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Ultrafast reactions dynamics; surface femtochemistry; substrate mediated photochemistry; phonon-electron and phonon-phonon interactions; two-temperature-model; dip in surface temperature; vibrational heating; phonon-adsorbate and electron-adsorbate interactions; energy transfer time; fs-Laser-induced desorption and oxidation; dynamical cooling in translation; isotope effect; vibrations at surfaces; broadband-IR
Abstract:
The energy transfer between metal substrates and adsorbates occurs on an ultrafast timescale, mediated by substrate electrons and phonons. Its mechanism and timescale has been studied for the desorption and oxidation reaction of CO over the Ruthenium(001) single-crystal surface. Experiments include fluence-dependent and two-pulse correlation measurements of the reaction yield after femtosecond(fs)-laser excitation, time-of-flight spectroscopy, isotopic substitution and a new technique for fs-time-resolved vibrational spectroscopy by sum-frequency generation (SFG).
Analysis is based on the two-temperature model, providing the time evolution of the electron and phonon temperatures after substrate excitation by fs-laser pulses. The validity of the model is demonstrated by the prediction of a dip in the surface phonon temperature at temporal overlap in two-pulse correlation experiments, which has been observed experimentally. A friction-coupled heat bath model describes the subsequent vibrational heating of the adsorbate-substrate-complex that promotes the reaction.
The desorption of CO is determined to be driven by substrate phonons. Significant deviation of the reaction kinetics from equilibrium behaviour is found in the rate and the translational energy of the products; the findings are discussed in a dynamical picture.
By fs-laser excitation of CO/O/Ruthenium(001) with the oxidation of CO a novel reaction pathway is observed for this coadsorbate system. The activation of O is found to be electronically driven and rate-determining. The oxidation occurs prior to the competing desorption, allowing the oxidation in spite of a higher activation energy.
By vibrational spectroscopy, insight in the adsorbat motion can be gained. Therefore a new broadband-IR/narrowband-VIS SFG-probe scheme has been implemented, providing transient spectra of the intramolecular C-O-vibration under excitation conditions leading to desorption.
