Microscopic understanding of an ultrafast photochemical surface reaction: Hads + Hads → H2,gas on Ru(0 0 1)

As a prototypical surface reaction initiated by excitation with an ultrafast light pulse, the associative desorption of hydrogen from a Ru(0 0 1) surface has been studied. Femtosecond laser techniques reveal detailed information on the underlying reaction mechanism, the pathway and time scales of energy flow and the partitioning of excess energy between different degrees of freedom within the reaction product. A response time of the desorption yield of only few hundred femtoseconds as manifested in two-pulse correlation measurements and a pronounced isotope effect in the H2 versus D2 yield unambiguously indicate a hot substrate electron-driven reaction mechanism. All experimental results can be well reproduced within the framework of electronic friction between the ruthenium substrate and the hydrogen adlayer. In addition, it is found that adsorbate-adsorbate interactions crucially influence the reaction as seen in promotion effects within isotopically substituted surroundings of the reactants. Finally, energy partitioning into various degrees of freedom is investigated by time-of-flight measurements and state-resolved detection of desorbing hydrogen molecules via resonance enhanced multiphoton ionization. The results show a pronounced preference for translational energy versus intramolecular excitations like vibrations and rotations. These studies provide an ideal benchmark for theoretical calculations of potential energy surfaces and photoinduced reaction dynamics of this adsorbate-substrate system.