The theory of double-beam three-photon absorption

This paper deals with the theory of double‐beam three‐photon absorption, in which the excitation process involves the concerted absorption of three photons from two laser beams which simultaneously irradiate the sample. Equations are derived for the rate of absorption under various polarization conditions, and the dependence on the angle between the two beams is examined in detail. It is shown how a set of experiments at a suitable angle with specified polarizations allows the determination of five molecular parameters which characterize the symmetry of the excited state. It is demonstrated how the results thereby provide for an unambiguous assignment of the excited state to one of six different symmetry classes. It is also shown that the double‐beam experiment allows access to excited states which are forbidden under single‐beam excitation.