PHOTODISSOCIATION OF THE BENZENE DIMER CATION IN THE GAS-PHASE

The photodissociation of the benzene dimer cation, (C6H6)2+, has been investigated by crossing a mass-selected ion beam with a linearly polarized laser beam and measuring the kinetic energy distribution of the charged photodissociation products with an electrostatic energy analyzer. The C6H6+ + C6H6 product channel was the only channel observed. A photodissociation onset of 2.56 ± 0.03 eV is observed. The average relative translational energy of the C6H6+/C6H6 photofragments was determined to be 0.070 ± 0.007 eV at a photon energy of 2.71 eV. The small fraction of the available energy (EAV ≅ 2.0 eV) partitioned into the kinetic energy of the photofragments indicates that the upper state in the photoabsorption process is a bound state. Comparison of the experimental results with predictions of statistical phase space theory indicates that after the bound upper state is accessed via photon absorption, dissociation occurs by internal conversion to highly vibrationally excited levels of the ground state, followed by statistical vibrational predissociation. The observation of a photodissociation threshold allows an estimate of the binding energy of the (C6H6)2+ bound excited state accessed by the photon to be made. We obtain D00(C6H6+ - C6H6)* = 0.50 ± 0.17 eV. The relevance of the results to previous condensed-phase investigations of (C6H6)2+ is discussed. © 1990 American Chemical Society.