Origin of New Bands in the Raman Spectra of Dye Monolayers Adsorbed on Nanocrystalline TiO2

The adsorption of dye monolayers on nanocrystalline titanium dioxide surfaces leads to high photoelectrochemical yields in the visible range, which gives a renewal of interest in the use of this system in solar energy. A high resonance effect in these dyes enabled the attainment of the Raman spectra of the adsorbed molecules; further, a new series of Raman peaks appeared during the polarization of TiO{sub 2} modified by Ru-bipyridinium compounds in the presence of an electron donor. They correspond to a molecular configuration in which an electron is exchanged during the measurement between a particular ligand and a substrate titanium atom. This configuration appears then to be similar to the short-lived excited state of the metal ligand charge-transfer complex (MLCT).