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Abstract:
Absorption of femtosecond near-infrared laser pulses in the surface near region of Ru(001) covered with atomic hydrogen induces recombinative desorption of its molecular species. The ultrafast time scale of this surface reaction as evidenced by two-pulse correlation measurements (fwhm of ~1 ps) together with a nonlinear dependence of the reaction yield on the absorbed laser fluence reveals an electron-mediated reaction pathway involving nonadiabatic coupling between adsorbate vibrational degrees of freedom and transient electron-hole pair excitations in the substrate. A pronounced isotope effect in the H2 vs D2 yield exhibits a strong fluence dependence (H2/D2 ratio changes from 5:1 at 120 J/m2 to ~20:1 at 50 J/m2). All experimental findings can be well described within the framework of electronic friction between the substrate and adsorbate degrees of freedom, with an effective activation energy of 1.35 eV and coupling times of 180 and 360 fs for H2 and D2, respectively, as parameters. A pronounced coverage dependence of the desorption yield underlines that adsorbate-adsorbate interactions play a crucial role in the hydrogen recombination reaction.
