Photo-Induced CO Desorption Dominates over Oxidation on Different O+CO Covered Ru(0001) Surfaces

The photo-induced desorption and oxidation of CO on Ru(0001) is simulated us- ing ab initio molecular dynamics with electronic friction that accounts for the non- equilibrated excited electrons and phonons. Different (O,CO) coverages are consid- ered, the experimental room temperature coverage consisting in 0.5ML-O+0.25ML- CO (low coverage), the saturation coverage achieved experimentally at low tempera- tures (0.5ML-O+0.375ML-CO, intermediate coverage), and the equally mixed mono- layer that is stable according to our calculations but not experimentally observed yet (0.5ML-O+0.5ML-CO, high coverage). The results of our simulations for the three coverages are consistent with femtosecond laser experiments showing that the CO photo-desorption largely dominates over CO photo-oxidation. These results cannot be explained in terms of the distinct activation energies calculated for the relaxed sur- faces. Different (dynamical) factors such as the coupling to the laser-excited electrons and, more importantly, the interadsorbate energy exchange and the strong surface distortions induced in the more crowded surfaces are fundamental to understand the competition between these two processes under the extremely non-equilibrated condi- tions created by the laser.