Book of Abstracts Book of Abstracts - Universität Konstanz

B - 32
Fragmentation of small alkali cluster attached to helium nanodroplets
72
Patrick Claas 1 , Georg Droppelmann 1 , Claus-Peter Schulz 2 and Frank Stienkemeier 1
1 Fakultät für Physik, Universität Bielefeld, Universitätsstr. 25, D-33615 Bielefeld, Germany
email: franks@physik.uni-bielefeld.de
2 Max-Born-Institut, Max-Born-Str. 2a, D-12489 Berlin, Germany
We use helium nanodroplet isolation (HENDI) to form cold small alkali clusters. In
particular, potassium atoms are picked up successively by superfluid helium droplets in order to
aggregate to clusters at temperatures of 380 mK. In a femtosecond pump-probe setup we
determine fragmentation times of the clusters upon electronic excitation. In contrast to earlier
experiments on free clusters (cf. Figure 1) we find substantially longer times in the
fragmentation processes. The following aspects have to be considered: (a) population of highspin
states [3]; (b) missing thermal activation because of the low temperature; (c) cage effects
by the helium environment. Studies performed at different photon energies, droplet sizes, etc.
allow a closer look at the fragmentation mechanism and the items given above.
Furthermore first results are presented on the formation of complexes of alkali clusters with
foreign atoms or molecules (e.g. H2O) attached to helium droplets.
decay time [ps]
Figure 1.: Fragmentation times upon femtosecond laser excitation of potassium clusters K n attached to helium
nanodroplets as a function of cluster size. As a comparison results of gas-phase clusters by Wöste et al. are also
included.
References
10
1
2 4 6 8 10
cluster size n
[1] H. Kühling et al., J. Phys. Chem. 98, 6679 (1994).
[2] A. Ruff et al., Z. Phys. D 37, 175 (1996).
[3] C. P. Schulz, et al., Phys. Rev. Lett. 92(1), 013401 (2004) .
this work, 1.55eV, K
this work, 1.47eV, K
[1], 1.47 eV, K
[1], 2.94 eV, K
[2], 2.92 eV Na