The 2 1Ag state of isolated cis-trans-1,3,5,7-octatetraene: two-color resonance enhanced two-photon ionization studies

Vibrationally resolved 1 1Ag2 1Ag excitation spectra and decay times for cis,trans-1,3,5,7-octatetraene seeded in a supersonic He expansion have been measured by two-color resonance enhanced two-photon ionization spectroscopy. The excitation energy of the 1 1Ag2 1Ag 0-0 band (29 035 cm-1 ) is ~6500 cm-1 lower than the 35 484 cm-1 excitation energy of the 1 1Ag1 1Bu 0-0 band. The intensity pattern of the vibronic development of this spectrum is qualitatively similar to the pattern observed previously in solid state experiments. However, a detailed analysis of the vibrational structure reveals that the electronic structure of the 2 1Ag state is more susceptible to external perturbation than previously suspected. The decay times measured for vibronic levels in the 2 1Ag state decrease with increasing vibrational energy, most dramatically for vibrational energies 1200 cm-1 and higher. This indicates the increasing importance of a nonradiative decay channel which is most reasonably associated, at least in part, with cis-trans isomerization in the 2 1Ag state. The Journal of Chemical Physics is copyrighted by The American Institute of Physics.