Coverage dependence of surface diffusion of hydrogen and deuterium on Rh{111} by laser induced thermal desorption

Abstract

The coverage dependence of the surface diffusion of hydrogen and deuterium on Rh{111} has been studied by laser-induced thermal desorption. Diffusion across a step-like boundary created by a rapid interlaced laser raster was monitored at a series of temperatures by an orthogonal laterally resolved laser-induced desorption scan. Relative surface coverages as a function of distance were estimated from the integrated time-of-flight desorption yield and the coverage dependence of the surface diffusion coefficient calculated via a Boltzmann-Matano analysis. The activation energy for deuterium diffusion was found to be 13.8 ± 2 kJ mol -1, independent of coverage within the accuracy of the analysis in the range 0.3 ⩽ θ D ⩽ 0.9 ML. Corresponding pre-exponentials varied from 5.7 × 10 -4 cm 2 s -1 at θ D = 0.4 ML to 7.1 × 10 -4 cm 2 s -1 at θ D = 0.8 ML. Hydrogen exhibited similar values with activation energies and pre-exponentials from E d = 11 ± 2 kJ mol -1, D 0 = 5.9 × 10 -4 cm 2 s -1 at θ H = 0.8 ML to E d = 13.4 ± 2 kJ mol -1, D 0 = 6.5 × 10 -3 cm 2 s -1 at θH = 0.3 ML. The “hole-refilling” method was also used to study diffusion, and similar values were obtained for E d and D 0. The weak lateral adatom-adatom interactions apparent in the diffusion data are suggestive of the coverage dependent population of two distinct binding sites to explain the pronounced shift to lower temperature of peak maxima in thermal desorption spectroscopy.