Of course, the em field of the excitation radiation is incident on the analyte. This is the case for normal Ramanscattering and the Ea is relatively small since the source of the em radiation is relatively far away (the laser). This isone of the reasons why RS is a spectroscopy with notoriously weak signal. However, in SERS, the presence of theroughness feature provides another em field, that of the metal particle itself, Ep, and this em field is quite proximateas the analyte is adsorbed onto the roughness feature, of which the surface is composed. Because of thisproximity, any em field from the particle will contribute a large factor to E. The magnitude of the em field of theparticle is, thus, crucial to the intensity of Raman signal. Since the surface electrons of the metal are confined tothe particle whose size is small, the plasmon’s excitation is also confined to the roughness feature. The resultingem field of the plasmon is very intense.10 The largeem energy density on the particle effects the IR ofthe scatterer as shown in the above relationships.
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