MULTIPHONON HE ATOM SCATTERING IN HE-]XE CU(111) COLLISIONS/

We have investigated the collision between He-atoms and the xe(root 3x root 3)R30 degrees overlayer on Cu(111) under the conditions in which multiple phonons of the Einstein-like, vertically polarized Xe-vibrat ion are excited and annihilated. The measurements are compared to theo retical results which were obtained using a recently developed formali sm for calculating the multiphonon scattering spectra in the collision regimes in which both the motion of the scattered particle and surfac e vibrations must be treated quantum-mechanically. The energy and late ral momentum resolved scattering spectra are expressed in the form of a cumulant expansion in terms of uncorrelated and correlated phonon ex citation amplitudes and the correlated ones can be neglected under the scattering conditions typical of He atom scattering. By introducing t he one phonon interaction parameters into the obtained expressions we can calculate the multiphonon scattering spectra which are in a good a greement with the experimental data.