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

Abstract

We revisit the vibronic band systems of benzene cluster isotopomers by mass-selective resonant two-photon ionization (R2PI) and ultraviolet-ultraviolet hole burning $spectroscopies.^{a}$ Hole burning spectra on the transitions previously assigned to the $trimer^{b}$ in the $C_{6}H_{6}$-localized $0^{0}_{0}$ region show that there are five distinct isotopomers having $C_{6}H_{6}$ molecule(s). R2PI spectra which have similar band structure are recorded in the $(C_{6}D_{6})_{3}^{+}$ mass channel as well. The results indicate that the transitions are due to the cluster larger than the trimer, and thus the mass assignments that have been accepted for a couple of decades must be corrected. The number of the isomers and observed splittings are discussed in terms of the tetramer that have the geometry with four equivalent sites belonging to $S_{4}$ point group, which is also consistent with a NEMO $calculation^{c}$ of its most stable structure.