  In time-resolved pump-probe vibrational spectroscopy the internal stretch
mode of polar molecules is utilized as a key observable to characterize the
ultrafast dynamics of adsorbates on surfaces. The adsorbates non-adiabatic
intermode couplings are the commonly accepted mechanisms behind the observed
transient frequency shifts. Here, we study the CO/Pd(111) system with a robust
theoretical framework that includes electron-hole pair excitations and
electron-mediated coupling between the vibrational modes. A mechanism is
revealed that screens the electron-phonon interaction and originates a
blueshift under ultrafast non-equilibrium conditions. The results are explained
in terms of the abrupt change in the density of states around the Fermi level,
and are instrumental for understanding dynamics at multi-component surfaces
involving localized and standard $s$ or $p$ states.
