Photodissociation dynamics of (C₆H₆)₃⁺: Role of the extra benzene molecule weakly bound to the dimer core

The benzene trimer ion has a charge-localized structure, (C₆H₆)₂⁺⋯C₆H ₆, where (C₆H₆)₂⁺ is the dimer core. The trimer ion is photodissociated by excitation of the charge resonance transition of the dimer core. Branching ratios and translational energies of the product ions, (C₆H₆)₂⁺ and C₆H₆⁺, are measured as functions of photon energies (hν=0.99-1.80eV). At the lowest photon energy studied, the dominant dissociation channel is the formation of (C₆H₆)₂⁺ and C₆H₆. In this process, only a small fraction (7%-8%) of the available energy is partitioned into the translational energy of the products. The branching ratio of the (C₆H₆)₂⁺ product decreases smoothly with increasing photon energy from 0.90 at hν=0.99eV to 0.04 at 1.80 eV. The behavior is consistent with the following two-step model for the formation of C₆H₆⁺. The photoexcited (C₆H₆)₃⁺ ion first ejects one C₆H₆ molecule, yielding the transient dimer ion. If the dimer ion has sufficient internal energy, it further dissociates into C₆H₆⁺ and C₆H₆. Statistical theories for unimolecular reactions are applied to predict the translational energies and the branching ratios. A comparison of the theoretical branching ratios with the experimental data suggests that a part (30%-35%) of the product internal energy is distributed to the intramolecular vibrations of the extra C₆H₆ molecule. As far as the energy partitioning is concerned, the extra C₆H₆ molecule is no longer a spectator.