Channeling of low-energy ions on hydrogen-covered single-crystal surfaces

Abstract

Low-energy ion beam techniques such as direct recoil spectroscopy (DRS) are well-suited for directly detecting adsorbed hydrogen. However, with this approach it has previously only been possible to detect the hydrogen configuration on single-crystal surfaces for a narrow range of conditions (e.g., when the adsorbate atoms reside close to the surface). In this study we investigate the experimental and modeling tools needed to extend DRS to a much wider range of adsorption geometries. At grazing incidence, we show how adsorbed hydrogen affects ion focusing along open surface channels, thereby revealing fine details about the binding geometry even for adsorbates residing high above the substrate. Under such conditions, the scattering process is characterized by ions interacting with many atoms simultaneously. Therefore interpreting these types of measurements properly requires realistic molecular dynamics (MD) simulations with accurate scattering potentials at large distances. As an illustration of the progress that can be made, we consider how hydrogen is recoiled from W(100)$+$H(ads) during exposure to a low-energy (1 keV) Ne${}^{+}$ analysis beam at grazing incidence. The closest approach distance of the ions is $>$1 $\AA{}$, making hydrogen located high above the substrate readily accessible. By mapping the hydrogen recoils over different crystallographic directions using DRS, we identify focusing mechanisms that provide information on the adsorbed hydrogen configuration. When W(100) is continuously dosed with H${}_{2}$(g) to maintain a saturation coverage, we observe enhanced recoil signals along the $\ensuremath{\langle}$100$\ensuremath{\rangle}$ and $\ensuremath{\langle}$110$\ensuremath{\rangle}$ azimuths consistent with hydrogen residing in bridge sites. The reproduction of the complex experimental scattering spectra by our MD modeling techniques offers a pathway to locate hydrogen well within its vibrational envelope.