FEMTOSECOND DYNAMICS OF ADSORBATE CHARGE-TRANSFER PROCESSES AS PROBEDBY HIGH-RESOLUTION CORE-LEVEL SPECTROSCOPY

When adsorbate core-hole states are excited with photon bandwidths bel ow their natural linewidths, the corresponding decay spectra exhibit a branching into two channels: a coherent resonant ''Raman''-type chann el and an incoherent ''normal'' Auger-type channel. Using adsorbed Ar atoms in situations of varied adsorbate-substrate coupling, we demonst rate 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 time s on the energetics of the resonance relative to E-F and discuss possi ble reasons for these variations. [S0163-1829(98)09319-9].