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

Vibronic interactions in the first five energetically lowest electronic states ((X) over tilde (2)Pi(u)-(A) over tilde (2 Sigma)(+)(g)(B) over tilde2 Sigma(+)(u)-(C) over tilde (2)Pi(g)-(D) over tilde (2)Pi(u)) of dicyanoacetylene radical cation (C4N2 center dot+) and their effect on the nuclear dynamics are examined in this article. The spectroscopy of C4N2 center dot+ 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 theprogressions are identified in terms of vibrational modes and compared with experimental slow photoelectron spectroscopy results. Renner-Teller (RT) coupling within the degenerate electronic states ((X) over tilde, (C) over tilde (D) over tilde) and 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 ((X) over tilde) electronic state. While the dynamics of the (X) over tilde and (D) over tilde states is somewhat simple, the same of the (A) over tilde, (B) over tilde and (C) over tilde states is outstandingly complex due to strong RT and PRT interactions.