Advantages of molecular ions for the study of electronic radiationless transitions are
discussed. Several coincidence techniques for measuring key relaxation parameters are
briefly described. The different cases of radiationless transition studies of molecular
ions are outlined and examples mentioned. Coincidence measurements between threshold
photoelectrons and ion fluorescence photons emitted by VUV-excited
hexafluorobenzene are presented. The radiative, kr, and nonradiative, knr, relaxation rates for specific vibrational levels in the B∼2A2u and C∼2B2u states of C6F6+ are derived from the measured lifetimes and quantum yields of ion fluorescence. The kr rate for excitation of the C∼, 0° level is found to be about 25% of that for the B∼2A2u state where kr = (18±3) × 106 s−1. The knr rates for the 1n and 1n21 progressions in the B∼ state increase with n but exhibit a mode selective behavior. Model calculations of knr confirm the mode selectivity.
These calculations were carried out on a noncommunicating, harmonic oscillator basis.
Progressive deviations between experimental and calculated results are interpreted as indicating the
gradual onset of vibrational redistribution with increasing vibrational energy.
The results illustrate a new method for demonstrating the growing in of intramolecular vibrational relaxation.
