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

Electron transfer at organic/metal interfaces is a fundamental issue of interest to a large number of problems in chemistry. We use two-photon photoemission (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 dispersion measurements reveal the image-like property of this electron-transfer state. The binding energy of this state is dependent on the thickness of the adsorbate layer. This observation is in agreement with simulation based on the dielectric continuum model. The simulation also shows that, while partially distributed in the adsorbate layer at one monolayer coverage, the excited electron is confined within the adsorbate layer at high coverages, an effect which can be attributed to the electron affinity of naphthalene.