The electronic excitation occurring on adsorbates at ultrafast timescales from optical lasers that initiatesurface chemical reactions is still an open question. Here, we report the ultrafast temporal evolution of x-rayabsorption spectroscopy (XAS) and x-ray emission spectroscopy (XES) of a simple well-known adsorbateprototype system, namely carbon (C) atoms adsorbed on a nickel [Ni(100)] surface, following intense laseroptical pumping at 400 nm. We observe ultrafast (similar to 100fs) changes in both XAS and XES showing clearsignatures of the formation of a hot electron-hole pair distribution on the adsorbate. This is followed byslower changes on a few picoseconds timescale, shown to be consistent with thermalization of the completeC=Ni system. Density functional theory spectrum simulations support this interpretation
