The 2 1A g state of isolated c i s,t r a n s-1,3,5,7-octatetraene: Two-color resonance enhanced two-photon ionization studies

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Abstract
Vibrationally resolved 1 1Ag→2 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 1Ag→2 1Ag 0–0 band (29 035 cm−1 ) is ∼6500 cm−1 lower than the 35 484 cm−1 excitation energy of the 1 1Ag→1 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.