Two-photon photoemission study of heterogeneous electron transfer:: C6F6 on 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 C6F6, which likely adsorbs flat on the surface, the ET resonance is not visible in 2PPE spectrum. However, for second-layer C6F6, a strong molecular resonance is observed. Photon energy dependence shows that the shift in electron kinetic energy (Delta E-kin) from the molecular resonance scales with Delta hv, unlike that from the occupied surface state, which scales with 2 Delta hv. 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 sigma* 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.