Trapping, molecular adsorption, and precursors for nonactivated chemisorption.

Many fundamental questions are still unanswered regarding the very existence of precursor states and the microscopic mechanism of its population in the case of dissociative adsorption of light molecules on metal surfaces. We report results of classical trajectory calculations for H(2)/Pd(110) based on a six-dimensional potential energy surface obtained from ab initio calculations and a generalized Langevin oscillator model to account for energy exchange with the surface and dissipation. A quantitative characterization of the dynamic process is obtained which elucidates the respective roles played by direct dissociation, dynamic trapping, and precursor mediated adsorption. We predict the existence of H2 molecular chemisorption and provide the precise adsorption conditions for its experimental observation.