PHOTOELECTRON SPECTROSCOPY OF LASER EXCITED Ca ATOMS

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PHOTOELECTRON SPECTROSCOPY OF LASER EXCITED Ca ATOMS

M. Meyer, T. Prescher, E. V. Raven, M. Richter, B. Sonntag, B. Müller, W.

Fiedler, P. Zimmermann

To cite this version:

M. Meyer, T. Prescher, E. V. Raven, M. Richter, B. Sonntag, et al.. PHOTOELECTRON SPEC-

TROSCOPY OF LASER EXCITED Ca ATOMS. Journal de Physique Colloques, 1987, 48 (C9),

pp.C9-547-C9-550. �10.1051/jphyscol:1987991�. �jpa-00227412�

PHOTOELECTRON SPECTROSCOPY OF LASER EXCITED Ca ATOMS

M. MEYER, T. PRESCHER, E. v. RAVEN, M. RICHTER, B. SONNTAG, B. R. M~~LLER' , ^W. FIEDLER* and P. ZIMMERMANN'

II. Institut fifr Experimentalphysik, Universitdt Hamburg, Luruper Chaussee 149, 0-2000 Hamburg 50, F.R.G.

'Institut fifr Strahlungs- und Kernphysik, rechnische Universitdt Berlin, Sekr, PN 3-2, ~ardenbergstrasse 36, 0-1000 Berlin 12, F.R.G.

Abstract: Ca I atoms were excited by pumping the Ca I 3p64s2 l.S0 + Ca I 3 ~ ~ 4 s . 4 ~ lpl transition using a cw ring-dye- laser. In a second step the atoms were excited by synchrotron radiation 'to core resonances 3p53dnlln111' which mainly decay by autoionization. Varying the photon energy of the synchrotron radiation the partial cross-section 3p64s4p lpl + 3p64sek were studied.

INTRODUCTION

Core excitations of atomic Ca induce dramatic changes of the outer shell configuaration leading to complicated absorption and ion

spectra and to prominent satellite structures in the photoelectron spectra /1,2,3/. Therefore, Ca is ideally suited to test predictions based on atomic-many-body theory. The rearrangement of the outer electrons upon core excitation is expected to strongly depend on the initial configuration. The study of core excitations of excited Ca I.

atoms promises detailed insight in the many electron effects.

3p-absorption spectra of excited Ca I atoms and of Ca I1 ions have been determined by Sonntag et a1 /4/ using a flashlight pumped dye

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987991

C9-548 JOURNAL DE PHYSIQUE

laser to induce the 3p64s2 + 3p64s4p 3p1 ( A = 657.2 nm) intercombination transition and the W V radiation of a ruby laser produced plasma. The photoionization cross section of Ca I1 ions has been obtained by Lyon et a1 /5/. In all spectra strong autoionizing resonances have been detected. The intense radiation of a laser can be used to achieve a considerable fraction of atoms in well prepared excited states /6,7/. Combining a cw ring-dye-laser with synchrotron radiation we have studied the Ca I 3p64s4p lpl + 3p5n~n'~1n"!L"

autoionization resonances.

EXPERIMENTAL

The Ca atomic beam was produced from an oven heated by electron impact. The oven was operated at temperatures of about 800 K to produce a density of about 1011 Ca atoms/cm3 in the interaction region. The excitation of the Ca atoms was achieved by pumping the resonance transition Ca I 3p64s2 ls0 + Ca I 3p64s4p lpl with a cw ring-dye-laser at X = 422.7 nm. An output power of about 90 mW with Stilbene 3 in single mode operation was obtained by 3W UV pump power of an Ar-ion-laser. The fraction of the excited atoms was estimated to 5%. In order to lock the cw ring-dye-laser to the

resonance transition the resonance fluorescence emitted perpendicular to the atomic and the laser beam was monitored by a photomultiplier.

The Ca I 3p64s4p lpI + Ca I 3p5n!Ln'R'n"$" excitations were obtained by synchrotron radiation emitted from the electron storage ring BESSY. The synchrotron radiation was monochromatized with a toroidal grating monochromator /8/. The bandwidth of 0.06 eV of the

monochromator was achieved with a slit width of 0.5 mm. Both beams were focussed in the interaction region with a focal spot gi < 1 mm. A good illumination of the interaction region by both radiation sources was obtained when the laser beam, propagating towards the storage ring, coincided with the beam of the synchrotron radiation.

The kinetic energy of the photoelectrons was determined by a cylindrical mirror analyzer (CMA). The CMA had an angular acceptance of 0.8% of 4 n and an energy resolution of AE = 0.8% of the pass energy of the electrons. Only electrons emitted at angles close to the magic angle of 54O44' relative to the polarization vector of the synchrotron light were accepted by the CMA.

Fig. 1 shows a part of the photoelectron spectrum of Ca atoms at the energy (hV = 33.03 eV) of the dominant Ca I (1p1)3p + 3d

resonance. In the absence of laser radiation, one observes photolines 4s, 3d, 4p corresponding to the final ionic states of Ca 11. When the laser is on, new photoelectron lines appear. They are shifted by the

2 4 2 6 2 8 3 0

kinetic energy/eV

600.

.

Fig. 1: Photoelectron spectral of Ca atoms taken at 33.03 eV photon energy (top: laser off; bottom: laser on). The marked lines are due to synchrotron excitation from laser excited Ca atoms.

500

laser excitation energy of 2.93 eV. The relative strengths of these photoelectron lines markedly differ for the ground state and the excited'state spectrum. Fig. 2 shows the Ca I 3p64s4p lpl + 3p64s&E partial cross section. There is a strong resonance at 33.03 eV. The

CaII

I " ' I

-48

hu

BP = 0.06eV

-

400 -

^{Laser off} -

4 V)

300 - I ^-

0 v

200 ^{- -}

-

C9-550 JOURNAL DE PHYSIQUE

shoulders at both sides are located at 32.89 eV and 33.17 eV. The experimental points were best fitted with three Gaussian profiles at 32.89 eV, 33.03 eV and 33.17 eV photon energy with halfwidths of 0.12 eV, 0.13 eV and 0.18 eV. The 3pabsorption spectra of Ca I 4s2 ls0 and Ca I 4s4p 3p are dominated by similar maxima positioned at 31.4 eV and 32.29 eV respectively. Going to Ca I1 the corresponding 3 p 6 4 s 2 ~

-

Fig. 2

3 d

2

O

1.0 2

Q) V)

vl k

0.5

0

+3

2

P,

0.0

32.6 32.8 33.0 33.2 33.4

photon energy/eV

: Partial cross section of the transition Ca I 3p64s4p lpl * Ca I1 3p64sa~

The authors acklowledge the support of the BESSY staff. This work has been funded by the German Federal Minister of Research and Technology (BMFT)

.

REFERENCES

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