The phenomenon of single-molecule detection by the SERS method and the SERS quadrupole theory

On the basis of quadrupole SERS theory, developed by the author, the enormous SERS coefficients observed in the works of Kneipp and co-workers are obtained. For some models of single particles the enhancement coefficient may reach enormous values in the range 10(10)-10(16). These models are similar to the particles of rod-like and faceted forms revealed in the experiments of Emery and Nie and previously used in quadrupole SERS theory. In rudimentary terms the enormous SERS arises as a consequence of the large molecules situated near the wedges and tips of the large particles. The results demonstrate, that single-molecule detection with SERS cannot be explained simply by the enhancement of the electric field and by the dipole interaction, but that the strong quadrupole light-molecule interactions also have to be taken into account. Moreover, it appears that one cannot use the multiple expansion due to the singular behaviour of the electromagnetic held near the crossing of facets and sharp points of some separate particles. However, in spite of the fact that our estimations based on the quadrupole interaction can explain the experimental results, due to their crudeness we are forced to suppose the possible influence of the precise configuration of the electromagnetic field near the crossing of facets and rod-like particles.