Analysis of H-2 dissociation dynamics on the Pd(111) surface

We perform a detailed analysis of the dynamics of the dissociative adsorpti on of H-2 molecules on a Pd(111) surface using ab initio data for the molec ule-surface interaction and classical trajectory methods. We show that the reaction probability is completely determined by the molecule-surface inter action in the approach toward the surface before it reaches a critical dist ance of 1.5 Angstrom. The corresponding dynamics can be reduced to a 2D one , involving only the translational and rotational degrees of freedom, excep t in the lower energy range where an important role is played by dynamic tr apping. We establish the relation between the dissociation probability and the shape of 2D cuts of the potential energy surface using a simple model o f the evolution of orientational forces as the molecule approaches the surf ace. Whereas above 1.5 Angstrom the molecule evolves "as a whole," below 1. 5 Angstrom the dynamics has the character of independent atom-surface inter actions which explains why it dissociates with a probability close to one o nce it has reached the critical distance of 1.5 Angstrom. (C) 2001 American Institute of Physics.