Vibrational dynamics of large clusters from helium atom scattering: Calculations for Ar55

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Abstract
Vibrational excitation of an Ar55 cluster in collisions with He (Ekin=25 meV) is investigated by using classical trajectories and a highly approximate quantum mechanical method (vibrationally sudden approximation). The energy transfer from the helium atom to the cluster (ΔE) is calculated as a function of the scattering angle θ. It is found (i) that predominantly the modes corresponding to the cluster atoms in the outer shell are excited and (ii) that the probability for multi-phonon (Δn⩾2) excitations steadily increases with the scattering angle. The results of both sets of calculations are generally in good agreement with measured energy loss spectra over the entire range of scattering angles. In the region θ≈15°–30°, which—because single-phonon transitions dominate—is most important for determining the frequency distribution 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 ΔE≲2 meV.