Atom-Specific Probing of Electron Dynamics in an Atomic Adsorbate by Time-Resolved X-Ray Spectroscopy

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