Electronic structure at organic/metal interfaces: Naphthalene/Cu(111)

Electron transfer at organic/metal interfaces is a fundamental issue of int erest to a large number of problems in chemistry. We use two-photon photoem ission (2PPE) spectroscopy to investigate heterogeneous electron transport in a model system: naphthalene adsorbed on Cu(111). The dependence of 2PPE spectra on photon energy establishes the occurance of photoinduced electron transfer to an unoccupied state at 3.1 eV above the Fermi level (or 1.1 eV below the vacuum level) at one monolayer coverage. Polarization and disper sion measurements reveal the image-like property of this electron-transfer state. The binding energy of this state is dependent on the thickness of th e adsorbate layer. This observation is in agreement with simulation based o n the dielectric continuum model. The simulation also shows that, while par tially distributed in the adsorbate layer at one monolayer coverage, the ex cited electron is confined within the adsorbate layer at high coverages, an effect which can be attributed to the electron affinity of naphthalene.