Structure and dynamics of excited electronic states at the adsorbate/metal interface: C6F6/Cu(111)

Abstract

Excited state electron transfer at the adsorbate/metal interface represents a key step in molecular electronic devices. The dynamics of such processes are governed by ultrafast energy relaxation which can be probed directly by time-resolved two-photon photoemission (2PPE). Using 2PPE spectroscopy we investigate the energetics and lifetimes of the unoccupied electronic states of C₆F₆ adsorbed on Cu(111) as a model system for electron transfer at organic/metal interfaces. With increasing C₆F₆ layer thickness we find a pronounced decrease in the energetic position of the lowest unoccupied state, which is accompanied by a strong increase in its lifetime as well as a decrease in the effective electron mass. The frequently employed dielectric continuum model which describes delocalized (quantum well) states within adsorbate layers does not give a consistent explanation of these findings. By adsorption of Xe overlayers onto C₆F₆/Cu(111) we can show that, even for one monolayer of C₆F₆, the excited state must be localized predominantly inside the C₆F₆ layer and thus originates from a molecular state (presumably an antibonding σ* orbital). With increasing coverage this state becomes more delocalized within the adsorbate layer, which reduces the coupling to the metal substrate and thus enhances the excited state lifetime.