Renner-Teller and pseudo-Renner-Teller interactions in the electronic ground and excited states of the dicyanoacetylene radical cation: Assignment of vibronic spectrum and elucidation of nonradiative and radiative decay mechanisms

Abstract

Vibronic interactions in the first five energetically lowest electronic states (X̃2Πu-Ã2Σg+-B̃2Σu+-C̃2Πg-D̃2Πu) of dicyanoacetylene radical cation (C4N2·+) and their effect on the nuclear dynamics are examined in this article. The spectroscopy of C4N2·+ is a subject of outstanding complexity and addressed by the [see, the recent article, J. Chem. Phys. 139, 184304 (2013)]. The energetic ordering of electronic states and the emission mechanism of the states seem to have ambiguity. Here we have undertaken a detailed theoretical study in an attempt to resolve it. A vibronic coupling Hamiltonian of the five electronic states mentioned above is constructed in a diabatic electronic basis and with the aid of ab initio quantum chemistry calculations. Quantum nuclear dynamics studies are carried out by both time-independent and time-dependent quantum mechanical methods. The vibronic spectrum is calculated and the progressions are identified in terms of vibrational modes and compared with experimental slow photoelectron spectroscopy results. Renner-Teller (RT) coupling within the degenerate electronic states (X̃, C̃ and D̃) and the pseudo-Renner-Teller (PRT) coupling among states are examined in detail to elucidate their impact on the vibronic structure of the state and its decay mechanism to the ground (X̃) electronic state. While the dynamics of the X̃ and D̃ states is somewhat simple, the same of the Ã, B̃ and C̃ states is outstandingly complex due to strong RT and PRT interactions.