Unimolecular Dissociation Kinetics of Benzene Cluster Ions

The "unimolecular" dissociation processes of benzene cluster ions prepared by resonant 2-photon ionization (R2PI) of neutral clusters were investigated by using a time-of-flight mass spectrometer equipped with a reflection. The dissociation rates were determined for fast-decay processes (k_fast) in the acceleration region and slow dissociations (k_slow) in the drift region of the spectrometer. The ejection of one neutral molecule was a dominant dissociation channel of small (C₆H₆)_n⁺ in the drift region; in addition, the ejection of two molecules was observed for n - 2≥14. These size-specific dissociation channels and the size dependence of the k_slow values imply a closing of the first shell of (C₆H₆)_n⁺ at n = 14. The magnitude of both k_fast and k_slow was found to be independent of the excess energy upon R2PI, suggesting rapid cooling of the nascent hot ions by prompt dissociation within the laser focus. For slow dissociation within a well-defined time window, the measured values of k_slow were compared with those calculated according to a statistical theory of unimolecular reaction. The comparison deduced the average internal energies of (C₆H₆)_n⁺ at ca. 1 μs after preparation.