Raman enhancement (SERS) of the surface phonon modes of TiO2

We report the first observation of the enhancement of Raman forbidden surface phonon modes (SO) of A2u symmetry in TiO2 nanoparticles on the adsorption of 2,2′-bipyridine and N-719 dye molecules. We identify the observed surface modes in conjunction with HREELS studies and attribute their enhancements to (Herzberg-Teller) vibronic coupling between the charge-transfer and the molecular transitions. We conclude that the vibronic coupling is responsible for the enhancements of the surface modes through intensity borrowing. Furthermore, using SERS selection rules, we examine possible orientations of the molecules relative to the surface confirming the Raman forbidden A2u symmetry of the surface modes.