Elementary photoprocesses in benzene clusters

This article reports the use of the resonant two-photon ionization technique to selectively excite a molecular ion within a cluster and observe the dynamical outcome. We have excited clusters containing up to 14 benzene molecules to energies of 10.00 or 12.84 eV and measured the probability that an initially formed C₆H₆⁺ attacks a neighboring benzene unit of the cluster according to the vapor-phase reaction C₆H₆⁺ + C₆H₆ → C₇H₇⁺ + C₅H₅, ΔH = 0.63 eV. The C₅H₅ radical is expelled from the cluster. At either energy excitation proceeds through an X¹1² vibrational level of benzene (X = 6 or 8) but in the latter case the benzene cation is also produced electronically excited. Accordingly, at low-energy excitation the above pathway is entirely absent, while the 12.84-eV excitation leads to reaction with a probability increasing with cluster size, as predicted by solvation models. This result makes it appear quite likely that >12-eV excitation of condensed benzene will lead to transient tropylium ion centers for conduction electrons, accompanied by variable trapping of C₅H₅.