A symmetry analysis of electric-field-induced spectra

The application of an electric field is shown to allow access to previously unobserved vibronic states when the perturbed molecules are probed by a laser. The appropriate absorption rate is calculated using standard quantum electrodynamics, in the electric-dipole approximation. Random orientation of the molecules is assumed, corresponding to the case where the molecules have no permanent dipole moment. Reduction of the resulting expression to its irreducible, second-rank tensor components allows selection rules for this electric-field-induced spectroscopy (EFIS) to be derived. These are similar to those for the two-photon spectroscopies, such as Raman scattering and two-photon absorption. Variation of the angle between the two fields is shown to give definite identification of the transition symmetry in most cases.