Symmetry-resolved CO desorption and oxidation dynamics on O/Ru(0001) probed at the C K-edge by ultrafast X-ray spectroscopy

J. LaRue, B. Liu, G.L.S. Rodriguez, C. Liu, J. A. Garrido Torres, S. Schreck, E. Diesen, M. Weston, H. Ogasawara, F. Perakis, M. Dell'Angela, F. Capotondi, D. Ball, C. Carnahan, G. Zeri, L. Giannessi, E. Pedersoli, D. Naumenko, P. Amann, I. Nikolov, L. Raimondi, C. Spezzani, M. Beye, J. Voss, H.-Y. Wang, F. Cavalca, J. Gladh, S. Koroidov, F. Abild-Pedersen, M. Kolb, P. S. Miedema, R. Costantini, T. Heinz, A. Luntz, L.G.M. Pettersson, A. Nilsson
Year of publication: 
J. Chem. Phys.

We report on carbon monoxide desorption and oxidation induced by 400 nm femtosecond laser excitation on the O/Ru(0001) surface probed by time-resolved X-ray absorption spectroscopy (TR-XAS) at the carbon K-edge. The experiments were performed under constant background pressures of CO (6x10-8 Torr) and O2 (3x10-8 Torr). Under these conditions, we detect two transient CO species with narrow 2π* peaks, suggesting little 2π* interaction with the surface. Based on polarization measurements, we find that these two species have opposing orientations: (1) CO favoring a more perpendicular orientation and (2) CO favoring a more parallel orientation with respect to the surface. We also directly detect gas-phase CO2 using a mass spectrometer and observe weak signatures of bent adsorbed CO2 at slightly higher X-ray energies than the 2π* region. These results are compared to previously reported TR-XAS results at the O K-edge where the CO background pressure was three times lower (2x10-8 Torr) while maintaining the same O2 pressure. At the lower CO pressure, in the CO 2π* region, we observed adsorbed CO and a distribution of OC-O bond lengths close to the CO oxidation transition state, with little indication of gas-like CO. The shift towards 'gas-like' CO species may be explained by the higher CO exposure, which blocks O adsorption, decreasing O coverage and increasing CO coverage. These effects decrease the CO desorption barrier through dipole-dipole interaction, while simultaneously increasing the CO oxidation barrier.

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