Book of Abstracts Book of Abstracts - Universität Konstanz

High resolution electron ionization study of helium-clusters
S. Denifl 1 , S. Ptasińska 1 , F. Martinez 2 , K. Głuch 3 , S. Feil 1 , P. Scheier 1 , T. D. Märk 1
A - 22
1 Insitut für Ionenphysi,, Universität Innsbruck, Technikerstrasse 25, 6020 Innsbruck, Austria
2 Institut für Physik, Ernst Moritz Arndt Universität Greifswald, Domstrasse 10a,17487 Greifswald,
Germany
3 Institute of Physics, Maria Curie-Sklodowska University, Pl. Marii Curie-Sklodowskiej 1, 20-031 Lublin,
Poland
After many years of research the determination and interpretation of appearance energies
(AE’s) for clusters using electron impact ionization is still difficult and involves large error bars.
The reasons for this are a number of technical obstacles to prepare electrons with high-energy
resolution. In addition we are confronted with a complicated physical situation of a reaction
complex involving a quantum mechanical many body system. The use of photoionization
sometimes appears to be less difficult but nevertheless the AE values obtained are not directly
comparable to those obtained by electron impact studies. Both processes are intrinsically
different, because the transition complexes in the ionization process are not the same.
This work presents the high resolution study of the threshold behavior for small helium cluster
ions. The crossed electron/cluster beams apparatus used for these measurements consists of a
cluster source, a hemispherical electron monochromator and a quadrupole mass spectrometer. A
standard home-built hemispherical electron analyzer produces electrons with a typical energy
distribution of 150 meV. The helium clusters are produced by supersonic expansion in a cluster
source which can be cooled down to 8.5 K. Previously already all other rare gases (Ne, Ar, Kr,
Xe), H2-, D2-, N2- and N2O-clusters have been studied with the present experimental setup
(except a different cluster source was used). The data evaluation is basing on an extension of
Wannier’s law for the ionization of atoms towards the case of clusters. The present results for
helium cluster up to size n = 10 are compared with those of previous experiments with small
helium clusters using electron impact [1] or photon impact [2], respectively.
This work was supported by FWF, Wien, Austria, and the European Commission,
Brussels.
References
[1] R. Fröchtenicht, U. Henne, J. P. Toennis, A. Ding, M. Fieber-Erdmann, T. Drewello, J. Chem.
Phys. 104, 2548 (1996).
[2] B. E. Callicoat, K. Förde, L. F. Jung, T. Ruchti, K. C. Janda, J. Chem. Phys. 109, 10195 (1998).
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