Internal energy dependent radiationless transitions in 1,3,5,-C6F3H+3 and C6F+6

Abstract

From fluorescence quantum yields φF and lifetimes τ measured by the threshold photoelectron–fluorescence photon coincidence (T-PEFCO) method, reported in a companion paper, the radiative kr and nonradiative knr relaxation rates have been determined for energy selected states of 1,3,5,-C6F3H+3 and C6F+6. knr is less than 106 s−1 in the B̃ state energy region of 1,3,5,-C6F3H3+; an exponential dependence of knr as a function of the selected energy was found in the C̃ and D̃ energy regions of this ion. The nonradiative relaxation of fluorescent states of 1,3,5,-C6F3H+3 and C6F+6 involves coupling to high vibrational levels of their ground states. Emission properties, in particular the kr rates, are used to demonstrate the existence of radiationless coupling between the C̃ and B̃ states for both ions. For large vibrational energies in the B̃ 2A2u state of C6F+6, knr increases quasiexponentially with the internal vibrational energy Ev in agreement with results of Maier and Thommen who used a different experimental technique. Below 3100 cm−1, we find that knr no longer depends monotonically on Ev and is sensitive to the particular vibrational mode excited in the B̃ state. Results are given of model calculations of the relative knr rates as a function of specific vibrational levels of C6F6+ in the B̃ state. The results reproduce our experimentally observed mode selective behavior of knr in C6F+6. Comparison with experiment shows that with the increasing values of Ev the excited vibronic state gradually loses its memory of initial mode composition, exhibiting vibrational redistribution to a degree which increases with internal vibrational energy.