For a microscopic understanding of chemical reactions at surfaces, it is essential to obtain detailed knowledge on the underlying elementary processes. The reaction mechanism, the pathways and time scales of energy flow and the energy partitioning between different degrees of freedom of the reaction products are of key interest. Reactions of species adsorbed on a metal surface are generally mediated through electron and/or phonon excitations of the substrate. Since thermal equilibration between these excitations occurs on a femto- to picosecond time scale, chemical reactions initiated by ultrashort-laser pulses provide the base to investigate processes beyond equilibrium conditions. The recombination of two hydrogen atoms forming an H2 molecule, which leaves the surface, represents one of the most basic surface reactions one could think of and thus may serve as a prototype system for femtosecond- laser induced surface chemistry. In particular, the Hads + Hads -> H2;gas associative desorption from a Ru(001) surface has been studied in great detail. Ultrafast energy transfer times of less than 200 fs in conjunction with a pronounced isotope effect between H2 and D2 unambiguously indicate a hot substrate electron-driven reaction mechanism. Measurements of the energy partitioning between external (translational) and internal (vibrational, rotational) degrees of freedom of the product molecule reveal predominantly translational excitation of the desorbing hydrogen. Theoretical modelling based on a multidimensional frictional description of energy transfer between the ruthenium substrate and the hydrogen layer excellently reproduces the experimental findings. Furthermore, peculiar characteristics like a threshold-like coverage dependence of the desorption yield and promotion e®ects in isotopically substituted adlayers have been observed in the experiment which demonstrate the importance of strong adsorbate-adsorbate interactions in the H2 / D2 association, yet still awaiting a quantitative theoretical treatment.
The original publication is available at www.springerlink.com by link DOI: 10.1088/0953-8984/20/31/313002