15min:

T. IIMORI, Y. AOKI AND Y. OHSHIMA, Department of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa-Oiwakecho, Sakyo-ku, Kyoto 606-8502, Japan.

As the second part of our recent reinvestigation on the electronic spectra of benzene clusters, we present the trimer vibronic system studied by two-color (2C) resonance enhanced two-photon ionization (R2PI) and UV-UV holeburing experiments. This band system is observed only in the dimer-ion channel even with 2C-R2PI because of extensive fragmentation after photoionization, and thus it has been incorrectly assigned to an isomeric form of the dimer. Detailed examination with mixed samples of C₆H₆ and C₆D₆ has revealed that the parent neutral has a single isomeric form for each isotopomer, (C₆H₆)_m(C₆D₆)_n with m + n = 3. This observation confirms the equivalency in three benzene sites, which is consistent with the most stable cyclic form predicted by a NEMO calculation. One of the intermolecular modes shows prominent Franck-Condon activity, implying a substantial conformational change via photoexcitation. Other details observed in the vibrionc spectra, e.g. , exitonic splitting, will also be discussed.