Coverage-dependent hydrogen adsorption site determination onRh(100)by high-resolution core-level spectroscopy

Abstract

We show that high-resolution real-time x-ray photoelectron spectroscopy can be used to determine hydrogen adsorption sites as a function of coverage on $\mathrm{Rh}(100)$. The measurement of the surface core-level shifts does not suffer from the lack of direct sensitivity of other surface probes due to the low scattering cross section and high mobility of atomic hydrogen. At low temperatures $(70--140\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ and coverage (below $0.25\phantom{\rule{0.3em}{0ex}}\mathrm{ML}$), we find that hydrogen adsorbs in fourfold hollow sites on $\mathrm{Rh}(100)$, while at higher coverage the bridge site is preferred. Using Monte Carlo simulations, we unequivocally associate each surface component of the $\mathrm{Rh}\phantom{\rule{0.3em}{0ex}}3{d}_{5∕2}$ core level with a specific adsorption configuration. We obtain a value of $0.74\ifmmode\pm\else\textpm\fi{}0.08$ for the hydrogen initial sticking coefficient, in very good agreement with previous reports.