Two-photon absorption study of 1,3,5-hexatriene by cars and CSRS

Two-photon absorption spectra of trans- and cis-1,3,5-hexatrienes in the liquid state have been observed in the frequency range of 2ω = 30000–50000 cm−1, by analyzing the interference effect of Raman and two-photon resonances in the CARS and CSRS lineshape. In both isomers, a broad two-photon absorption with a maximum absorptivity of about 10−50 cm4 s photon−1 molecule−1 was determined to be a few thousand cm−1 higher than an intense dipole-allowed transitions (11Bu ← 11Ag or 11B2 ← 11A1) near 40000 cm−1. This band is most probably assigned to the two-photon-allowed state corresponding to the lowest excited singlet already established to be 21Ag or 21A1 in larger homologues. The relative ordering of the 11B and 21A levels is discussed.