Two-photon photoselection in rigid solutions: A study of the B2u←A1g transition in benzene

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Abstract
The first detailed study of simultaneous two‐photon photoselection is reported, using the benzene B 2 u ←A 1 g two‐photon forbidden transition to illustrate this new technique. Polarized emission from the B 2 u fluorescent state is used to probe the symmetries of excited molecular states reached by two‐photon absorption in an initially isotropic solution. Three independent polarization measurements are reported for each of the e 1 u , e 2 u , and b 2 u false origins. These transitions span all possible D 6 h molecular symmetry species which are two‐photon allowed from the ground state in a single color experiment. It will be seen how the photoselection method complements the conventional circular/linear polarization measurements in fluid solutions by providing empirical symmetry assignments for the nontotally symmetric transitions. The partial randomization parameter ε, commonly used in one‐photon photoselection studies to rationalize nonideal polarization values, is introduced into the two‐photon theory. This considerably improves the agreement between experimental and theoretical two‐photon polarization ratios.