Electric-field-induced two-photon processes

In this paper we outline the novel features associated with two-photon processes in the presence of intense static electric fields. Both electrical polarisation effects and non-linear electro-optical effects are discussed, and their likely orders of magnitude estimated. It is illustrated how the interaction of the electric field with a molecular centre can lead to a relaxation of conventional two-photon selection rules, allowing spectroscopic access to otherwise forbidden excited states. Selection rules for the electro-optical channel are tabulated for some of the more common point groups.