Electric field gradient effects in Raman spectroscopy

Raman spectra of materials subject to strong electric field gradients, such as those present near a metal surface, can show significantly altered selection rules. We describe a new mechanism by which the field gradients can produce Raman-like lines. We develop a theoretical model for this gradient-field Raman effect, discuss selection rules, and compare to other mechanisms that produce Raman-like lines in the presence of strong field gradients. The mechanism can explain the origin and intensity of some Raman modes observed in SERS and through a near-field optical microscope (NSOM-Raman).