# Hannover 2013 – wissenschaftliches Programm

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# A: Fachverband Atomphysik

## A 50: Atomic systems in external fields II

### A 50.7: Vortrag

### Freitag, 22. März 2013, 15:30–15:45, B 302

**Angular distribution effects in multi-photon ionization** — •Gregor Hartmann^{1,2}, Markus Braune^{3}, Toralf Lischke^{1,2}, André Meissner^{1}, André Knie^{4}, Arno Ehresmann^{4}, Markus Ilchen^{3}, Omar Al-Dossary^{5,6}, and Uwe Becker^{1,2,5} — ^{1}Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany — ^{2}Max Planck Institute of Microstructure Physics, Weinberg 2, 06120 Halle, Germany — ^{3}DESY Notkestraße 85, 22067 Hamburg, Germany — ^{4}Institut für Physik and CINSaT, Universität Kassel, Heinrich-Plett-Str. 40, 34132 Kassel, Germany — ^{5}Physics Department, College of Science, King Saud University, Riyadh 11451, Saudi Arabia — ^{6}National Center for Mathematics and Physics, KACST, Saudi Arabia

The angular distribution of emitted photoelectrons is determined by the transfer of the angular momentum of the ionizing photon to the ejected electron. In the case of two-photon ionization another angular momentum of one comes into play giving rise to a Legendre polynomial of fourth order, the so called β_{4} term. Interestingly this additional term does not only effect the electrons emitted by two photons but also the one being ejected by one photon only. This shows that both processes are coherently coupled. We show this effect for the first time for the two-photon ionization of the rare gases measured at FLASH. Even more interesting is the fact that this phenomenon is even observed for the case of He although 1s-shell photoionization should exhibit no β_{4} term at all because the remaining hole
is isotropic. We interpret this unexpected result as due to autoionizing doubly excited resonances in He.