  We use a pump-probe scheme to measure the time evolution of the C K-edge
X-ray absorption spectrum (XAS) from CO/Ru(0001) after excitation by an
ultrashort high-intensity optical laser pulse. Due to the short duration of the
X-ray probe pulse and precise control of the pulse delay, the
excitation-induced dynamics during the first ps after the pump can be resolved
with unprecedented time resolution. By comparing with theoretical (DFT)
spectrum calculations we find high excitation of the internal stretch and
frustrated rotation modes occurring within 200 fs of laser excitation, as well
as thermalization of the system in the ps regime. The ~100 fs initial
excitation of these CO vibrational modes is not readily rationalized by
traditional theories of nonadiabatic coupling of adsorbates to metal surfaces,
e. g. electronic frictions based on first order electron-phonon coupling or
transient population of adsorbate resonances. We suggest that coupling of the
adsorbate to non-thermalized electron-hole pairs is responsible for the
ultrafast initial excitation of the modes.
