Electron-hole scattering resonances

No doubt, the present author has his own private consensus which, in spite of his efforts, may inject itself into the review. In order to offset such an undesirable bias, as much as possible, and perhaps putting the cart before the horse, the author will state here his own conclusions and beliefs I am convinced that electric field amplification and enhanced emission near SERS-active surfaces due to resonating metal excitations (surface-plasmon polaritons, plasmonlike modes, shape resonances, or electron-hole pairs) is an active mechanism in most of the systems studied. However, in most systems, this contribution, though an important one, is minor compared to the total enhancement possible in SERS. The major mechanism, in my opinion, must be a resonance mechanism, in the sense of a resonance Raman process, i.e., a mechanism by which a part of the system (molecule, molecule-metal atoms, metal surface) becomes a strong scatterer by virtue of its large resonance polarizability and not as a result of strong fields exerted by the other parts of the system . 

One may classify the various proposed models in several ways. One way is to differentiate between models that focus on the role of the electric field E and the emission G terms (these two are related), on the one hand, and those that emphasize the role of changes in the Raman polarizability tensor, on the other. The former discuss the enhancement in terms of amplified fields, due to the presence of the surface, which act on the scattering molecule and its emission being further amplified by the surface. These are the local field and emission enhancement models (LFE). The difference between the various models which belong to this group is in the identification of the specific excitation in the solid which is responsible for the amplification plasmon polaritons, shape resonances, electron holes, etc. 

To this group belong several models of very different kinds such as the image model (RE-IE), the charge transfer (RE-CT) model, the electron-hole excitation model (RE-EH), and the Raman reflectivity model (RE-RF). These models have very little in common except that they all lead to enhancements by virtue of a resonance scattering mechanism. The validity of the last statement is not always realized by people, but it will be shown below to hold true. 

Fig. 3. Schematic, one-electron view of resonant magnetic scattering at the Ljjj absorption edge. The linearly polarised incident photon promotes a Ipj/j core electron into an empty state above the Fermi level. In the lanthanides there are 5d states available in the dipole approximation, and unfilled 4f states available through a quadrupole transition. Magnetic scattering results when the virtually excited electron decays, thereby filling the core hole and coherently emitting an elastically scattered photon.

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