Raman spectroscopy of molecular monolayers without giant field enhancements

Raman spectra have been obtained from molecular monolayers of nitrobenzoic acid and aminobenzoic acid on AlO_x and in Ag and Sn tunnel junctions. The large electromagnetic field enhancement necessary for surface enhanced Raman scattering experiments from roughened noble metal surfaces is replaced by the enhanced sensitivity of a cooled, optical multichannel detector. The behavior of the Raman intensity for a molecule adsorbed on a nonresonant metallic substrate has been studied and compared with theory. The evolution of the vibrational spectrum of the adsorbate during the fabrication of an inelastic electron tunneling spectroscopy junction has been observed and the dominant spectral changes assigned to the initial adsorption of the molecule on the oxide surface.