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 e.ect 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 non-reacting adatoms. A concentration dependent rate
constant which accounts for the faster excitation of H versus D is proposed.
PACS numbers: 82.65.+r, 82.53.-k, 68.43.Mn, 78.90.+t