Theoretical investigation of à 2Σg+–X 2Πu vibronic-coupling and ultrafast internal-conversion dynamics in the acetylene cation Origina

The dynamics and spectroscopy of the X 2Πu and à 2Σg+ states of C2H2+ and C2D2+ have been investigated in the framework of a vibronic-coupling model. The model includes six of the seven vibrational degrees of freedom and has been constructed on the basis of CASSCF/CASPT2 ab initio electronic-structure calculations. The main feature of the model is a conical intersection of the 2Σg+ state with the 2Πu state. Time-dependent quantum wave-packet calculations have been performed for this model and photoelectron spectra have been obtained by Fourier transformation of the so-called autocorrelation functions. The calculations provide the explanation of the observed extremely short lifetime of the à 2Σg+ state in terms of an ultrafast internal-conversion process caused by the 2Σg+–2Πu conical intersection. The calculated photoelectron spectra and autocorrelation functions are compared with experimental data of Reutt et al. [J. Chem. Phys. 84 (1986) 3022]. The comparison provides evidence that the ultrafast dynamics of the 2Σg+ state is affected by an additional curve crossing involving the 2Πg shake-up state.