CH4 dissociation on Ru(0001):: A view from both sides of the barrier

This paper reports measurements of both dissociative adsorption on and associative desorption from CH4 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 dissociative adsorption 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. (C) 2002 American Institute of Physics.