Two-Photon Photoemission Study of Heterogeneous Electron Transfer:  C₆F₆on Cu(111)

Photoinduced heterogeneous electron transfer (ET) constitutes the key step in a number of chemical processes, such as solar energy conversion, photocatalysis, and surface photochemistry. We present a general strategy to investigate photoinduced heterogeneous ET using two-photon photoemission (2PPE) spectroscopy. In this report, we focus on the energetics in photoinduced heterogeneous ET in monolayer and bilayers of hexafluorobenzene on Cu(111). We found that for monolayer C 6 F 6, which likely adsorbs flat on the surface, the ET resonance is not visible in 2PPE spectrum. However, for second-layer C 6 F 6, a strong molecular resonance is observed. Photon energy dependence shows that the shift in electron kinetic energy (Δ E kin ) from the molecular resonance scales with Δ h ν, unlike that from the occupied surface state, which scales with 2Δ h ν. This provides evidence that the observed molecular resonance results from electron transfer to an unoccupied molecular orbital located at 2.9 eV above the Fermi level (or 1.8 eV below the vacuum level). We attribute this ET resonance to the occupation of the σ* LUMO level. Compared to the condensed phase, this negative ion state is lowered by 0.7 eV on the metal surface because of charge−image interaction.