Theory of electro-optical effects in two-photon spectroscopy

This paper develops the general theory of electro‐optical effects in two‐photon molecular spectroscopy. Two distinct mechanisms can play a role in these nonlinear interactions with an external static electric field. One relates specifically to polar fluids and is associated with the partial molecular alignment produced by the applied field. The resultant anisotropy produces a relaxation of symmetry restrictions on the allowed two‐photon transitions. The other mechanism directly involves a nonlinear electro‐optical channel and is associated with quite distinct selection rules. Both mechanisms induce changes in line intensities and can under suitable polarization conditions enable certain transitions to be ‘‘switched into’’ the spectra. Generalized rate equations are derived using standard quantum electrodynamical procedures. The results, which are applicable to any required laser beam configuration, are cast in terms of irreducible Cartesian tensors. This facilitates the elucidation of the selection rules appropriate for molecules of any given symmetry. A comprehensive tabulation of the transformation properties of the relevant molecular tensors under the operations of the common molecular point groups is also presented, paving the way for the subsequent application of the results specifically to two‐photon absorption and Raman scattering processes.