Regularities of the SER spectra of symmetrical molecules

On the basis of the SERS (Surface Enhancement of Raman Scattering) theory proposed by the author, a theory of the SER (Surface Enhancement Raman) spectra of symmetrical molecules is developed. The concept of main and minor moments for symmetrical molecules is introduced. It is demonstrated that the full scattering in a spectral line consists of some contributions caused by the scattering via various dipole and quadrupole moments. A qualitative classification of the contributions after the enhancement degree, based on the concept of the main and minor moments is made. On the basis of group theory, selection rules for these contributions are obtained. It is demonstrated that some bands, caused by the vibrations, transforming by the unitary irreducible representation are most pronounced in SERS. The lines caused by the vibrations, transforming as the d(z) moment, are also pronounced. It is pointed out that strong bands, usually forbidden in Raman scattering, can appear. The SER spectra of concrete symmetrical molecules ethylene, pyridine, PIMH, PIMK are considered. The theoretical results obtained fit the experimental results for these spectra and support the significance of the strong dipole and quadrupole light-molecule interactions in SERS.