THE 2(1)A(G) STATE OF ISOLATED CIS,TRANS-1,3,5,7-OCTATETRAENE - 2-COLOR RESONANCE ENHANCED 2-PHOTON IONIZATION STUDIES

Vibrationally resolved 1 1A(g) --> 2 1A(g) 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 1A(g) --> 2 1A(g) 0-0 band (29 035 cm-1) is approximately 6500 cm-1 lower than the 35 484 cm-1 excitation energy of the 1 1A(g) --> 1 1B(u) 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 1A(g) state is more susceptible to external perturbation than previously suspected. The decay times measured for vibronic levels in the 2 1A(g) 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 1A(g) state.