The electronic spectra of benzene cluster ions, (C6H6)n + with n = 3-6, are measured through mass-selected photodissociation spectroscopy. The spectra in the 400-1100-nm region show three distinct absorption maxima centered around 430, 590-620, and 950 nm, which are analogous to the spectrum of (C6H6)2 +. The 950-nm band is assigned to a charge resonance (CR) band characteristic of the dimer ion, while the other two bands are attributed to local excitation bands. The position of the CR band is found to be almost independent of cluster size. The result suggests that the cluster ions have a charge-localized structure involving a strongly bound dimer ion core. In addition, the number and translational energy of neutral molecules ejected following photoexcitation are measured for (C6H6)n + with n = 3-8 in the photon energy range of 0.5-3.0 eV. The average number of ejected molecules increases linearly with increasing photon energy, suggesting that the fragmentation proceeds via the sequential ejection of neutral monomers. The average translational energy carried by one monomer is determined to be 50-70 meV, which is comparable with the calculated value according to a statistical theory. A large part of the imparted photon energy is partitioned into internal energies of the products. These results indicate that the photofragmentation of (C6H6)n + can be regarded as a unimolecular decay of vibrationally hot clusters, despite the promotion of the chromophoric dimer core to the repulsive excited state.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry