Photodissociation dynamics of (C₆H₆)₂ ⁺

Photodissociation of (C₆H₆)₂ ⁺ is studied with photon energies between 1.29 and 2.81 eV. Massselected ion beam of (C₆H₆)₂ ⁺ is photodissociated by a pulsed laser beam in the field-free region of a reflectron-type time-of-flight mass spectrometer. The average relative translational energy, 〈ε_t〉, and the angular distributions of the photofragments (C₆H₆ ⁺ and C₆H ₆) are measured as a function of photon energy. With a photon energy of 2.81 eV, the (C₆H₆)₂ ⁺ ions are promoted to a bound upper state correlated to C₆H₆ ⁺(ππ) + C₆H₆(X). Only a small fraction (≈3%) of the available energy is partitioned into the translational energy of the fragments and the product angular distribution is isotropic. Absorption of a photon in the range of 1.29-2.14 eV induces a charge resonance transition to a repulsive upper state which correlates to C₆H₆ ⁺(X) + C₆H₆(X). The observed values for 〈ε_t〉 are at most 10% of the available energy, although the statistical phase space calculation shows that the complete randomization of the available energy is not achieved. As the photoexcited (C ₆H₆)₂ ⁺ moves apart on the dissociative potential surface, ≈90% of the available energy flows into the intramolecular modes of the fragments. The energy partitioning becomes almost statistical regardless of the photoexcitation to a dissociative state.