  The electronic excitation occurring on adsorbates at ultrafast time scales
from optical lasers that initiate surface chemical reactions is still an open
question. Here, we report the ultrafast temporal evolution of X-ray absorption
spectroscopy (XAS) and X-ray emission spectroscopy (XES) of a simple well known
adsorbate prototype system, namely carbon (C) atoms adsorbed on a nickel
(Ni(100)) surface, following intense laser optical pumping at 400 nm. We
observe ultrafast (~100 fs) changes in both XAS and XES showing clear
signatures of the formation of a hot electron-hole pair distribution on the
adsorbate. This is followed by slower changes on a few ps time scale, shown to
be consistent with thermalization of the complete C/Ni system. Density
functional theory spectrum simulations support this interpretation.
