Vibronic Coupling in the X̃²Π_g–òΠ_u Band System of Diacetylene Radical Cation

Abstract

Vibronic interactions in the two energetically lowest electronic states (X̃Π-ÃΠ) of the diacetylene radical cation (CH) are theoretically examined here. The spectroscopy of these two electronic states of CH 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.

Spin–orbit interaction and Renner–Teller effect in HCCCCH+studied by high-resolution photoelectron spectroscopy

“…1 at ≈ 82 cm −1 (827 cm −1 of internal excitation in the ion) and ≈ 83 630 cm −1 (1555 cm −1 ) are intriguing. In the case of a weak Renner-Teller effect along all π modes, as predicted by Gosh et al, 7 all bands should appear as doublets separated by about 30 cm −1 . Isolated bands separated by more than this interval could correspond to the two outer Π components of the overtone of a π mode, however, only for a relatively strong Renner-Teller coupling.…”

“…The assignments of levels involving the modes ν 6 , ν 7 and ν 9 discussed above would leave 8 2 as possible assignment. Such an assignment, which would imply 8 = 0.43, is incompatible with the predictions of Gosh et al 7 and thus appears very unlikely. We note that Ramos et al 10 have derived a large value for 6 (= 0.32), which is also incompatible with the predictions of Gosh et al.…”

“…Calculations predicted the Renner-Teller interaction to be weak along all four π modes, with close to negligible effects arising from the vibronic coupling of the A + and X + states. 7 Experimentally, the Renner-Teller effect in the X + state was found to be weak along ν 7…”

“…4 The study of the structural and dynamical properties of HCCCCH + by high-resolution spectroscopy is also of relevance to understand fundamental aspects of molecular structure and reactivity (see, e.g., Refs. [5][6][7]) and has a long history.…”

“…These experimental studies were complemented by several theoretical studies of the X + and A + states of HCCCCH + . 7,[23][24][25] In the present article, we report on a new measurement of the photoelectron spectrum of the X + ← X photoionizing transition of diacetylene and d2-diacetylene at high resolution using the technique of PFI-ZEKE photoelectron spectroscopy. 26 The present work was motivated by our hopes of carrying out a direct measurement of the spin-orbit coupling constant of the X + state and of studying the low-frequency π modes of HCCCCH + , which we thought might be observable at high-resolution and sensitivity because the Renner-Teller effect makes them weakly observable.…”