CH 4 dissociation on Ru(0001): A view from both sides of the barrier

Abstract

This paper reports measurements of both dissociativeadsorption on and associative desorption from CH 4 on Ru(0001). We consider the former a view of dissociation from the front side of the barrier, while the latter is considered as a view of dissociation from the back side of the barrier. A combination of both previous and new molecular beam measurements of dissociativeadsorption shows that S 0 depends on all experimental variables (E, T n , T s and isotope) in a manner similar to other close-packed transition metals. The interpretation of this behavior in terms of a theoretical description of the dissociation is discussed critically, with special emphasis on insights from new theoretical studies. The energy-resolved desorption flux D f (E,T s ) is obtained in associative desorption experiments using the technique of laser assisted associative desorption (LAAD). Measurements at several T s allow both a direct determination of the adiabatic barrier V * (0) and considerable insight into the dynamics of dissociation. The V * (0) obtained from D f (E,T s ) is in excellent agreement with density functional theory(DFT) calculations and with the value indirectly inferred from molecular beam experiments. The chief dynamic conclusion from an analysis of D f (E,T s ) is that both bending and stretching coordinates must be produced in associative desorption, although they are not populated statistically. The absence of an isotope effect in the shape of D f (E,T s ) argues against the importance of tunneling in the desorption/adsorption. When reactive fluxes are compared via detailed balance, both the molecular beam experiment and the LAAD experiment are in good agreement.