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)(+)... C6H6, 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 (h nu = 0.99-1.80 eV). 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 h nu = 0.99 eV 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. (C) 1998 American Institute of Physics. [S0021-9606(98)02833-5].