ENERGY-DISSIPATION OF FAST NEUTRAL ARGON AND HELIUM SCATTERED AT GLANCING INCIDENCE FROM AL(111)

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 degrees. Mean energy losses well in excess of those cal culated for direct energy transfer to the translational degrees of fre edom of the substrate atoms (0.37 eV for Ar, 0.11 eV for He) are obser ved. We see no evidence for discrete loss peaks which could be attribu ted to electronic excitation of the fast atoms or collective electroni c excitations of the substrate. We conclude that the primary loss mech anism is electron-hole pair excitation. The observed loss spectra are well described by a non-linear model of the electron-hole pair excitat ion process which leads to a Poisson-like distribution envelope, with on average 6 and 22 e-h pairs being resonantly excited during Ar and H e scattering, respectively. The high efficiency of this process is a d irect consequence of the nonadiabatic coupling of the 'slow' component of the substrate electronic response to the fluctuating charge or ind uced dipole on the adatom moving in the near surface region.