The coverage dependence of the sticking probability of Ar on Ru(001)

The coverage dependence of the sticking probability of argon on a cold Ru(001) surface is studied experimentally by thermal-desorption spectrometry, and simulated by classical molecular dynamics with an empirical pairwise-additive potential-energy function. Experimentally, a dramatic linear increase in sticking as a function of Ar coverage is observed between 0 and 0.7 monolayer for a 300 K thermal beam of Ar incident normal to the surface; at higher coverages the sticking probability gradually saturates to approach unity beyond 2 monolayers. The linear regime is reproduced with near-quantitative accuracy by the simulations, with a simple perfect islanding model. The origin of the enhanced sticking lies primarily in efficient energy transfer to the adsorbate, due to perfect mass matching and the deformability of the overlayer. In the simulations at incident energies above 50 kJ mol^-1 the sticking on the monolayer diminishes to almost zero, and collision-induced desorption is observed. Additionally, several interesting dynamical effects which have experimental relevance emerge from the simulations, such as prolonged impacts and sticking mediated by attractive interactions at the edge of adsorbate islands.