Vibrationally excited hydrogen molecules formation on a cesiated surface

Hydrogen atoms recombination via the Eley-Rideal mechanism on a cesiated surface was studied using a semiclassical collisional model. The Potential Energy Surface governing the reaction was evaluated by ab initio calculations in the framework of Density Functional Theory. The recent results obtained for H interaction potential with the same surface model were used together the first principle results obtained for H2 interaction potential with Cs and Mo surface atoms. Molecular Dynamics calculations have been performed for two different typical adsorption sites identified on the surface. The probabilities for recombination reaction and for the complete set of elementary surface processes arising from the assumed initial condition were evaluated. It appears that the recombination occurs only for one adsorption site on the surface and with low probability, while for the other site H atoms are mainly scattered from the surface with a partial negative charge. The vibrational distributions of formed H2 molecules exhibit a non-Boltzmann behaviour with peaks on medium-high lying vibrational levels. The considered surface appears promising for negative ion sources, contributing to the formation of ions by means of both usually considered mechanisms, surface production and volume production via dissociative attachment. The global and state-to-state recombination coefficients were also calculated to be used in kinetic modelling of negative ion sources.