Multiphonon processes in atom-surface scattering

Helium-atom time-of-flight spectra taken at 69-meV incident energy in scattering from Pt(111) show energy-loss peaks that cannot be assigned to one-phonon transitions, but are qualitatively explained by simple formulas for the multiphonon processes in the classical limit. This assignment is supported by trajectory-approximation calculations that correctly reproduce the experimentally determined density of states of Pt(111).