Normal bands in surface-enhanced Raman scattering (SERS) and their relation to the electron-hole pair excitation background in SERS

Normal bands in SERS are bands of Raman-active vibrations of adsorbates in the first layer, whose frequency is not shifted with respect to the condensed species. Here we show the experimental result, that the intensity of the N-bands of C2H4 on cold-deposited copper films in ultrahigh vacuum scales with the intensity of the broadband background of the SERS spectra at the frequency position of the N-bands. This is demonstrated by changing the background by temperature annealing of the Cu film. Changing the laser wavelength proves that the background is Raman scattering by electron-hole pair excitations. A simple model including electron scattering by surface roughness can explain the spectral shape of the background and the scaling of the N-bands with the background. Probably we also can explain the appearance of the N-bands in surface-enhanced infrared absorption (SEIRA) within this model.