Polar inelastic profiles in fast-atom diffraction at surfaces

Abstract

Elastic diffraction of fast atoms at crystal surfaces under grazing incidence $\ensuremath{\theta}\ensuremath{\approx}1$\ifmmode^\circ\else\textdegree\fi{} has strong similarities with atomic diffraction at thermal energies discovered almost hundred years ago. Here, we focus on the polar scattering profile, which does not exhibit diffraction features but shows well-defined elastic and inelastic components that are found to be essentially independent of the crystallographic axis. The width ${\ensuremath{\sigma}}_{\ensuremath{\theta}}$ of the inelastic component is very sensitive to the weak attractive forces responsible for the physisorption. This effect is visible on an energy range almost ten times larger than the depth $D$ of the physisorption well. Experimental data are analyzed using a binary collision model with a Morse potential where the width ${\ensuremath{\sigma}}_{\ensuremath{\theta}}$ of the scattering profile is connected to the classical energy loss and is governed by the surface stiffness, defined as the logarithmic derivative of the interaction potential along the surface normal. The main outcome is that the weak attractive forces make the mean surface potential almost twice harder at low energy.