The 2 ‘A, state Of frans,fmns-1,3,5,7=octatetraene in free jet expansions

Oneand two-photon fluorescence excitation and emission spectra of the S&?,, 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 ‘A’ +1 ‘A’ transition of cis,tuans-1,3,5,7-octatetraene. However, comparison of the oneand two-photon fluorescence excitation spectra shows clearly that the carrier of the spectrum has inversion symmetry, as expected for tram, trans1,3,5,7-octatetraene. The one-photon spectrum is built on b, 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 ‘A,+ 1 ‘A, electronic origin. The appearance of out-of-plane vibrations, possibly including torsions of the polyene framework, suggests large differences in force constants and perhaps in the geometries of the 2 ‘Ag and 1 ?4! potential surfaces. For 2 ‘A, vibronic levels with energies < 1000 cm-‘, the fluorescence lifettmes vary between 170 and 450 ns due to the dependence of radiative and nonradiative decay rates on the vibronic state. An abrupt increase in the nonradiative decay rates at -2100 cm-’ excess energy is tentatively ascribed to trans-+cis isomerization. This work demonstrates that the oneand two-photon cross sections of the 2 ‘Age 1 ‘A, transitions of all-trans linear polyenes are sufficiently large to allow the study of 2 ‘A, states under isolated, unperturbed conditions.