Electric Field and Emission Enhancement Mechanisms LFE

Gersten suggested that the strong electric fields produced near sharp metallic tips may cause some of the Raman enhancement. For an ellipsoid perfect conductor, the field at its tip can be even 10 times larger than the external electric field producing it. This was calculated for a point located 0.1 nm from the tip of an ellipsoid with a major axis of 50 nm and a minor axis of 2.5 nm. Lio and Wokaun obtain 10 Raman enhancement due to these effects at the tip of an ellipsoid of 3 1 axial ratio. At other points, on or near the ellipsoid, they find the electric field greatly reduced. 

These tips are suggested as simulations of the roughness on the SERS-active surfaces. On the average (considering the molecules distributed over the entire surface of the ellipsoid) large enhancements are not expected from this model. There is also nothing special about silver in this respect, and all molecules should exhibit this kind of enhancement, if it is really significant. 

The possibility of utilizing the surface electromagnetic field to enhance the Raman scattering from overlayers on metals in an ATR configuration was proposed before the SERS era by Chen, et 375,376 predicted, for silver, enhancements of about 300 when 

Gersten and Nitzan have presented a comprehensive treatment of the electric field and emission enhancement mechanism. In order to illustrate the main physical features of this mechanism a simple model, following Gersten and Nitzan, will be given here. 

Let the system consist of a molecule with (Rayleigh) polarizability tti, and a metallic body with the polarizability a2- Let the external field be Eq. The moments of the induced dipoles, in the molecule /jli and in the metal /X2, are given by the product of the polarizability and the total field operating on each moiety. Here one assumes the particles to be much smaller than the wavelength and, for simplicity, the tensorial properties are dropped. Thus, 

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