$S_{1}-S_{0}$ VIBRONIC SPECTRA OF BENZENE CLUSTERS REVISITED: II. THE TRIMER

Abstract

As the second part of our recent reinvestigation on the electronic spectra of benzene $clusters,^{a}$ 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 $dimer.^{b}$ Detailed examination with mixed samples of $C_{6}H_{6}$ and $C_{6}D_{6}$ has revealed that the parent neutral has a single isomeric form for each isotopomer, $(C_{6}H_{6})_{m}(C_{6}D_{6})_{n}$ 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 $calculation.^{c}$ 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.