Surface-enhanced hyper-Raman spectroscopy (SEHRS) and surface-enhanced Raman spectroscopy (SERS) studies of pyrazine and pyridine adsorbed on silver electrodes

Both surface-enhanced hyper-Raman spectroscopy (SEHRS) and surface-enhanced Raman spectroscopy (SERS) spectra of pyrazine and pyridine adsorbed on silver electrode surfaces were obtained at different potentials. Although there is a relaxation of the symmetry selection rule to a certain extent, it is demonstrated experimentally here that the SEHR spectrum for a centrosymmetric molecule (pyrazine) shows some new vibrational bands and significant relative intensity changes compared with its SER spectrum. By contrast, the SEHR spectrum for a non-centrosymmetric molecule (pyridine) resembles its SER spectrum and shows the absence of new vibrational bands. Our results also show that SEHRS is more sensitive to the interaction between the adsorbate and the substrate than SERS.