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Materie unter extremen Bedingungen Poster: Do., 13:00–15:30 D-P362
Multiphoton Ionisation and Excitation of Atoms and Fullerenes
Markus Braune 1 , Sanja Korica 1 , Axel Reinköster 1 , Daniel Rolles 2 , Jens
Viefhaus 1 , Uwe Becker 1
1 Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany
– 2 Lawrence Berkeley National Laboratory, 1 Cyclotron Road, MS 4R0230,
Berkeley, CA 94720, USA
At the new VUV-FEL light-source at DESY/Hamburg we performed experiments at
the monochromator beamline PG1 with a two chamber setup. In the front part of the
combined apparatus a spherical multi-detector vacuum chamber with a set of time-offlight
analysers at various angles with respect to the polarisation axis of the FEL was
adjusted to the focus of the beamline. With this setup we performed angle resolved
photoelectron spectroscopy measurements on various rare gases (He, Ne, Ar, Kr, Xe)
as well as on the small molecules N2 and H2. In the rear part - about one meter behind
the focal point - C60 molecules are evaporated in an oven assembly. Ions generated
by the FEL radiation were detected with an ion time of flight spectrometer. In both
parts data were acquired simultaneously by recording analogue signals of micro channel
plate detectors by means of a multi-channel digitizer card system and oscilloscope,
respectively, recording traces which have time duration of a full FEL pulse train.
An analysis of our recorded data shows clear indications for two-photon processes in
both cases. For the photoelectron spectroscopy part we restrict the presentation of
our results to experiments on neon only as it showed the strongest signals. Here, twophoton
processes can be identified by analysis of either the dependency of the signal
strength on the radiation intensity or the angular distribution of the signal. The former
should reveal a non-linear behaviour, the latter gives rise to angular patterns different
from a distribution of a dipole transition with momentum transfer of ∆l = 1 being
governed by the well-known β-distribution (a 2nd order Legendre polynomial and the
β-parameter). The observation of C60 ions can also gives hints about two-photon processes,
since one would expect just intact C q+
60 ions (q = 1–3) for photon energies of 38
eV under single photon absorption conditions, whereas other traces of fragments, like
, require higher energies.
C q+
58
or Cq+
56
This work was supported by the Bundesministerium für Bildung und Forschung (BMBF)
under promotion code no. 05 KS4EB1/3.