Resonances and critical kinematic effects

Abstract

The resonant interaction of the He atom with the bound states of the atom-surface potential can have a great influence on the elastic and inelastic intensities. Examples from experiment are used to illustrate how bound state resonances can be used to detect weakly coupled surface phonons by means of resonance enhancement. In addition there are a variety of focusing and surfing effects, arising from a coupling between the motion of the atomic projectile and that of the surface phonons. Several different focusing phenomena called critical kinematic effects are characterized by various tangency conditions among scan, resonance and phonon dispersion curves. Kinematical focusing, in particular, allows to determine envelopes of the surface phonon dispersion curves without resorting to time-of-flight measurements. Focused resonances provide a possible mechanism for an atom beam monochromator, while the surfing effect, allowing trapped atoms to ride a Rayleigh wave and leading to very sharp resonances, corresponds to a type of atomic polaron.