Direct observation of ultrafast charge transfer in relation to the surface enhanced Raman scattering activation detected by transient reflecting grating spectroscopy

Ultrafast charge transfer related to the surface enhanced Raman scattering (SERS) activation was found using the transient reflecting grating spectroscopic method. Two kinds of vapor-deposited gold films were prepared as SERS substrates. One of them induced the SERS effect (SERS active substrate), while the other did not (SERS inactive substrate). A charge transfer between gold and pyridine was observed only for the former and it occurred within 200 fs. Further, it was suggested that the surface roughness of the substrate not only induced electromagnetic enhancement, but also generated new adsorption sites that made a charge transfer occur easily.