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Free keywords:
Hydrogen; desorption; water; structure; Ru(001)
Abstract:
The goal of this work is to acquire detailed insight into the fundamental interaction mechanisms in surface chemistry and physics. Therefore the dynamic and static-structural properties of two prototypical adsorbate systems have been investigated: The ultrafast reaction dynamics of hydrogen on the Ru(001)-surface, and the interfacial structure of water on Ru(001). The investigation of the recombinative hydrogen formation according to Had+Had -> H2,gas was performed by irradiation of the hydrogen/Ru(001)-adsorbate-system with intense, ultrashort laser pulses (130 fs, 800 nm). This allows for the analysis of energy transfer processes between the three qualitatively different degrees of freedom of the system (adsorbate nuclear motion, electronic excitations and lattice vibrations of the surface). Compared to the thermally initiated process, the fs-laser induced reaction is unequivocally based on an electron-mediated energy transfer mechanism. This illustrates that the hydrogen desorption does not proceed in the conventional picture of being driving by collisions with surface atoms on the electronic ground state potential energy surface. The experimental data can be described very well in the framework of the electronic friction model, which yields the characteristic coupling times between the substrate electrons and the H- and D-Adsorbate to be telH=180 fs and telD=360 fs, respectively (with an activation energy of Ea=1.35 eV). Furthermore, a huge isotope effect in the reaction yield (characteristic for an electron-mediated energy transfer mechanism) of ~10:1 has been found between the H2- and D2-formation. Therefore, the fs-laser excitation is isotope selective and for isotopically mixed hydrogen coverages local insight into the reaction mechanism could be gained: The hydrogen formation exhibits an activation energy E_a which depends on the transient local surrounding U(t), and therefore the rate constant is given by k(Ea(U(t))). From a microscopic point of view the reaction mechanism is collective: Atoms not directly involved in the reaction alter the height of the reaction barrier. To resolve the interfacial structure of water (D2O) on Ru(001), ice films of a few molecular layers have been investigated by vibrational spectroscopy using the sum-frequency generation technique. The analysis of the coverage-dependent SFG-data of D2O/Ru(001) in the frequency range of the hydrogen-bonds (~2000-2800 cm-1) yields a structure for the first layer of water on the Ru(001) surface, which contradicts the originally accepted structure derived from bulk ice and also refutes the partly-dissociated structure recently derived from theory: When all available data are taken into account, an "H-down"-structure is most plausible, where water molecules form DOD-Ru-bonds (additional to the hydrogen bonds within the bilayer). Furthermore, experiments on slightly thicker ice films give insight into other properties of water molecules in the vicinity of interfaces.
