Femtosecond dynamics of adsorbate charge-transfer processes as probed by high-resolution core-level spectroscopy

When adsorbate core-hole states are excited with photon bandwidths below their natural linewidths, the corresponding decay spectra exhibit a branching into two channels: a coherent resonant ``Raman''-type channel and an incoherent ``normal'' Auger-type channel. Using adsorbed Ar atoms in situations of varied adsorbate-substrate coupling, we demonstrate charge-transfer times from as low as 1.5 fs up to above 70 fs for the Ar $4s$ resonance. We also show different dependences of these times on the energetics of the resonance relative to EF and discuss possible reasons for these variations.