Abstract
Photodissociation was observed and studied for a series of gas-phase cations of fluoro- and chlorobenzenes, using trapped-ion cyclotron resonance techniques. All of these dissociations were interpreted as involving absorption of two or more photons. Photodissociation (PDS) spectra of trichlorobenzene and trichloro-trifluorobenzene ions were compared with literature fluorescence emission spectra. In both cases, the sharp fluorescence peak at the 0-0 transition energy was not reflected in the PDS spectra; this was interpreted as showing high fluorescence quantum yield, and correspondingly low internal conversion quantum yield, for ions in the B̃ state vibrational ground state. Collisionless (infrared radiative) relaxation rates for several of the ions were determined by chopped-laser two-photon dissociation, showing that the presence of several chlorines, and particularly several fluorines, enhances the rate of infrared emission. The highest radiative cooling rate constant observed was 31 s−1 for trifluorobenzene ion. For those ions whose visible–UV fluorescence quantum yields have been reported as near unity, the observation of photodissociation was accounted for by a combined mechanism involving fluorescence-pumping heating of the ions, followed by two-photon photodissociation.
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