Energy dissipation of fast neutral argon and helium scattered at glancing incidence from Al(111)

Abstract

We have scattered highly collimated, monoenergetic neutral beams of 2 keV Ar and 5 keV He from an Al(111) surface under a glancing angle of incidence of 2°. Mean energy losses well in excess of those calculated for direct energy transfer to the translational degrees of freedom of the substrate atoms (0.37 eV for Ar, 0.11 eV for He) are observed. We see no evidence for discrete loss peaks which could be attributed to electronic excitation of the fast atoms or collective electronic excitations of the substrate. We conclude that the primary loss mechanism is electron-hole pair excitation. The observed loss spectra are well described by a non-linear model of the electron-hole pair excitation process which leads to a Poisson-like distribution envelope, with on average 6 and 22 e-h pairs being resonantly excited during Ar and He scattering, respectively. The high efficiency of this process is a direct consequence of the nonadiabatic coupling of the ‘slow’ component of the substrate electronic response to the fluctuating charge or induced dipole on the adatom moving in the near surface region.