s t a t e s . The spectrum i l l u s t r a t e s the large number of ionic channels t h a t are populated in t h i s excitation process: photo- excitation plus autoionization. Of course, d i r e c t photoionization of the 6s elec- tron i s possible, but t h i s channel has a very small cross section compared t o the resonant autoionization process t h a t i s a c t i v e here. Notice t h a t the ion i s l e f t p r e f e r e n t i a l l y in the 6s and 7s s t a t e s , suggesting a resonance of s f i n a l s t a t e sym- r n e t r ~ . ~ ~ The lower frame i s a spectrum obtained a t t h e same photon energy, but with

6

1 1 3

the l a s e r tuned t o t h e 5p66s2 '5, + ^{5p 6s6p}

^PI

t r a n s i t i o n . The 5d D2 and

D2

metastable s t a t e s become heavily populated even though the radiative t r a n s i t i o n ra- t e t o t h e s e s t a t e s i s 20 times smaller than t o the ground s t a t e . The ground s t a t e po- pulation i s reduced t o a small f r a c t i o n of the population with the l a s e r off ( l e s s than 30% here, i n some cases l e s s than lo%), indicating very e f f i c i e n t pumping of the barium atoms i n t o the excited s t a t e s . The spectrum shows t h a t , a t t h i s photon energy, resonant photoionization of !oth 6s and 5d electrons occurs with comparable

.,

i n t e n s i t i e s

i n

atoms excited in the

' D ~

as well as i n the intermediate s t a t e s . One a l s o observes a l a r a e number of intense s a t e l l i t e l i n e s 4n the autoionization

6 6 6

6

6 of t h e core-excited s t a t e s . They correspond t o

59

6p, 5p 7s, 5p 6d, 5p 7p, 5p 8s,

(12)

6 6

5p 7d and even 5p 8p i o n i c c o n f i g u r a t i o n . A t t h i s photon energy, i t seems t h a t t h e whole s a t e l l i t e s t r u c t u r e i s resonating simultaneously, t h e i n t e n s i t y o f a l l l i n e s being enhanced i n t h e a u t o i o n i z a t i o n process, i n marked c o n t r a s t w i t h t h e ground s t a t e . This was t h e f i r s t observation o f an almost pure s a t e l l i t e spectrum i n t h e p h o t o i o n i z a t i o n o f atoms i n e x c i t e d s t a t e s .

V. EXAMPLES OF RESULTS

A f t e r t h e success o f these f e a s i b i l i t y experiments, two main s e r i e s o f measurements were undertaken: a study o f t h e v a r i a t i o n o f 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

in

e x c i t e d barium atoms over a broad photon energy range and t h e d e t e r m i n a t i o n o f o s c i l - l a t o r s t r e n g t h s f o r t r a n s i t i o n s o f a c o r e - e l e c t r o n i n sodium o r bzrium atoms

,

prepa- r e d i n a s p e c i f i c e x c i t e d s t a t e , t o h i g h l y e x c i t e d a u t o i o n i z i n g f i n a l s t a t e s . Con- c u r r e n t l y , another group demonstrated i t s c a p a b i l i t y t o achieve s i m i l a r e x p e r i - ments.26 More r e c e n t l y , we succeeded t o produce and t o study two-electron h i g h l y e x c i t e d s t a t e s , u s i n g stepwise e x c i t a t i o n w i t h two cw l a s e r beams, t o e x c i t e an o u t e r e l e c t r o n , and synchrotron r a d i a t i o n t o e x c i t e an i n n e r e l e c t r o n i n sodium. I n t h e f o l l owing paragraphs, we wi 11 i 11 u s t r a t e t h e r e s u l t s obtained w i t h several examples.

I . PffOTOfONIZATION CROSS SECTIONS

'IN

EXCITED BARIUM ATOMS

From spectra such as t h e ones shown i n f i g u r e s 4 and 5, i t was p o s s i b l e t o f o l - low t h e v a r i a t i o n o f a photoelectron l i n e associated w i t h t h e i o n i z a t i o n o f a p a r t i - c u l a r subshell, and t o determine, f o r t h e f i r s t time, t h e v a r i a t i o n o f photoioniza- t i o n cross s e c t i o n s o f e x c i t e d atoms over a broad range o f photon energies i n c l u d i n g several i o n i z a t i o n t h r e s h c l d s . As an example, we show i n F i g u r e 7 t h e energy dependence o f t h e 5d p h o t o i o n i z a t i o n cross s e c t i o n i n e x c i t e d barium atoms.34 The absolute s c a l e o f t h e cross s e c t i o n was e s t a b l i s h e d by n o r m a l i z i n g t h e experimental data f o r t h e 5p p h o t o i o n i z a t i o n cross s e c t i o n f o r ground s t a t e barium atoms t o t h e l o c a l density-based random phase approximation (LDRPA) c a l c u l a t i o n s o f

endi in.^^

The e x p e r i - mental r e s u l t s a r e compared t o t h r e e d i f f e r e n t c a l c u l a t i o n s .

In

t h e low photon energy range (1C-4C eV), t h e one-electron t h e o r e t i c a l r e s u l t s 35y36 reproduce t h e general non-resonant behavior o f t h e cross s e c t i o n q u i t e w e l l , even b e t t e r i n f a c t than the LDRPA c a l c u l a t i o n s t h a t i n c l u d e c o r r e l a t i o n e f f e c t s . However, i f one takes i n t o account the s i z e o f t h e e r r o r bars, such an e x c e l l e n t agreement should be considered*

as somewhat f o r t u i t o u s . I n p a r t i c u l a r , t h e r e i s no a p r i o r i reason t o choose one c a l - c u l a t i o n over t h e o t h e r . The 5p

+

5d resonances, occuring below 20 eV photon energy, show up o n l y i n t h e LDRPA model, as expected. These r e s u l t s a r e c o n s i s t e n t w i t h t h e value o f t h e 5d cross s e c t i o n p r e v i o u s l y measured near threshold, which was about 20 Mb between 4 eV and 5 eV photon energies .48 I n our experiment, as i t w i l l be ex- p l a i n e d l a t e r , i t i s n o t p o s s i b l e t o determine the near-threshold behavior o f t h e cross s e c t i o n .

(13)

C9-464 JOURNAL DE PHYSIQUE

P

Fig. 7

-

V a r i a t i o n of t h e 5d photoio- n i z a t i o n c r o s s s e c t i o n i n e x c i t e d barium a s a f u n c t i o n of t h e photon e- nergy, from 10 t o 45 eV (upper p a r t ) and from 40 t o 140 eV (lower p a r t ) . One-electron t h e o r e t i c a l r e s u l t s : . a r e from Hartree-Fock-Slater c a l c u l a -

l o - \ . - t i o n s ( r e f . 3 6 , -,

-.-

⁾^{and from}

.

. ,

.

--.

.

^LDAc a l c u l a t i o n s (-

- - -

-

⁾

,

,' ;-,;-~3-=z6-*. - . Note t h a t , i n t h e o r i g i n a l paper

r n SP u ( r e f . 3 4 ) , t h e s e two c u r v e s have been

PHOTON ENERGY (.v) interchanged. C a l c u l a t i o n s t a k i n g in- t o account e l e c t r o n c o r r e l a t i o n s a r e o b t a i n e d u s i n g LDRPA approximation

2.I Mb

'

1 '

^I ^- (from Wendin, i n r e f . 34,

---- >.

(Fig. t a k e n from r e f . 34).

PHOTON ENERGY (.V)

A t higher photon energies, in- ner-shell interactinns enhance the cross section when the 4d-ionization channels open a t about

100

eV.

A

t h i r d one-electron calculation, 37 not shown, gives a slowly lraryincj curve i n t h i s energy region. These resonance e f f e c t s can be reproduced t h e o r e t i c a l l y only when one takes i n t o account inner-shell interactions between the 4d and t h e 5d electrons. 34 A resonant beha-

vior due t o interference between the d i r e c t photoioni zation process and i nner-shell 9 2 6

excitation t o d i s c r e t e final s t a t e s with 4d 5s 5p 5dnl electronic configuration i s also reproduced by the LDRPA calculations. A good agreement with experiment i s obtai ned when the Auger lifetime of the 4d-hole i s introduced i n t o the calculations. This broadens t h e 4d

-,

4f resonance and strongly reduces i t s strength i n t h e Sd-ioniza- tion channel.

Photoionization cross sections f o r

i

nner-shell ionization of excited bari um a- toms have a l s o been measured and a r e presented i n another paper.38 The main r e s u l t of these measurements i s t h a t inner-shell ionization i n 5s and 5p subshells of excited barium atoms i s even more strongly enhanced, via inter-shell correlations with t h e 4d electrons, than f o r barium atoms i n t h e ground s t a t e .

2.

OSCILLATOR STRENGTHS OF INNER-SHEL L EXCITATION TRANSITIONS IN EXCITED SOVlUM ATOMS

In the f i r s t of these new experiments,39 we have measured o s c i l l a t o r strengths

f o r t r a n s i t i o n s , i n excited atomic sodium, involving even-parity autoionizi ng l e -

v e l s . The t r a n s i t i o n s between a laser-excited i n i t i a l 2p 3p e l e c t r o n i c configuration

6

and autoionizing l e v e l s with 2p 3s3p configuration were systematically investigated. 5

(14)

When the Ep-electron i s e x c i t e d t o t h e 3s o r b i t a l , t h e parent c o n f i g u r a t i o n 3s3p cou- p l e s through t h e exchange i n t e r a c t i o n t o form 3~ terms ( 2 D5/2 and 2 ~ 3 3 2 l e v e l s a t 31.40 eV and 31.34 eV, 2 ~ 1 / 2 l e v e l a t 31.78 eV f o r t h e complete system) and P terms 1

2 2

( D2gI and D3/2 l e v e l s a t 32.69 eV and 32.85 eV, r e s p e c t i v e l y ) .

5

A t y p i c a l e l e c t r o n spectrum, obtained a t 31.40 eV, has been shown i n Fig.3.

The group o f resonances associated w i t h 2p 3s3p c o n f i g u r a t i o n was scanned by step of 5 0.03 eV, between 31.00 eV and 33.00 eV

.

Figure 8 shows t h e e x c i t a t i o n curve (convo-

T

f 20-

e 1

^16-

g

!! 1

5 LI

l u t i o n o f t h e cross s e c t i o n and the monochromator band pass) measured around 31.40

5 2 3

eV f o r t h e terms having 2p ( P)3s3p( P) coupling, w i t h two d i f f e r e n t values of t h e monochromator band pass

.

The spectrum i s n o t simple t o i n t e r p r e t

,

because several

hr. P ~ O T O ) ( mrnav (en 2p 3p + h 3 (SR) + 2 ~ 3 ~ 3l23 Sl ~

"

t ~ a ~ ~ ~ ~ ~ l ~

I*. I ~ I a. a. ( a m

I I I l l 1 ^.hO.*W.V

I

^"SR 1 ^{s D a R}1 ^'ps/*

-

"w. ==.,/, - $ / a u.O.*l.v

f ",

+ - + - A

I \

- -- -

- -

r.o.<a.v

16- ° .-O.~?.V - ~

/

d

P

\

' 0

,

^I ^~ ^I ^,

51.20 31.40 31.60

resonances o f s i g n i f i c a n t i n t e n s i t i e s a r e simultaneously e x c i t e d , even i n t h e smal- Fig.8

-

Excitation function of 2p 3s3p autoionizing resonances in Na as a func- tion of photon energy, around 31.40 eV,

- 0 for two values of the monochromator band

pass (BP = 0.12 eV and 0.19 eV). The scale of the ordinate has arbitrary u- nits. The configuration and terms of the

-16 autoionizing resonances in the spectral range shown in the figure are taken from Sugar et al. (ref.28). (From Ref.19).

10

The general two- step e x c i t a t i o n - and a u t o i o n i z i n g decay schemes a r e t h e

f o l l o w i n g :

6 2 6 2

2p 3s S2Il

+

h 3 ( l a s e r )

+

2p 3p PjI2

6 2 5 2

l e s t b a n d w i d t h o f the monochromator. However, i t i s c l e a r t h a t most o f t h e s t r e n g t h i s i n t h e D5/2,3/2 l e v e l s , w i t h some i n d i c a t i o n o f t h e presence o f the 2 4 ~ 3 / 2 l e v e l . I n s t r i c t L.S coupling, t h e 4~ l e v e l s should n o t a u t o i o n i z e . The small hump, around 31.20 eV, i n d i c a t e s t h a t t h e r e i s some m i x i n g w i t h t h e '0 l e v e l s t h a t do a u t o i o n i z e .

The value o f t h e o s c i l l a t o r s t r e n g t h was determined f o r each group o f i n n e r - s h e l l e x c i t a t i o n t r a n s i t i o n s , using t h e e x c i t a t i o n f u n c t i o n s s i m i l a r t o t h e curve shown i n f i g u r e 8. The data were normalized t o t h e value o f t h e d i r e c t Zp-photoioni- z a t i o n cross s e c t i o n a t 65 eV photon energy.39 The sum o f t h e o s c i l l a t o r s t r e n g t h s f o r a l l measured t r a n s i t i o n s was found t o be 0.22(4), assuming t h a t the r a d i a t i v e decay o f t h e two-electron e x c i t e d s t a t e s i s n e g l i g i b l e . A comparison w i t h t h e calcu- l a t e d values f o r t h e

zp6

⁹2p 3s t r a n s i t i o n i n t h e i s o e l e c t r o n i c sequence o f N e - l i - 5

(15)

C9-466 JOURNAL DE PHYSIQUE

ke i o n s (about 0.20) t h a t LS c o u p l i n g dominates and t h a t t h e excited-3p e l e c t r o n acts c h i e f l y as a spectator.

3. RESONANT PHOTOIONlZAT7ON CROSS SECTIONS 'IN EXCZTED-BARIUM ATOMS

We have already shown, i n Fig.6, an example o f photoelectron spectra t h a t a r e measured i n t h e energy range o f t h e 5 p - e x c i t a t i o n s . By c o n t i n u o u s l y scanning t h e photon energy range from 16 eV t o 22 eV, we have obtained p a r t i a l cross s e c t i o n s f o r photo- i o n i z a t i o n i n t o t h e various continuum channels o f Ba atoms i n t h e ground s t a t e and i n t h e 1 ' 3 ~ 2 e x c i t e d s t a t e s . 32y33 We present,in Figures 9,some s e l e c t e d examples o f t h e r e s u l t s t h a t have been obtained. I n Fig.9aY t h e two curves a r e t h e photoioniza- t i o n cross s e c t i o n s f o r l e a v i n g Ba atoms, i n i t i a l l y i n t h e i r 6s2

'so

ground s t a t e ,

6 2 6 2

i n one o f the 5p 6s S ( l e f t p a r t ) and 5p 5d D

l / 2 3/2,5/2 ( r i g h t p a r t ) i o n i c s t a t e s o f ~a'. The two curves i n Fig.9b a r e t h e corresponding r e s u l t s f o r Ba atoms i n i t i a l l y

6 1

i n t h e 5p 6s5d Dp e x c i t e d s t a t e . Without going i n t o d e t a i l s , t h e d i f f e r e n c e s i n t h e behavior of t h e ground s t a t e and o f the e x c i t e d s t a t e a r e s t r i k i n g . While, i n both cases, one observes t h e i n f l uence o f a u t o i o n i z i ng resonances, i n v o l v i ng core-exci t a - t i o n of 5p-electrons, whose e f f e c t i s t o s t r o n g l y enhance t h e p a r t i a l cross s e c t i o n s a t some photon energies ( f o r Ba atoms i n t h e ground s t a t e , t h i s e f f e c t has f i r s t been observed, u s i n g synchrotron r a d i a t i o n , by Rosenberg e t a1 .40), i t i s c l e a r t h a t the number o f resonances i s increased f o r atoms i n ehe Sd-excited s t a t e . Furthermore, thei:values o f t h e cross s e c t i o n s a t t h e maxima o f t h e resonances i s diminished f o r i o n i z a t i o n o f e x c i t e d atoms. Promoting one o f t h e 6s e l e c t r o n s i n t o a 5d o r b i t a l has t h e e f f e c t o f m u l t i p l y i n g t h e p o s s i b l e couplings, s i n c e they are several open sub- s h e l l s i n t h e i n t e r m e d i a t e 5p 5 &n'&'nlZ" resonant s t a t e s . The o s c i l l a t o r s t r e n g t h f o r these t r a n s i t i o n s a r e spread over a l a r g e r number o f resonances. From these data,

PHOTON ENERGY (*V)

WAVELENGTH (A)

Fig.9a

-

Partial photoionizrtion cross sections for leaving Ba atoms, initially in the ground state, in the 5p66s (leftpart) of tne 5p65d (right art) ionic states(33)-

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PHOTON ENERGY (8V) PHOTON ENERGY (eV) WAVELENGTH (A)

8 0 0 7 5 0 7 0 0 6 5 0 6 0 0 5 5 0

Fig.9b

-

P a r t i a l c r o s s s e c t i o n s f o r p h o t o i o n i z a t i o n of barium atoms i n t h e 5p 6d5d 6

ID^

e x c i t e d s t a t e . A s i n Fig.9a, t h e two f i n a l i o n i c s t a t e s a r e t h e 5p66s 2s ( l e f t p a r t ) and 5p65d 2~ ( r i g h t p a r t ) s t a t e s . The v a l u e s of t h e resonance maxima a r e s t r o n g l y dependent on t h e monochromator band p a s s

(0.11 eV h e r e ) , b u t t h i s e f f e c t does n o t a f f e c t t h e d e t e r m i n a t i o n of t h e o s c i l l a t o r s t r e n g t h s . (From Ref. 32 and 3 3 ) .

5 0

one can conclude33 t h a t a l l resonances present i n the ground s t a t e can a l s o be seen in the excited s t a t e s , suggesting t h a t a t l e a s t part of the d i s c r e t e intermediate s t a t e s involving core-exci t a t i o n of a 5p-electron i n neutral bari um are the same, i . e . they have to be represented by a strong mixing of configuration such as 6s

2

,

6s5d and 5d2.41 Additional transitions are, of course, observed i n the i n i t i a l l y excited atom. I t i s worthwhile to note t h a t the modest resolution (compared t o the

4

2

most recent photoabsorption data

)

t h a t had t o be used here t o g e t high enough pho- ton flux enabled us t o show c l e a r l y where the o s c i l l a t o r strength i s mainly concen- t r a t e d , especially f o r t r a n s i t i o n s from ground s t a t e atoms.

I I I I I

Ba

5 ~ ' Sd 6s 'Cl+hv4p8 Sd 2 ~ * r

- -

The highly complex spectrum of barium requires extensive computational e f f o r t for a detailed interpretation of the 5p-excitation spectrum. Even f o r the ground s t a t e , our data show33 t h a t t h e detailed interpretation of the photoabsorption spec-

does not account f o r a l l experimental features observed i n these photoemis- sion experiments. The interpretation of the photoabsorption spectrum of excited ba- rium15 i s f a r from being a t end. The next generation of photoemission experiments must be carried out with a higher resolution, i n ground s t a t e as well as in excited s t a t e atoms, t o b e t t e r understand these complex interactions. This can be achieved only with t h e high brightness expected from undulators mounted on new storage rings.

4 . 'iHREE-STEP EXCTTATION OF HlGHLY -EXClTEV AUJOTONlZlhG STATES IN SOVIUM

The t u n a b i l i t y range of cw l a s e r s i s r a t h e r limited, when one takes i n t o ac-

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C9-468 JOURNAL DE PHYSIQUE

count t h e power d e n s i t y t h a t i s needed i n t h i s photoemission experiments (several hundreds of m i l l i w a t t s )

.

Thus, i t was important t o t e s t i f several l a s e r s c o u l d be e f f i c i e n t l y combined w i t h synchrotron r a d i a t i o n t o study atoms promotrd t o an extended number o f h i g h l y e x c i t e d s t a t e s . Na was chosen f o r t h i s t e s t experiment because o f t h e i n t e n s i t y o f synchrotron r a d i a t i o n a v a i l a b l e i n the energy range o f i n n e r - s h e l l e x c i t a t i o n o f a 2p-electron and o f t h e t u n a b i l i t y range o f the l a s e r s . Also, t h e experience gained i n e a r l i e r s t u d i e s o f t h e f i r s t step o f l a s e r - e x c i t a t i o n t o t h e 2p 3p resonant l e v e l s was a p r e r e q u i s i t e t o the success o f t h i s more complex 6 experiment.

Detai 1 s on t h e experiment have been a1 ready pub1 i ~ h e d ~ ~ o r a r e given i n another paper o f t h i s volume .45 B a s i c a l l y

,

monochromatized synchrotron r a d i a t i o n and two cw l a s e r beams a r e focussed i n t o t h e source volume o f the CMA already shown i n F i - gure 1. The l a s e r e x c i t a t i o n i s achieved i n t h e f o l l o w i n g way: f i r s t , a r i n g c a v i t y

6 2

i s used t o prepare Na atoms e x c i t e d i n the 2p 3p P3,* state;then, the second step o f t h e p h o t o e x c i t a t i o n i s produced by a s t a t i o n n a r y wave dye l a s e r which i s tuned

6 2 6 2 6 2 6

e i t h e r t o the 2p 3p

P -+

2p 4d D o r t o t h e 2p 3p P312

-+

^{2p 5s}2 ~ 1 / 2

3/2 512 6 2

t r a n s i t i o n s . I n t h e r a d i a t i v e decay o f these s t a t e s , atoms i n the 2p 4p P 1/2,3/2 s t a t e s a r e formed i n t h e beam. The synchrotron r a d i a t i o n i s used t o promote a 2p-

e l e c t r o n t o t h e 3s-empty o r b i t a l

.

^{F i g .}¹⁰

shows an example o f e l e c t r o n s p e c t r a ob- BINDING ENERGY in 2nd order (eV)

40 38 36 34 t a i n e d d u r i n g the f i r s t s e r i e s o f e x p e r i -

8000

4000

2

-

Fig.10

-

Fhotoelectron spectra o f Na atoms i n various inLtiaT s t a t e s taken a t 33.06

-

eV photon energy. Top: peak 2 i s due t o photoionization o f ground s t a t e atoms i n - the Zp-shell by photons o f 66.12 eV. Mid-

d l e : spectrum taken with l a s e r I turned on:

8000

2 ~ 3 / 2 e x c i t e d s t a t e (peak noted 1, Laser I) 0

0 S R + Laser I ( 3 s - 3 0 )

8000

- -

ments, a t a photon energy o f 33.06 eV.

The upper p a r t and the middle p a r t o f t h e f i g u r e shows t h e 2p-photoelectron spectrum o f Na atoms i n t h e ground s t a t e (peak no- t e d 2, 1 asers o f f ) and i n the' f i r s t 2p 3p 6

1 ~ 1 1 1 1 1 1

7 5 3 1

BINDING ENERGY in 1st order (eV) S R + Lamer I ^(3s-3p) + ^LaserP. (3p-4d)

- -

l o o o O : d :

-

.. :.. . . _.

.

. . . ..

-

Na

-

2 ~ ~ 3 s

I

-

peak 1 i s due t o photoionization of the Zp-shell e l e c t r o n s i n 2p63p e x c i t e d atoms.

Bottom: spectrum with both l a s e r s turned on: peaks 3 and 4 are due t o a u t o i o n i z a t i o n

5 2 5 2

o f 2p 3s4p D and 2p 3s4d F s t a t e s , res- p e c t i v e l y . (From Ref . 4 4 ) .

SR on:

12

:: ^1:::::::

^-

Lasers O f f

-

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r e s p e c t i v e l y . These spectra a r e b a s i c a l l y the same as t h e ones presented i n F i g u r e 2 a t a d i f f e r e n t photon energy (75 eV). When the second l a s e r i s a l s o switched on (lower p a r t o f t h e f i g u r e ) , new e l e c t r o n l i n e s , noted 3 and 4

,

appear. Peak 3 i s due t o a u t o i o n i z a t i o n o f the d i s c r e t e s t a t e s r e s o n a n t l y e x c i t e d by t h e synchrotron r a d i a t i o n i n t h e h i g h l y o p t i c a l ' l y - e x c i t e d sodium atoms, according t o t h e scheme:

. . --t 2p6

'so

(~a')

+

^e-

5 2

S i m i l a r l y , peak 4 r e s u l t s from the a u t o i o n i z a t i o n o f t h e 2p 3s4d F 5/2,7/2 excited s t a t e s .

Peaks 3 and 4 disappear when t h e synchrotron r a d i a t i o n monochromator i s s h i f t e d o f f t h e resonance region, because the non-resonant 4p- and 4d-photoionization cross sections a r e small a t these photon energies, f a r above the i o n i z a t i o n thresholds, o r when l a s e r I1 i s detuned. By continuously scanning t h e photon energy range o f i n t e - r e s t , between 32.5 eV and 33.5 eV, i t was p o s s i b l e t o measure the energy o f several o f these newly observed a u t o i o n i z i n g s t a t e s and t o determine the r e l a t i v e o s c i 1 l a t o r strengths o f the i n n e r - s h e l l t r a n s i t i o n s having populated these e x c i t e d s t a t e s i n the o p t i c a l l y - e x c i t e d atoms.

VI. FUTURE D E V E L O P M E N T S

The experiments described i n t h e various s e c t i o n s o f t h i s paper are s t i l l d i f - f i c u l t . Higher photon f l u x i n a narrower band pass are e s s e n t i a l i f one wishes t o go t o a more q u a n t i t a t i v e a n a l y s i s o f p h o t o i o n i z a t i o n processes i n e x c i t e d atoms and t o measure processes w i t h small cross s e c t i o n s . Large gains a r e expected from t h e undulators t h a t are going t o be i n s t a l l e d on new storage r i n g s e s p e c i a l l y b u i l t f o r t h e p r o d u c t i o n o f synchrotron r a d i a t i o n , such as Super ACO o r the ALS. As brightness increases by several orders o f magnitude, a l a r g e number o f new e x p e r i - ments w i l l become f e a s i b l e such as:

1. Experiments w i t h h i g h e r r e s o l u t i o n , f o r the synchrotron r a d i a t i o n monochromator as w e l l as f o r the e l e c t r o n spectrometer. The case o f barium described p r e v i o u s l y i s a p a r t i c u l a r l y good example o f a study f o r which such improvements a r e e s s e n t i a l . 2. Experiments a t lower atomic d e n s i t i e s . This w i l l make p o s s i b l e t h e study o f metals

w i t h lower vapor pressures such as most o f t h e t r a n s i t i o n metals and t h e r a r e ^'

~ a r t h s . Furthermore, a s i g n i f i c a n t decrease of t h e atomic d e n s i t i e s i n t h e i n t e r - a c t i o n volume would considerably reduce t h e r a t e o f low-energy e l e c t r o n s produced i n c o l l i s i o n a l i o n i z a t i o n o f e x c i t e d atoms. F i g u r e 11 i s an i l l u s t r a t i o n o f t h e present experimental problem, i n the case of l a s e r - e x c i t e d barium atoms .46 A t l o w - k i n e t i c energies, huge peaks, produced by Penning i o n i z a t i o n o f e x c i t e d atoms f o l l o w e d by s u p e r e l a s t i c c o l l i s i o n s , make i t impossible t o study t h e photoioniza-