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Interfacial Charge Transfer from Molecules to Metals

Optically induced charge transfer between adsorbed molecules and a metal or semiconductor electrode has been explored. The absorption of a redox couple on the surface of an electrode was predicted by Hush to lead to new electronic absorption features, but has not been experimentally observed. However, Gerischer characterized photocurrents arising from such absorption between adsorbed metal atoms and semiconductor conduction bands.

Interfacial charge transfer absorption (IFCTA) provides information concerning the barriers to charge transfer between molecules and the metal/semiconductor and the magnitude of the electronic coupling and could thus provide a powerful tool for understanding interfacial charge-transfer kinetics. We have developed a framework for modeling and predicting IFCTA spectra. The key feature of optical charge-transfer to or from a band of electronic levels (taken to have a constant density of states and electronic coupling element) is that the absorption probability reaches half intensity at λ + ΔG0 , where λ and ΔG0 are the reorganization energy and free energy gap for the optical charge transfer, attains >90% intensity at λ + ΔG0 + 0.9(4λkT)1/2 and remains essentially constant until the top (bottom) level of the band is attained. However, when the electronic coupling and transition moment are assumed independent of photon energy (Mulliken-Hush model), a peaked, highly asymmetric absorption profile is predicted.

We conclude that, in general, the electronic coupling between molecular adsorbates and the metal levels is so small that absorption is not detectable, whereas, for semiconductors, there may be intense features involving coupling to surface states.