J.E. Valdés

Surface channelling and energy losses of 4 keV hydrogen and fluorine ions in grazing scattering on Au(111) and missing row reconstructed Au(110) surfaces

We present the results of an experimental and theoretical study of surface channelling and energy loss of 4 keV hydrogen and fluorine ions in grazing scattering on a missing row reconstructed Au(110) surface and a Au(111) surface. We performed measurements of energy losses for grazing angle scattering in surface channelling conditions for various azimuthal orientations of the crystal. Experimental results are discussed in the light of trajectory calculations of hydrogen and fluorine ions scattered under grazing incidence conditions on the surface. A nonlinear model is used in order to calculate the ion energy loss in these crystalline systems. Ab initio electronic crystal structure calculations and semi-classical simulations are performed and allow us to delineate various trajectory classes that correspond to different contributions in the energy loss spectra for various azimuthal orientations of the surface.

Threshold effect in the energy loss of hydrogen and helium ions transmitted in channeling conditions in gold single crystal

The energy loss of hydrogen and helium ions in the low energy range (<10keV/u) in single crystal gold targets is investigated experimentally. We found that the stopping power for helium ions shows a deviation from the proportionality with ion velocity predicted theoretically. This behavior has been also observed for protons in previous experiments. Additionally, we found that for a small range of velocity the energy loss for helium ions has the same magnitude as for protons for the same velocity. This last finding is in agreement with early theoretical prediction for the ion energy loss interacting with low electron densities, beyond of metallic densities, in the frame of the free electron gas model and the density functional theory. Both effects are explained due to two particular phenomena: particle channeling in monocrystalline targets, where the inhomogeneity of the spatial electron density distribution plays a fundamental role and the well-known threshold effect in the stopping power which is explained considering the electronic band structure properties of metallic targets.

Energy losses of slow ions traveling through crystalline solids and scattered on crystalline surfaces

ABSTRACT This progress report focuses on recent work on energy losses undergone by keV slow ions through crystals and on solid surfaces. We treat cases where strong corrugations of the electron density along ion trajectories occur and where an averaged electron density along the ion track cannot be assumed. We present experiments which reveal the existence of a threshold effect in stopping on d-electron metals and discuss experiments on scattering on surfaces where ion trajectories pass through regions with exponentially decreasing electron densities. The modeling realistically describes the inhomogeneous electron density in and above a crystal, especially in directions with high symmetry. We strive to highlight the most important features in these cases.

Proton energy loss in multilayer graphene and carbon nanotubes

ABSTRACT Results of a study of electronic energy loss of low keV protons interacting with multilayer graphene targets are presented. Proton energy loss shows an unexpectedly high value as compared with measurements in amorphous carbon and carbon nanotubes. Furthermore, we observe a classical linear behavior of the energy loss with the ion velocity but with an apparent velocity threshold around 0.1 a.u., which is not observed in other carbon allotropes. This suggests low dimensionality effects which can be due to the extraordinary graphene properties.

Simulation of the energy loss of proton beams interacting with few layer graphene foils

Synopsis We have simulated the passage of proton beams, having energies E < 10 keV, through graphene targets, considering the case of few layer foils. For this purpose and for comparison, we implement Monte Carlo and deterministic semi-classical approaches to describe the interaction of protons with carbon atoms in graphene structures. Our results show that the energy loss through a single layer graphene foil is smaller than recent ab initio calculations based on time dependent density functional theory (TD-DFT). Besides, for E > 6 keV the energy loss into a graphene target composed of n layers is n times the energy loss through a single layer graphene foil.

Energy losses of H and F ions in grazing scattering on a missing row reconstructed Au(110) surface

A joint experimental and theoretical study of low-keV H and F ions in grazing scattering on a missing row reconstructed Au(110) surface is presented. We show the influence of surface electronic corrugation and trajectory effects on energy-loss spectra. Measurements of energy losses for grazing angles scattering in surface channeling conditions for various azimuthal orientations of the crystal have been performed and discussed in semi-classical simulations, which allow us to delineate various trajectory classes that correspond to different contributions in the energy-loss spectra for various azimuthal orientations of the surface.