LAUSR

Abstract Vibronic interactions in the first five energetically lowest electronic states ( X 2 Π u - A 2 Σ g + - B 2 Σ u + - C 2 Π g - D 2 Π u ) of dicyanoacetylene radical cation ( C 4 N 2 · + ) and their effect on the nuclear dynamics are examined in this article. The spectroscopy of C 4 N 2 · + 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 ( X , C and D ) 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 ) electronic state. While the dynamics of the X and D states is somewhat simple, the same of the A , B and C states is outstandingly complex due to strong RT and PRT interactions.