Complex dynamics at conical intersections: vibronic spectra and ultrafast decay of electronically excited trifluoroacetonitrile radical cation

Abstract

An ab initio quantum dynamical study is performed here to examine the complex nuclear motion underlying the first two photoelectron bands of trifluoroacetonitrile. The highly overlapping structures of the latter are found to originate from transitions to the five lowest electronic states (viz., X̃²E, òA₁, B̃²A₂, C̃²A₁, and D̃²E) of the trifluoroacetonitrile radical cation. The Jahn−Teller (JT) instability of the doubly degenerate X̃ and, D̃ and their pseudo-Jahn−Teller (PJT) interactions with the nondegenerate Ã, B̃, and C̃ electronic states along the degenerate vibrational modes lead to multiple multidimensional conical intersections and complex nuclear trajectories through them. It is found that the JT splitting is very weak in the X̃ and relatively stronger in the D̃ state. However, the PJT couplings play the pivotal role in the detailed shape of the vibronic bands of the radical cation. Ultrafast nonradiative decay of electronically excited radical cation has been examined. The findings of this paper are compared with the experimental data and are also discussed in relation to those observed for the methyl cyanide radical cation.