Electric-field-induced Raman spectroscopy

In this final paper of a series on electric‐field‐induced spectroscopy, the general theory of electro‐optical effects in vibrational Raman spectroscopy is developed. It is demonstrated that the electrical polarization of a dipolar fluid can lead to appreciable intensity enhancement of certain lines in its Raman spectrum, and a modification of the polarization rules normally applicable to an isotropic medium. Exact intensity expressions are presented, and it is shown how the complete symmetry analysis of all Raman transitions can be accomplished with five intensity measurements, introducing new possibilities for species characterization close to electrode surfaces. It is also demonstrated that in intense electric fields a novel nonlinear electro‐optical channel exists, whereby the electric field perturbs the stationary states of the system and gives rise to selection rules completely different to those which normally apply in Raman scattering. This can result in the appearance of entirely new lines in the spectrum. Detailed examination of the intensity expressions for the electro‐optical route reveal that in this case five intensity measurements are required for a complete elucidation of the Raman spectrum.