INNER SHELL EXCITATION IN HIGHLY EXCITED
SODIUM ATOMS FROM THE COMBINED USE OF
SYNCHROTRON RADIATION AND TWO LASER
BEAMS
D. Cubaynes, J. Bizau, T. Morgan, F. Wuilleumier, M. Ferray, F. Gounand, P.
D ’Oliveira, P. Fournier
To cite this version:
D. Cubaynes, J. Bizau, T. Morgan, F. Wuilleumier, M. Ferray, et al.. INNER SHELL EXCITATION IN HIGHLY EXCITED SODIUM ATOMS FROM THE COMBINED USE OF
SYNCHROTRON RADIATION AND TWO LASER BEAMS. Journal de Physique Colloques,
1987, 48 (C9), pp.C9-513-C9-521. <10.1051/jphyscol:1987984>. <jpa-00227405>
HAL Id: jpa-00227405
https://hal.archives-ouvertes.fr/jpa-00227405
Submitted on 1 Jan 1987
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JOURNAL DE PHYSIQUE
Colloque C9, suppl6ment au 11'12, Tome 48, d6cembre 1987
INNER SHELL EXCITATION IN HIGHLY EXCITED SODIUM ATOMS FROM THE
COMBINED USE OF SYNCHROTRON RADIATION AND TWO LASER BEAMS
D. CUBAYNES, J.M. BIZAU, T.J. MORGAN, F.J. WUILLEUMIER,
M. FERRAY' , F. GOUNAND' , P. D'OLIVEIRA* and P.R. FOURNIER'
Laboratoire de Spectroscopie Atomique et ~ o n i q u eand LURE,
Universite Paris-Sud, BSt. 350, F-91405 Orsay Cedex, France
'service de Physique des Atomes et des Surfaces, Centre
dlEtudes Nucleaires de Saclay, F-91191 Gif-sur-Yvette Cedex,
France
RESUME
Nous avons demontre qu'il
etait possible de
combiner
I'utilisation de deux
rayonnements laser. agissant en cascade pour exciter I'electron
externe d'un
atome
alcalin (sodium) sur un niveau optique ClevC. avec celle du rayonnement synchrotron
pour exciter un tlectron interne sur la premiere orbitale inoccupte. Les etats trCs
excites ainsi form&s, ayant pour configuration Clectronique 2p5 3s4p. 2p5 3s4d or
2p5 395s. autoionisent vers I'etat fondamental 2p6 IS,
Cmis dans ce processus d'autoionisation
de I'ion Na+. Les Clectrons
sont analysCs en Cnergie avec un analyseur
mirroir cyllndrique. Les energies de plusieurs de ces Ctats autoionisants et les
forces d'oscillateur
relatives associees h I'excitation
de ces etats ont CtC
mesurtes.
ABSTRACT
We have successfully combined the use of two laser beams with synchrotron
radiation to produce and to study highly core-excitea autoionizing states in an
alkali atom (sodium). Stepwise excitations of the outer electron in atomic sodium
produce highly excited optical states with 2p6nl configurations tnl = 4p. 4d, 5s).
inner-shell 2p
--+
3s excitation of a 2p electron with synchrotron radiation in the
optically excited atom creates autoionizing states witn 2p5 3s4p. 2p5 3s4d or 2p5
395s electronic configurations. The electrons emitted in the autoionization of these
states to the 2p6
ground state of Nat are energy analyzed by
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987984
electron
C9-5 14
JOURNAL DE PHYSIQUE
spectrometry. The excitation energies and the relative oscillator strengths of the
Inner-shell transitions in the excited atoms have been measured.
The autoionizing states of an atom play a role of significant importance in many physical fields such as the understanding of the processes occuring
In astrophysical plasmas or the search for generation of X-UV laser radiation. Since
an autoionizing state is, by definition, a state of an atom whose internal energy is
higher than the first ionization threshold, autoionization has long been knobm in
inner-shell atomic physics. The production of a single hole by photoionlzation of an
atom in an inner shell creates in f a c t a very highly excited atomic state. In
addition to the radiative decay of this hoie, this excited state may also decay by
autoionization. When a n inner electron is e~eCted into the continuum, creating an
autoionizing state of the ion, the non radiative decay i s usually called Auger
thus leading
effect ; when it is selectively excited to m empty optical level
to autoionizing states of the neutral atom, the subsequent decay i s preferentially
called autoionization. However, it has been recently pointed out. both
experimentally and theoretically, that this classification i s schematic and that
the processes are, in fact, more complex./ 1.2 / To create a hole in an inner shell
requires, usually, photon energies of at least several tens of an eV. Since the
dynamics of the interaction is also of great importance to fully understand the
process of photoionization, synchrotron radiation has been for a long time the most
valuable source of information in inner- shell atomic physics.
.
In the photolonization or the photoexcitation of an inner electron,
another electron could also be excited via shake up effects, or, more generally, via
correlation effects. Thus, autoionization of two-electron highly excited atomic
states has been the subject of a large number of experimental and theoretical
studies by inner-shell atomic physicists. After the simplest case of the rare
gases,/ 3-5 / more complex atoms have become the subject of investigations. / 6 i
follcuing the significant improvements occuring in the production and in the use of
synchrotron radiation.
Wore recently, another approach has been followed by using several laser
beams, in cascade, to produce two-electron excited states./ 7-10 / However, all
these studies involved multiphoton or stepwise one-photon excitation of only Outer
electrons and have been limited, up to now. to alkaline earths, because of the 1in.i.:<::i t u n a b i l i t y of present day lasers. These investigations have produced interesting
results, almost exclusively on barium atoms, / 11-19 / and many laboratories are
trying to develop this approach to study autoionization.
The possibility of combining one laser beam with a beam of synchrotron
radiation to study photoionization processes in exclted atoms was demonstrated
successfully several years ago for alkali(sodium)- /20,21/ and for alkaline-earth
(barium) /22.23/ atoms. R e c e n t l y , t h i s n e t h o d h a s been used t o measure a number o i
oscillator strengths for inner-shell excitations /24,25/ and photoionization cross
sections /26/ in excited atoms. However, the number of states accessible with
one-laser excitation is limited and i t was felt important to test if several laser
beams could be used, in combination with synchrotron radiation, to produce and to
study atoms excited in a widely extended number of highly excited states. Sodium was
chosen for this feasibility experiment for several reasons: i) the final state of
ground state
the autoionization process is the simplest. since it is the 2p6
of ~ a +ion; i i) the experience gained in earlier studies of the first step of
excitation /25/ was a prerequisite to the success of this more complex experiment:
i i i ) the tunability range required of the lasers was easily achievable;
iv) the intensity of synchrotron radiation available in the proper photon energy
range was high.
We have already presented part
of the results of
this successful
experiment. /27/ Here, we would like to give a little more information on the
experimental procedure and to complement the results previously published.
Figure 1 shows the experimental apparatus used in this work. Two laser
beams are combined with the synchrotron radiation from the Anneau de Collisions
d'0rsay (ACO). The three breams are focused onto a weakly colllmated sodium beam at
the source volume of an electron cylindrical mirror analyzer (CMA). Electrons
emitted at an angle of 54'44' with respect to the CMA axis are energy analyzed. For
an isotropic initial state, the number of electrons ejected at this so-called maglc
angle is independent of the polarization of synchrotron radiation and of the angular
F i 7 : Schma2ic v i w od t h e expehimentd b e t up. ACO=Omaq btohage hing phoducing
&qneirnofion
nnlGation which ia enehgq a d e e t e d wLth t h e g a t i n g and id nedocmbed i n t o t h e bowlce volume 04 t h e CMA; CMA=cqfindhicd mirc/roa andqzeh; VS=voLtage
bowlce; Ah= C n ' k n h a ; DL=dqe laneh; ==beam bpLi,tta; ,M=m.Luoh; F=intehdetence tJ.2t e h ; PMT=phoRornlLeti~~
tube; SS=behvo-nt~otem;CR=chaht hecohdm; FP-Fabhq-Pehot.
The imehf: nhom t h e m o - o t e p e x c i t a t i o n 06 t h e 5s oh the. 40 . t e v d 06 Na an w e l l
an t h e m d i a t i v e cancade t o t h e 4P l e v e l . I ( 1 1 ) h e d m t o t h e Int (2nd) ntep o< the.
h e h excitation.
JOURNAL DE PHYSIQUE
C9-516
distribution of the emitted electrons.
Na atoms are photoexcited to the
2p6 5s 2~1/2
or zp64d 2~5/2state in two steps by cw single frequency dye lasers. A
ring cavity, working with Rh 6G, is used to preoars Na atoms excited in the
2p6 3p 2~3/2 state with a typical output power of 600 mW. As in the previous
experiments, /25/ it is frequency stabilized by monitoring the fluorescence of
anauxiliary Na beam illuminated by a fraction of the laser beam. The second step of
the photoexcitation is achieved by a stationary wave dye laser of lower powerCaround
60 mW) working with either Rh 6G, at 6160 A (3p->5s)
or Rh 110, at 5687 A
( 3 ~ 3 4 d ) . It is frequency stabilized with an external Fabry-Perot cavity. A small
amount of the beam is also sent to the auxiliary Na beam after mlxing with the first
laser beam. Its frequency Is tuned in order to optimize the intensity of the
21s64p32p63s UV line, which is one of the second steps of the radiative cascade
occuring when the 2p65s 2~1/2or 2p6 4d 2~5/2states are photoexcited. The recording
of this fluorescence allows for continuous monitoring of the efficiency of the
two-step excitation during the data acquisltion. The two laser beams are mixed with
a set of mirrors and focussed by a cylindrical lens in order for the interaction
region to properi y match the source volume of the CMA. Atoms excited in the 2p65s
(or 2p64d> and 2p64p states are present in the vapor. The third step of the
excitation is produced by the monochromatic SR : it is used, between 30 and 35 eV,
to promote an inner 2p electron to the 3s empty orbital As a consequence of this
three-photon excitation, autoionizing states with 2p53s5s (or 2p53s4d) and 2p53s4p'
.
BINDING ENERGY in 2 ndorder (eV)
Laser. oft
0
u!
t"
SR.La8.r
1 ( 2 ~ ~ 3-2v03p>
.
BINDING ENERGY in 1 s t order (eV)
Top : A photodecttton bpeothwn 06 Ma
Laken at hg = 33.74 eV photon enehgy
LLlith both Labeh t u n e d odd. Peak 2 LA
due .to i o n i z d o n 0 5 .the 2p-nh& d e c &onn by photon4 06 e n w y equal t o
h i c e h 9 oh 66.28 eV. UiddLe : A opec&ttlum
.taken at t h e name h$ wQh t h e Laneh 1 t u n e d on. Peak 1 O due t o photoioianizdan 06 t h e 2p-nh& eRectttonb i n
2p6 3 p atomts. BoMom : A npe&wn .taken
LIA t h e name hv wiXh both
.tuned
on, w i ~ h ~ h e n m n d o b&npZuned
ne
fi .the
3p+ 4d a%an4iXion. Peak 3 .LA due t o
a u t o i o n i z d o n 06 a 2~53b4p ~ C L t e d
dtate and pah 4 to a u t o h n k d h n 0 d
f yS3b4d ntate.
configurations are produced in the vapor. The electrons emitted in the non-radiative
decay of these states to the 2p6
ground state of ~ a +are observed with the CMA.
Since the density of Na atoms (loi2 to 1013 / cm3) is high, any laser-induced
alignment of the excited Na atoms would be destroyed and the initial state can be
considered as being isotropic.
A typical set of electron spectra is
presented in figure 2, in the case
of 4d excitation. In the upper panel is shown a spectrum with both lasers off and
the SR monochromator set at a photon energy h3 equal to 33.14 eV. Photons of twice
the first-order photon energy (66.28eV) are also transmitted through the
monochromator and photoionize the 2p-shell electrons in ground state Na atoms (peak
labeled 2 at 38 eV binding energy. BE, energy scale at the top of the figure). Idith
the first laser switched on (middle panel ), photoelectron peak 1 appears at about
40eV BE. It is due to the photoionization of 2p-shell electrons with the 3s electron
laser-excited to a 3p orbital. Since about 80 % of Na atoms are still in the ground
state, peak 2 is still present. When the second laser is also switched-on (tuned
here to the 3 p 3 4 d transition energy), new electrons lines, noted 3 and 4, appear
(lower panel). When laser I1 is detuned, or when the SR monochromator is shifted off
the resonance reaions, lines 3 and 4 disappears. This last effect occurs because the
non resonant 4p -and 4d -photoionizat ion cross sect ions are small at these photon
energies.
From the kinetic (KE) scale, it is possible to establish the binding
energy (BE) scales shown in figure 2 according to BE = hJ -KE, in order to identify
the observed structures. However, these BE scales are valid only for direct
photoionization into the continuum. On such a scale, the position of an electron
line due to resonant photoionization of a nl electron (i.e. autoionization of a
2p53snl state) may not coincide exactly with the BE of the nl electron, because of
the large band pass of the monochromator (0.2eV). In addition, several autoionizing
states of different configurations (2p53s3p, 2p53s4p and 2p53s4d in the case of the
4d-laser excitation) can be simultaneously formed in the vapor, which complicates
the interpretation of the spectrum. The kinetic energy of peak 3 is 31.60 eV. Since
the BE of 3p, 4p and 4d electrons are 3.03eV, 1.39eV and 0.86eV. respectively, the
attribution of this line to autoionization of a 2p53s3p, 2p53s4p or 2p53s4d state
would require photons of 34.03eV, 32.99eV or 32.46eV, respectively, to be present in
the photon beam transmitted by the monochromator. Since the monochromator was set
at a photon energy hJ = 33.14eV with a band pass of 0.2eV, it is clear that peak 3
is due to the decay of a 2 ~ ~ state.
3 ~ 4Similarly,
~
peak 4, at 32.30eV kinetic
energy, can be attributed to the decay of a 2p53s4d state.
In the present experiments, it was not possible to observe the low
intensity signal due to photoionization of 2p electrons in 2p64d atoms because of
the small population of atoms in the 4 2~5/2state, typically a few percent , and of
the highly increased background when both lasers were tuned on resonance. This
JOURNAL DE PHYSIQUE
C9-5 18
background is due to the high rate at ionization aclruring in Penning ianizatlon or
associative ionization collisions involving sodium atoms in the ground state and in
the various excited states present in the vapor.
In the case of laser-excitation to the 2p6 5s 2~1,,2state. simi lar
spectra were observed. We show one of them in figure 3, taken at 33.17 eV photon
energy. Here the interpretation is simpler because we have determined that,at this
photon energy, only the two unresolved 2p5 395s 2~1/2,3/2states are accessible by
inner-shell excitation of a 2p electron from the 2p6 5s 2~1/2
state.
BlNDlNG ENERGY in 2nd order (eV)
40
OOoOt
I
'
I
"
38
'
36
"
I
1
34
I
I
SR + Laser I ( 3 ~ - 3 p ) + Laser 11 (3.-5s)
32
i
BINDING ENERGY In 1st order (eV)
a.- A
3
photaelecttlon specttlum 0 6 csodiwn .taken at h v = 33.17 eV w i t h both L a e m
almzed an, Laos 11 being .tuned t o t h e 3p
56 . t h a n n i l i o n . The p u k at
about 1 eV b i n d i n g enengy (Loweh 6 c d e ) .in due t o acLtoionization 0 6 t h e
2p53o5n n=e
-+
.
In order to measure the energy and the excitation function of the
autoionizing states, we scanned the photon energy range between 32.50eV and 33.50eV
in both cases of laser excl tatlon. At each photon energy. we determined the energy
and the normalized intensity of the autoionlzatlon lines. In the absence of a 2p
photoelectron signal proportional to the density of atoms in the 2p5 3snl states, we
measured the ratio between the integrated area under lines 3-4 and under line 1.
Our results for the 4d excitation case were already pub1 ished /27/. Here
we present, in figure 4, the curve for the case of 5s excitation. The solid line is
the result of a computation based
on the measured energies of the autoionizing
states, lines marked A, B and F in the figure. Lines A and B correspond to
transitions from the 2p6 4p 2~1,2,3/2 to the (2p5 3s) I P 4p 2~5,/2,3,,.2
or 'sl,i2
hv .PHOTON
ENERGY (.v)
V)
32.80
3300
33.20
33.40
F&.
4 -
Excitation ,$unction od .the
photon enetcgq, don a SR
monochomaton band pahb 06
0 . 2 0 eV, i n t h e cacre 0 6 5 b
e x c i t a t i o n . A, 8 and F mmk
.the. enehgq 0 6 .the a d o i o n i zation 4 f u - t ~meawred i n
t k i n expehiment (see. .text
,$oh deXaieed exp.tanation1.
states, line F corresponds to the two unresolved dipole-allowed transitions
2p6 5s 2~1/2-+2~5 395s 2~1/2,3/2 at 33.17(4>eV. Since the natural width of the
autoionizing states is negligible compared to the band pass of the monochromator. a
gaussian line profile was centered on each state energy ; then, the relative
intensity of each bar (which is proportional to the oscillator strength of the
corresponding transition, after correction for the different populations of atoms in
the various excited states) was adjusted to obtain the best fit to the experimental
data. The resulting line Is quite sensitive to small variation of the relative
intensities. The sum of the
relative oscillator strengths for these 2p+3s
transitions. (in 4d and 5s excited atans) obtained in that way, as well as the
energy of the autoionizing states, remains practically constant and Is independent
of the orbital in which the outer electron has been excited. /25,27/
We have successfully demonstrated that the combination of several laser
beams with synchrotron radiation widely extends the field of investigation open to
photoionization of atoms in excited states. Ultimately, with the large improvements
st11 I occuring in the new sources of synchrotron radiation, yet unexpected further
developments might become possible.
J O U R N A L DE PHYSIQUE
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