In-out asymmetry of surface excitations in reflection-electron-energy-loss spectra of polycrystalline Al

We present experimental evidence for differences in surface energy losses between (a) electrons entering a solid from vacuum and (b) electrons leaving the solid into vacuum. Although these so-called in-out asymmetries have been long assumed to exist on theoretical grounds, the present work constitutes a clear experimental observation of the phenomenon. The effect has been exposed by comparing reflection-electron-energy-loss spectra of polycrystalline Al for pairs of conjugate scattering geometries where the directions of the source and the detector were interchanged. Differences of up to 30% in the peak height of surface energy-loss features are observed. The experimentally observed in-out asymmetry has been examined within the semiclassical dielectric formalism using state-of-the-art models for surface scattering of charged projectiles. The theoretical analysis suggests that in-out asymmetry effects are most accentuated for surface-crossing directions close to the surface normal and for high kinetic energies, in good agreement with the observed behavior. The effect is assumed to be present not only for electrons, but in principle for any charged particle.