Surface-Enhanced Electronic Raman Scattering at Various Metal Surfaces

Surface-enhanced Raman scattering (SERS) has been extensively used to obtain vibrational information at metal/dielectric interfaces because of its extremely high chemical sensitivity and surface selectivity. Since the discovery of this effect in 1970s, however, the origin of the spectral background has remained questionable. Because the "background" signals dominate in the low-frequency region, it is necessary to deeply understand this phenomenon for analyzing SERS spectra properly in the wide spectral range. Herein, scattering spectra for various metal surfaces are measured with and without excitation of plasmonic resonances in the wide frequency ranges covering both Stokes and anti-Stokes branches, including very low-frequency region. The comparison of the spectra between smooth surfaces and plasmonic rough surfaces of Au, Ag, and Pt and the theoretical evaluation of local fields at the metal/air interfaces indicate that electronic Raman scattering accounts for the background continuum in SERS spectra. This is also confirmed by the comparison of the spectra with and without ultrathin Ag films on a same Au surface. An analytical method of SERS spectra is demonstrated in which both contributions of electronic Raman scattering and vibrational Raman scattering are considered.