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A: Fachverband Atomphysik
A 2: Interaction with VUV and X-Ray Light I
A 2.1: Hauptvortrag
Montag, 2. März 2009, 10:45–11:15, VMP 6 HS-E
Angular distributions and continuous intensity behavior in multi-photon processes — •Markus Braune1, Axel Reinköster1, Jens Viefhaus2, Sanja Korica1, and Uwe Becker1 — 1Fritz-Haber-Insitut der MPG, 14195 Berlin — 2DESY, 22607 Hamburg
Using angle-resolved photoelectron spectroscopy at the FLASH facility we were able to distinguish various sequential and simultaneous pathways of multi-photon double-ionization of rare gases. 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 multi-photon processes differ from a distribution of a dipole transition showing contributions of higher order Legendre polynomials. The corresponding higher order parameters β4 could be determined and compared to recent calculations [1,2]. Especially for argon and krypton, these predict a dramatic change in the β4-values at a photon energy around 50eV due to the Cooper minimum in the cross section. Surprisingly, this effect depends on the coupling of the ionic final states. A substantial variation of β4 is in fact observed in our preliminary data analysis of the sequential double ionization of krypton.
For very short time delays between the steps of sequential double-ionization a dynamical screening effect is theoretically predicted which should give rise to continuous photoelectron intensity between the pair of photoelectron lines caused by the sequential process. So far, only weak indications of such a screening effect is found in our helium data.