Photo-induced electron transfer to molecular quantum structures on a metal surface

Electron transfer at organic–metal interfaces is important in molecule-based electronic and optoelectronic devices. We probe interfacial electron transfer using two-photon photoemission in two model systems: (i) hexafluorobenzene (C6F6) weakly adsorbed on Cu(111); and (ii) SAM of thiophenolate (C6H5S) on Cu(111). In the first system, the σ∗ LUMO in C6F6 is found to be located around 3 eV above the Fermi level. With increasing overlayer thickness, the position of this molecular resonance decreases layer by layer, suggesting the formation of molecular quantum wells, each of which extends to the total thickness of the adsorbate layer. In the second system, which is a model interface for self-assembled molecular quantum wires, electron transfer to σ∗ states is found to dominate the two-photon photoemission spectrum. The two σ∗ states (LUMO and LUMO+1) are localized to the CSCu anchor and are strongly coupled to the metal substrate. These interfacial σ∗ states may play important roles in electron transport.