A critical examination of data on the dissociative adsorption and associative desorption of hydrogen at copper surfaces

Abstract

We examine experimental data on the activated dissociative adsorption and associative recombination of hydrogen at copper surfaces with respect to the role played by molecular vibrational states. We use models describing the variation of the adsorption probability with the vibrational state, kinetic energy, and angle of the incident molecules, establishing the parameters of the models by a nonlinear least squares fit to adsorption data. Using the principle of detailed balance, we apply these models to the corresponding data on associative desorption thus comparing adsorption data with desorption angular, velocity, and internal state distributions. The most consistent picture resulting from this analysis is that the adsorption has significant contributions from both H2(v=0) and H2(v=1) and that these components have markedly different translational thresholds. Within the framework of this picture we are able to resolve the apparent contradiction between the strong angular dependence of the kinetic energy required for adsorption and the lack of angular dependence of the mean kinetic energy of desorption. We also partially resolve the apparent discrepancy in interpretation of the role of H2(v=1) in recent adsorption experiments.