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 photo dissociated by excitation of the charge resonance transition of the di mer core. Branching ratios and translational energies of the product i ons, (C6H6)(2)(+) and C6H6+, are measured as functions of photon energ ies (h nu = 0.99-1.80 eV). At the lowest photon energy studied, the do minant dissociation channel is the formation of (C6H6)(2)(+) and C6H6. In this process, only a small fraction (7%-8%) of the available energ y is partitioned into the translational energy of the products. The br anching ratio of the (C6H6)(2)(+) product decreases smoothly with incr easing photon energy from 0.90 at h nu = 0.99 eV to 0.04 at 1.80 eV. T he behavior is consistent with the following two-step model for the fo rmation of C6H6+. The photoexcited (C6H6)(3)(+) ion first ejects one C 6H6 molecule, yielding the transient dimer ion. If the dimer ion has s ufficient internal energy, it further dissociates into C6H6+ and C6H6. Statistical theories for unimolecular reactions are applied to predic t 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 t he energy partitioning is concerned, the extra C6H6 molecule is no lon ger a spectator. (C) 1998 American Institute of Physics. [S0021-9606(9 8)02833-5].