Multiphonon He atom scattering from Xe overlayers on Cu(111) and Cu(001) surfaces

We have studied the scattering of lie atoms from ordered monolayers of Xe atoms adsorbed on Cu(111) and Cu(001) surfaces. Both Xe overlayers, the commensurate (root 3 x root 3)R30 degrees on Cu(lll) at substrate temperature above 47 K and the incommensurate one on Cu(001), exhibit weak diffraction. On both surfaces the Xe overlayers sustain Einstein-like, vertically polarized Xe vibrations which can be multiply excited and annihilated even at low He atom incident energies and substrate temperatures. In spite of the anharmonic Xe-substrate potentials, the energies of multiphonon excitations are not found to exhibit any noticeable anharmonic shifts in either system. The results of the measurements are compared to theoretical energy and lateral momentum resolved scattering distributions, which were calculated by using the recently developed formalism for treating multiphonon scattering in the collision regimes, in which both the motion of the scattered particle and surface vibrations must be treated quantum mechanically. We can interpret the multiphonon scattering spectra and obtain good agreement with experiments by assuming linear He atom-phonon coupling, delocalized phonons in Xe adlayers, and by employing the one-phonon interaction parameters determined from the He-Xe interaction potentials. Effects of the quantum recoil on the motion of the scattered He atom, which give rise to a difference between phonon emission and absorption probabilities, and thereby non-Poissonian scattering distributions, were assessed, (C) 1997 American Institute of Physics.