Ionic State Relaxation Processes in VUV‐Excited Polyatomic Molecules

Advantages of molecular ions for the study of electronic radiationless transitions arediscussed. Several coincidence techniques for measuring key relaxation parameters arebriefly described. The different cases of radiationless transition studies of molecularions are outlined and examples mentioned. Coincidence measurements between thresholdphotoelectrons and ion fluorescence photons emitted by VUV-excitedhexafluorobenzene are presented. The radiative,k r , and nonradiative,k n r, relaxation rates for specific vibrational levels in theB ∼ 2 A 2 uandC ∼ 2 B 2 ustates ofC 6 F 6 +are derived from the measured lifetimes and quantum yields of ion fluorescence. Thek rrate for excitation of the C ∼ , 0° level is found to be about 25% of that for theB ∼ 2 A 2 ustate wherek r= (18±3) × 10 6 s −1 . Thek n rrates for the1 nand1 n 2 1progressions in the B ∼ state increase with n but exhibit a mode selective behavior. Model calculations ofk n rconfirm 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.