Electric-field-induced two-photon absorption

This paper provides a theoretical treatment of the electrochromic effect of a static electric field on molecular two‐photon absorption, based on a full quantum electrodynamical formulation of the interactions. The results include a description of changes in the spectrum resulting both from electro‐optical interactions and, in the case of polar fluids, from the anisotropy associated with energetically favored molecular orientations. A detailed consideration of the effect of varying laser polarization and field direction is included in the analysis. The results demonstrate that on application of the field, weak but potentially detectable new lines should appear in the two‐photon spectrum. These lines are associated with transitions to vibronic states which would normally require three‐photon excitation. The application of the static electric field also results in the selective enhancement of other lines normally present in the two‐photon spectrum. The effect may thus offer scope for the accurate determination of the frequency and symmetry character of weak vibronic transitions when they occur under the wing of strongly allowed transitions.