MULTIPHONON ATOM-SURFACE SCATTERING IN THE COLLISION SYSTEM HE-]CU(001)

Recent measurements [1] of the angular distributions and time-of-fligh t spectra of monoenergetic He atom beams scattered from a clean Cu(001 ) surface under multiphonon conditions (incident He atom energy 100 me V, surface temperature between 100 and 1000 K) were interpreted by usi ng the Glauber-van Hove approach to neutron scattering by crystal latt ices conveniently adapted to atom-surface collision problems. From thi s the authors have drawn a number of conclusions regarding the propert ies of inelastic He-surface interactions in general, and the values of the interaction parameters which would be specific to multiphonon sca ttering in the system studied in particular. Here we comment on the li mitations of such an approximate treatment and show that an alternativ e approach, based on the calculation of the multiphonon scattering spe ctra in the exponentiated Born approximation, which preserves optical theorem, gives better agreement between experiment and theory upon usi ng interaction parameters consistent with the one-phonon scattering sp ectra. This implies that, regarding the interaction parameters, there exists a smooth transition between the one- and multiphonon scattering regimes.