Ultrafast reaction dynamics and vibrational spectroscopy at surfaces

A mechanistic understanding of the surface femtochemistry leading to the formation of hydrogen and the desorption and oxidation of CO from a Ru(001) single crystal surface is obtained by application of a variety of experimental methods. Among these are measurements of the translational energy distributions, two-pulse-correlations, isotope effects, and vibrational spectra of the reaction products after excitation with near-infrared (800 nm) femtosecond laser pulses. It is demonstrated that both the formation of hydrogen and the oxidation of CO are initiated by an electron-mediated excitation mechanism leading to novel reaction pathways only accessible via excitation with femtosecond laser pulses. The observed characteristics are an ultrafast response of the two-pulse-correlation with a full width at half maximum of 3 ps and 1 ps, pronounced isotope effects of 2.2 and 9, and high translational temperatures of the reaction products of 1600 K and 2300 K for the CO oxidation and hydrogen formation, respectively. The nonlinear optical technique of broadband-IR sum-frequency generation (SFG) spectroscopy is found to be a highly sensitive and surface-specific method. The C-O stretch vibration of CO/Ru(001) can therefore be studied at CO coverages even below 0.001 ML. The high intensity and the use of broadband-IR pulses lead to a strong excitation (saturation) of the fundamental transition and make it possible to simultaneously observe the fundamental and subsequent hot-band transitions at conditions where lateral interactions are negligible. From the anharmonicity the dissociation energy of the C-O bond is determined to be 9.1±0.1 eV. The v=1->2 hot-band transition serves as a sensitive indicator for vibrational energy localization. With increasing coverage the lateral interactions (dipole-dipole coupling) between the adsorbed molecules, and therefore the delocalization of vibrational energy, increases and leads to the disappearance of the hot band at a coverage of 0.025 ML at 90 K. Thus, the transition from local oscillators to delocalized phonons is observed directly by changing the CO coverage. This behavior can be described by a modified exchange model with residence times of the hot-band excitation on a single oscillator down to 2.5 ps. The technique of broadband-IR SFG spectroscopy has also been applied to study the dynamics of the C-O stretch vibration under conditions of laser-induced desorption of CO. A large transient redshift and a broadening of the resonance is observed after excitation with 110 fs laser pulses at 800 nm. This originates from anharmonic coupling of the C-O stretch vibration to the frustrated translation and rotation that are highly excited under these excitation conditions