The 2 1Ag state of trans,trans‐1,3,5,7‐octatetraene in free jet expansions

One‐ and two‐photon fluorescence excitation and emission spectra of the S1↔S0 transition of trans,trans‐1,3,5,7‐octatetraene have been measured for the first time in free jet expansions. The one‐photon excitation spectrum is the same, with the exception of significant differences in the intensities of a few lines, as the two‐color, resonance‐enhanced, two‐photon ionization spectrum, previously assigned to the 2 1A’←1 1A’ transition of cis,trans‐1,3,5,7‐octatetraene. However, comparison of the one‐ and two‐photon fluorescence excitation spectra shows clearly that the carrier of the spectrum has inversion symmetry, as expected for trans,trans‐1,3,5,7‐octatetraene. The one‐photon spectrum is built on bu Herzberg–Teller promoting modes, which are origins of progressions in ag modes, while the two‐photon spectrum is due to a single progression in ag modes starting from the 2 1Ag←1 1Ag electronic origin. The appearance of out‐of‐plane vibrations, possibly including torsions of the polyene framework, suggests la...