S 1 –S vibronic spectra of benzene clusters revisited. I. The tetramer

Abstract

We report the reinvestigation of the S1–S0 electronic transitions of (benzene)n clusters by two-color mass-selective resonantly enhanced two-photon ionization (R2PI) and UV–UV (ultraviolet) hole burning spectroscopies. The present paper describes the band system that has been assigned to the trimer for two decades. Hole burning measurements by monitoring the trimer ion isotopomer channels in the expansion of a mixture of C6H6 and C6D6 have shown the contribution of six spectral carriers in the R2PI spectra, two of which are isotopically pure clusters. The other hetero isotopic species containing at least one C6H6 moiety appear in two adjacent isotopomer channels. It is argued that the band system should be reassigned to the neutral tetramer having four equivalent sites, which are detected in daughter ion mass channels due to efficient fragmentation after ionization even with two-color excitation. The experimental results are consistent with a distorted tetrahedral structure with S4 symmetry predicted as the global minimum by several model calculations. This conclusion is further supported by an analysis of exciton splittings in the C6H6-localized origin band, and this analysis provides coupling constants for the excitation exchange in the S1 state. Other experimental results reported so far pertaining to this species are reconsidered on the basis of the new assignment.