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

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 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 K? and coverage (below 0.25 ML), we find that hydrogen adsorbs in fourfold hollow sites on Rh?100?, while at higher coverage the bridge site is preferred. Using Monte Carlo simulations, we unequivocally associate each surface component of the Rh 3d5/2 core level with a specific adsorption configuration. We obtain a value of 0.74±0.08 for the hydrogen initial sticking coefficient, in very good agreement with previous reports.