Trajectory straggling and nonlinear effects in the energy loss of surface-channeled ions

We have performed surface-channeling experiments with He+ ions in the energy range between 0.5 keV and 3.5 keV scattering off a flat Pd(110) surface. The energy-loss spectra are measured by a time-of-flight analysis at grazing incidence for different surface directions. The shape of the energy-loss spectra is found to depend strongly on the azimuthal direction. Along axial channeling directions, the energy spectra are broadened and multipeak structures are found. The analysis of these features allows insight into the inelastic interaction process with the surface electrons, which is found to depend strongly on the projectiles' trajectories. Deviations from a linear dependency of the mean energy loss on the primary energy are observed and partly explained by the introduced surface-channeling model including the analysis of detailed trajectory calculations. The significant broadening of the energy spectra is mainly attributed to trajectory straggling