The Raman spectra of 4-pyridine-carboxaldehyde-doped Al-${\mathrm{Al}}_2$${\mathrm{O}}_3$-Ag tunnel junctions evaporated onto Ca${\mathrm{F}}_2$ films and optical diffraction gratings show that the molecular Raman scattering is strongly enhanced under conditions which permit the direct excitation of the surface plasmon modes of Ag. This suggests that the surface plasmon is an intermediate state in surface-enhanced Raman scattering.

• Received 3 July 1979

DOI:https://doi.org/10.1103/PhysRevLett.43.772