PHOTODESORPTION DYNAMICS OF BA ATOMS FROM LARGE AR CLUSTERS

Barium atoms at the surface of large argon clusters are excited into t he 6s6p(1)P(1) state leading either to ejection and subsequent fluores cence of the free barium atom or to fluorescence of solvated barium. R elative probabilities for these two processes as a function of the exc itation energy and of the cluster size are presented. By comparison wi th the binding energy from a molecular dynamics (MD) calculation, we f ind that the barium atom is not excited above the limit for direct dis sociation. The nonzero desorption probability is attributed to the gai n of surface energy of the argon cluster which is partially transferre d to the barium atom. Using linearly polarized laser light for the exc itation, a positive polarization of the fluorescence of the ejected ba rium atoms is observed, whereas the polarization fluorescence of the s olvated barium is negative. This indicates that the solvated barium at oms undergo a relaxation process which rotates the orientation of the excited orbital before the fluorescence. The degree of polarization of the fluorescence from ejected barium does not depend on the excitatio n energy or the cluster size.