Femtosecond dynamics of image potential states of C6F6/Cu(111) studied with two-photon photoemission

The energetics and dynamics of electronic states of C6F6/Cu(111) are investigated with time-resolved two-photon photoemission spectroscopy. An unoccupied σ∗ molecular resonance is observed from submonolayer up to multilayer coverages. For one monolayer (ML) of C6F6, image potential states are observed with binding energies of 0.37 and 0.18 eV below the vacuum level, which decrease slightly above 2 ML. The lifetime of the lower-lying image state is only 12±4 fs for 1 ML and 5±4 fs for 2 ML, and thus much shorter than those in other adsorbate–metal systems. This is attributed to an efficient decay from the image state into the σ∗ molecular resonance of the C6F6 layer.