High-resolution one and two photon excitation spectra of t r a n s , t r a n s -1,3,5,7-octatetraene

One and two photon excitation spectra for the lowest energy singlet transition (2 1Ag←1 1Ag) of trans, trans 1,3,5,7-octatetraene in n-octane at 4.2 K(0–0 at 28 56l cm−1) have been measured and analyzed. The one photon spectrum is found to be vibronically induced while the two photon spectrum is electronically allowed. The contribution of the higher lying 1 1Bu←11Ag transition (0–0 at approximately 32 100 cm−1) to the observed two photon excitation spectrum is found to be negligible allowing vibrational overtones of the 2 1Ag←1 1Ag system to be observed. On the bases of frequency, intensity, and one or two photon allowedness, 128 measured lines are assigned as combinations of 4 antisymmetric and 11 symmetric normal modes.