Vibronic Coupling in the X²Π_g-òΠ_u Band System of Diacetylene Radical Cation

Vibronic interactions in the two energetically lowest electronic states (X²Π_g-òΠ_u) of the diacetylene radical cation (C₄H2•+) are theoretically examined here. The spectroscopy of these two electronic states of C₄H2•+ has been a subject of considerable interest and measured in the laboratory by various groups. Inspired by numerous experimental data, we attempt here a detailed investigation of vibronic interactions within and between the doubly degenerate Π electronic states and their impact on the vibronic structure of each state. A vibronic coupling model is constructed in a diabatic electronic basis and with the aid of ab initio quantum chemistry calculations. The vibronic structures of the electronic states are calculated by time-independent and time-dependent quantum mechanical methods. The progression of vibrational modes in the vibronic band is identified, assigned, and compared with the literature data. The nonradiative internal conversion dynamics is also examined and discussed.