Theoretical Models of SERS

In Section II it was shown that there is a large number of experimental studies of SERS phenomena. It is not surprising that there is also an abundance of theoretical models proposed to explain these results. Several excellent reviewsand, in addition, some more specific papers (see, e.g.. Refs. 21-23) have been published. The reader is referred to them for a more complete and rigorous presentation, or for a different point of view. The aim here is to present the theories in as simple terms as possible in order to facilitate comparison of their predictions with the experimental evidence. Also, an attempt is made to point out the strengths of each model and its limitations. 

Just as the previous section dealt with experiment alone, trying to exclude theoretical interpretation, thus, in the present section, we try to discuss theory devoid of the experimental results. 

Our theoretical sojourn begins with the following expression, which describes the intensity of the Raman-shifted light 

(1) denotes an average over all possible orientations and positions of the species which scatter the light. For instance, we may envisage molecules adsorbed on a heterogeneous surface, each sensing a local field, characterized by a polarizability and associated with a propagator, all of which may be position dependent. 

The enhancement of the Raman scattering, R, is given in these terms by the ratio 

---