  The photo-induced desorption and oxidation of CO on Ru(0001) is simulated
using ab initio molecular dynamics with electronic friction that accounts for
the non-equilibrated excited electrons and phonons. Different (O,CO) coverages
are considered, the experimental room temperature coverage consisting in
0.5ML-O+0.25ML- CO (low coverage), the saturation coverage achieved
experimentally at low temperatures (0.5ML-O+0.375ML-CO, intermediate coverage),
and the equally mixed monolayer 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 surfaces.
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 conditions created by the laser.
