Electronic excitation and dynamic promotion of a surface reaction.

The mechanism of recombinative desorption of hydrogen from a Ru(0001) surface induced by femtosecond-laser excitation has been investigated and compared to thermally initiated desorption. For the laser-driven process, it is shown that hot substrate electrons mediate the reaction within a few hundred femtoseconds resulting in a huge isotope effect between H2 and D2 in the desorption yield. In mixed saturation coverages, this ratio crucially depends on the proportions of H and D. Deviations from second order desorption kinetics demonstrate that the recombination is dynamically promoted by excitation of neighboring, but nonreacting adatoms. A concentration dependent rate constant which accounts for the faster excitation of H versus D is proposed.