Observation of collective inner-shell effects for protons backscattered from the Al(110) surface

Abstract

The surface peak of 98-keV protons backscattered from a clean Al 110 surface has been studied in detail for three different scattering geometries. The corresponding energy-loss distributions are asymmetric due to the single and double ionization of the Al inner-shell electrons L shell. Effects beyond the independent-electron model inner-shell collective effects have been observed by using Monte Carlo simulations for the ion ballistic and coupled-channel calculations for the inelastic energy loss. These effects decrease the width by up to 10% for the full width at half maximum. Correspondingly, the low-energy tail of the surface peak is reduced in intensity by more than 30% in the case of a large number of near central collisions as in shadowing and blocking geometries. Many aspects of the energy-loss processes of ions in solids are well understood at high projectile energies, but important issues related to the energy loss in the polarization field are still unclear 1. In particular, for atoms having many electrons in inner shells, there is no clear evidence of the role of collective response and its effect of dressing the projectile interaction. Only for the weakly interacting fast light ions and for valence-band electrons is the polarization field successfully described by means of the dielectric function of the medium 2. Energy-loss distributions under single-collision conditions have been measured for gases 3‐7 as well as for solids 8,9. For the case of multiple ionization, deviations from the independent-electron model have already been identified 6,7, but an influence of dynamic inner-shell screening is presented here for the first time. An investigation of the full energy-loss distribution allows for a better understanding of inner-shell collective effects in ion-atom interactions and is a prerequisite for monolayer resolution in ion-beam techniques used for depth profiling such as nuclear reaction analysis NRA and medium-energy ion scattering MEIS10. Measurements in solids under shadowing and blocking conditions involving collisions with very small impact parameters are used in this work to study energy-loss processes involving inner-shell electrons. These conditions are realized in high-precision resolution and statistics measurements of the so-called surface peak, a highenergy structure that appears in backscattering experiments for crystalline materials. Here we report on measurements of the energy-loss distribution of the surface peak for protons impinging along the shadowing directions 0°, 30°, and 60° =36° and being backscattered along the blocking direction of 60° with respect to the normal of clean Al110. These special scattering geometries have some advantages compared to outgoing random directions. They provide the best scenario for the applicability of advanced atomic-physics models, such as coupled-channel calculations, since solid-state effects are of minor importance due to the large energy transfers involved. Moreover, collective effects due to inner shells are amplified in a sequence of near-central collisions since the energy transfer to inner-shell electrons is enhanced. In this paper, we apply improved experimental and theoretical methods in order to explicitly extract the influence of the dynamic polarization of inner shells as well as the influence of multi-inner-shell ionization on the energy transfer. Furthermore, we provide a Monte Carlo simulation of the surface peak which goes far beyond a stochastic treatment 11.