Mass-selective two-color photoionization of benzene clusters

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Using two-color mass-selective photoionization, we have obtained the S1 ← S0 excitation spectrum of the benzene dimer, trimer, and tetramer in a supersonic beam. All clusters show a weakly induced 000 band red shifted from the forbidden monomer origin by an amount which increases monotonically with cluster size. The 601 band for all clusters is split into two features corresponding to a crystal-field-induced lifting of the degeneracy of this e2g benzene vibration. Exciton splitting of the 000 band in these species was found to be less than 6 cm-1. The trimer and tetramer spectra show strong progressions in van der Waals modes which ultimately correspond to the optically active phonon side bands in the bulk benzene crystal. Excited-state lifetime, fluorescence quantum yield, and band profile measurements indicate the dimer rearranges from a T-shaped geometry into a sandwich excimer within several picoseconds after laser excitation from the ground T-shaped van der Waals well. This process does not occur readily in the higher clusters. © 1981 American Chemical Society.

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Journal of Physical Chemistry

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