Vibronic Coupling in the First Five Electronic States of Dicyanodiacetylene Radical Cation

We examine vibronic coupling in the first five electronic states (X̃²Π_g-òΠ_u-B̃²Σ_g⁺-C̃²Σ_u⁺-D̃²Π_g-Ẽ²Π_u) of dicyanodiacetylene radical cation (C₆N₂^•+) in this article. Prompted by the prediction of its existence in the astrophysical environment, the vibronic band structure of these electronic states of C₆N₂^•+ has been probed in spectroscopic measurements in laboratory by various groups. Inspired by numerous experimental data, we undertook the task of investigating topographical details of electronic potential energy surfaces, their coupling mechanism and nuclear dynamics on them. The degenerate Π electronic states of this radical are prone to Renner-Teller instability, and in addition symmetry allowed Σ-Π and Π-Π vibronic coupling is expected to play crucial role in the detailed vibronic structure of each of the above electronic states. A vibronic coupling model is developed here and first-principles nuclear dynamics study is carried out employing quantum mechanical methods. The vibronic band structure thus calculated is compared with experimental results and the progressions are identified and assigned. The nonradiative internal conversion dynamics among electronic states is also examined and discussed in relation to the various coupling of electronic states.