Peculiarities of the sers spectra of 4,4'-bipyridine molecule in a single molecule detection regime

The paper describes briefly main statements of the theory of the SERS spectra with regards to the single molecule regime, when the enhancement achieves the values $\sim$10$^{14}$–10$^{15}$. Analysis of the spectra of 4,4'-bypyridine, obtained on the dimer lattice of sharp nanoparticles points out that the observed enhancement is caused exclusively by a strong quadrupole light-molecule interaction, which manifests in the presence of lines, caused by vibrations with the unit irreducible representations of the $D_2$ and $D_{2h}$ symmetry groups, which apparently describe the symmetry properties of the molecule. The study of the spectra, obtained by tip enhanced spectroscopy demonstrates that the strong quadrupole light-molecule interaction still has a governing role, however the strong dipole interaction still manifests in the existence of very weak forbidden lines. This result apparently is associated with another experimental geometry.