S 1 –S vibronic spectra of benzene clusters revisited. II. The trimer

Abstract

We present a reexamination of the S1–S0 transition of the (benzene)n cluster that appears only in the dimer ion channel and thus has been assigned to an isomer of the neutral dimer other than the extensively studied T-shaped form. Mass-selective resonantly enhanced two-photon ionization (R2PI) excitation and UV–UV (ultraviolet) hole burning spectra are measured in the 000 and 601 vibronic regions. It is established from the observed spectra monitoring three isotopomer channels, (C6H6)2+, [(C6H6)(C6D6)]+, and (C6D6)2+, that efficient fragmentation following ionization prohibits one to observe the two-color R2PI spectra in the parent ion channels, similar to the case for the benzene tetramer as presented in Paper I [J. Chem. Phys. 117, 3656 (2002)]. Three neutral isotopomers containing at least one C6H6 moiety are identified, and this result argues the reassignment of the band system to the neutral benzene trimer with a cyclic form, where the three benzene sites are equivalent. The spectra of the homo isotopomers [(C6H6)3 and (C6D6)3] in the two vibronic regions exhibit small splittings due to the excitation exchange interactions, and this observation is discussed on the symmetry of the cluster geometry. Energetics pertaining to neutral and ionic benzene clusters are also discussed with previous experimental studies to assess a reason of the efficient fragmentation in the ionic states after R2PI for the trimer and larger clusters but not for the dimer.