  CO oxidation on Ru(0001) is a long-standing example of a reaction that, being
thermally forbidden in ultra-high vacuum, can be activated by femtosecond laser
pulses. In spite of its relevance, the precise dynamics of the photo-induced
oxidation process as well as the reasons behind the dominant role of the
competing CO photo-desorption remain unclear. Here we use ab initio molecular
dynamics with electronic friction that account for the highly excited and
non-equilibrated system created by the laser to investigate both reactions. Our
simulations successfully reproduce the main experimental findings: the
existence of photo-induced oxidation and desorption, the large desorption to
oxidation branching ratio, and the changes in the O K-edge X-ray absorption
spectra attributed to the initial stage of the oxidation process. Now, we are
able to monitor in detail the ultrafast CO desorption and CO oxidation
occurring in the highly-excited system and to disentangle what causes the
unexpected inertness to the otherwise energetically favored oxidation.
