Author: Uwe Becker

Affiliation(s): Fritz-Haber-Institut der MPG, Berlin, Germany

Anisotropy parameters of angular distributions in sequential and simultaneous few-photon processes M. Braune1, A. Reinköster1, S. Korica1, J. Viefhaus2, and U. Becker1,3 1 Fritz-Haber-Institut der Max-Planck-Gesellschaft Faradayweg 4-6, 14195 Berlin GERMANY 2 HASYLAB, Deutsches Elektronen-Synchrotron, Notkestr. 85, 22607 Hamburg, GERMANY 3 Department of Physics, King Saud University, Riyadh, 11451, Saudi Arabia Various sequential and simultaneous pathways of few-photon double-ionization of rare gases could be distinguished by means of angle-resolved photoelectron spectroscopy in experiments at the FLASH facility. The results show that sequential double-ionization is the dominant process if the required photon energy threshold is exceeded. The photoelectron angular distributions of these two-photon processes differ from a distribution of a dipole transition showing contributions of higher order Legendre polynomials. Values of the corresponding higher order anisotropy parameter β4 could be determined and compared to recent calculations [1,2]. Our data reveal good agreement in the case of neon. Especially for argon and krypton, the calculations predict a dramatic change in the β4-values at a photon energy around 50eV and 80eV, respectively, due to the Cooper minimum in the cross section. Adaption of the ionic β2 values regarding the Cooper minimum to the position of this minimum in single ionization of the neutral atom moves the pronounced Cooper minimum in the β4-values of the 3P ionization channel by about 10eV in photon energy. Taking this into account, our experimental β- values for argon and krypton agree quite well with theory. Surprisingly, in this energy range the β4-values depend strongly on the coupling of the ionic final states although the Cooper minimum is basically a radial effect of the wave function of the outgoing electron. This is a clear signature of differences in the anisotropic interactions of the photoelectron depending on the angular momentum coupling of the ionic terms. Our data give first evidence for this effect in argon for the 3P, 1S and 1D doubly charged final ionic states. A substantial variation of β4 is also observed in the preliminary data analysis of the sequential double ionization of krypton. In case of a direct double-ionization by two photons, simultaneous emission of two electrons leads to a continuous energy distribution ranging from zero kinetic energy up to a maximum value depending on the photon energy and the double ionization threshold. Correspondingly shaped signatures are not clearly observed in the spectra indicating that simultaneous double ionization must be very weak compared to the sequential one. This is in agreement with a preliminary calculation for e.g. helium predicting that the direct process should have at least two orders of magnitude less intensity then the sequential ionization. References 1. A.S. Kheifets, J. Phys. B 40, F313 (2007) 2. S. Fritzsche et al., J. Phys. B 41, 165601 (2008)

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