The ultrafast timescale of electron transfer processes is crucial to their role in many biological systems and technological devices. In dye-sensitized solar cells, the electron transfer from photo-excited dye molecules to nanostructured semiconductor substrates needs to be sufficiently fast to compete effectively against loss processes and thus achieve high solar energy conversion efficiencies. Time-resolved laser techniques indicate an upper limit of 20 to 100 femtoseconds for the time needed to inject an electron from a dye into a semiconductor, which corresponds to the timescale on which competing processes such as charge redistribution and intramolecular thermalization of excited states occur. Here we use resonant photoemission spectroscopy, which has previously been used to monitor electron transfer in simple systems with an order-of-magnitude improvement in time resolution, to show that electron transfer from an aromatic adsorbate to a TiO(2) semiconductor surface can occur in less than 3 fs. These results directly confirm that electronic coupling of the aromatic molecule to its substrate is sufficiently strong to suppress competing processes.

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Articles you may be interested inA single centre water splitting dye complex adsorbed on rutile TiO2(110): Photoemission, x-ray absorption, and optical spectroscopy J. Chem. Phys. 135, 114703 (2011); 10.1063/1.3637497 X-ray absorption and photoemission spectroscopy of zinc protoporphyrin adsorbed on rutile TiO 2 ( 110 ) prepared by in situ electrospray deposition J. Chem. Phys. 132, 084703 (2010); 10.1063/1.3336747 Photoemission, resonant photoemission, and x-ray absorption of a Ru(II) complex adsorbed on rutile Ti O 2 ( 110 ) prepared by in situ electrospray deposition

By exploiting the core-excitation-induced dissociation of O2, we find that the Auger emission exhibits a Doppler-like energy shift. We show this to be a manifestation of localization of the core hole and propose that the problem of core-hole localization versus delocalization in core-hole spectroscopies may be resolved by considering the nature of the measurement.

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