Spectroscopy of Cold Cs2 Molecules on Helium Nanodroplets

Wolfgang E. Ernst
Institute of Experimental Physics, Graz, Austria

Robert M. Beuc, Mladen Movre, Goran Pichler
Institute of Physics, Zagreb, Croatia

In recent years, experiments with alkali doped helium nanodroplets1 have shown that alkali metal atoms remain on the helium surface where they “skate” around and form molecules in cold collisions. The molecular binding energy is absorbed by the helium environment, which leads to evaporation of helium atoms until the cluster temperature has re-equilibrated to 0.4 K, the internal temperature of a stable helium nanodroplet. In a helium nanodroplet beam experiment, droplets with a newly formed van der Waals molecule survive whereas those with tightly bound reaction products “on board” evaporate and do not reach the measurement zone. This selectivity has helped to study a number of alkali dimer triplet states through direct excitation from the lowest 3^S_u^+ state that is formed in collisions of two atoms on the surface of a helium nanodroplet[1]. Many studies have focused on the lighter alkali species Li, Na, and K. First experiments with rubidium and helium showed a large abundance of molecular rubidium spectra[2] the analysis of which should be accompanied by theoretical calculations of potentials of sufficient accuracy.
Excitation spectra of cesium molecules adsorbed on the surface of cold helium nanodroplets were measured over a broad frequency range. In our theoretical analysis of these spectra we have used ab-initio potential curves by Spies and Meyer[3]. These theoretical potential curves when used in the simulation of cesium spectra exhibited satisfactory agreement with experimental spectra at thermal conditions[4].
Preliminary analysis of the new cesium spectra on helium points to some very interesting coincidences of experimental and theoretical spectra. However, there are some open questions regarding the possible role of symmetry-breaking processes.

References:
1 J. Higgins, C. Callegari, J. Reho, F. Stienkemeier, W. E. Ernst, M. Gutowski, and G. Scoles, J. Phys. Chem. A 102, 4952 (1998).
2 F. R. Brühl, R. A. Miron, and W. E. Ernst, J. Chem. Phys. 115, 10275 (2001).
3 N. Spies, Ph.D. thesis, Universität Kaiserslautern, 1989. W. Meyer, N. Spies, to be published
4 R. Beuc, H. Skenderovic, T. Ban, D. Veža, G. Pichler and W. Meyer, Eur. Phys. J. D 15,209 (2001)