VIBRATIONAL DYNAMICS OF LARGE CLUSTERS FROM HELIUM ATOM SCATTERING - CALCULATIONS FOR AR-55

Vibrational excitation of an Ar-55 cluster in collisions with He (E-ki n=25 meV) is investigated by using classical trajectories and a highly approximate quantum mechanical method (vibrationally sudden approxima tion). The energy transfer from the helium atom to the cluster (Delta E) is calculated as a function of the scattering angle theta. It is fo und (i) that predominantly the modes corresponding to the cluster atom s in the outer shell are excited and (ii) that the probability for mul ti-phonon (Delta n greater than or equal to 2) excitations steadily in creases with the scattering angle. The results of both sets of calcula tions are generally in good agreement with measured energy loss spectr a over the entire range of scattering angles. In the region theta appr oximate to 15 degrees-30 degrees , which-because single-phonon transit ions dominate-is most important for determining the frequency distribu tion of the cluster, the quantum mechanical calculations are superior to classical mechanics; the latter fails to conserve zero-point energy and therefore leads to unrealistic energy transfer below Delta E less than or similar to 2 meV. (C) 1997 American Institute of Physics.