Theoretical investigation of (A)over-tilde2Σg+-(X)over-tilde2Πu vibronic-coupling and ultrafast internal-conversion dynamics in the acetylene cation

The dynamics and spectroscopy of the (X) over tilde (2)Pi (u) and (A) over tilde (2)Sigma (+)(g) 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 (2)Sigma (+)(g) state with the (2)Pi (u) 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 (A) over tilde (2)Sigma (+)(g) state in terms of an ultrafast internal-conversion process caused by the (2)Sigma (+)(g)-(2)Pi (u) conical intersection. 9 9 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 (2)Sigma (+)(g) state is 9 affected by an additional curve crossing involving the (2)Pi (g) shake-up state. (C) 2001 Published by Elsevier Science B.V.