An experimental and theoretical study of the transition from single phonon to multiphonon atom-surface scattering

The angular distributions and time-of-flight spectra of a monoenergetic 100 meV He atom beam have been measured after scattering from a clean Cu(001) surface at temperatures between 100 K and 1000 K. With increasing temperatures sharp diffraction and phonon features of the quantum regime become broad as expected for the multiphonon classical regime. Good agreement with theory is achieved over the entire range of temperatures by assuming that the atoms are reflected by a smooth vibrating potential barrier rather than by a lattice of repulsive atom cores as previously frequently postulated.