Coverage dependent dissociative adsorption of HCl on Au(111)

Dissociative adsorption of HCl on Au(111) has become one of unsolved puzzles in surface chemistry. Despite tremendous efforts in the past years, various theoretical models still greatly overestimate the zero-coverage initial sticking probabilities (S₀). To find the origin of the large experiment-theory discrepancy, we have revisited the dissociative adsorption of HCl on Au(111) with a newly designed molecular beam-surface apparatus. The zero-coverage S₀ derived from Cl-coverage measurements with varying HCl doses agree well with previous ones. However, we notice a sharp change of the coverage/dose slope with the HCl dosage at the low coverage regime, which may result in some uncertainties to the fitted S₀ value. This seems consistent with a coverage-dependence of the dissociation barrier predicted by density functional theory at low Cl-coverages. Our results reveal the potential inconsistency of utilizing simulations with finite coverage to compare; against experimental data with zero coverage in this system, and provide guidance for improving both experiment and theory in this regard.