Ab initio molecular orbital studies on the structure, energies, and photodissociation of the electronic excited states of C2H

High level ab initio calculations have been carried out to study seven electronic states of C2H. The calculated equilibrium structure, energetics and vibrational frequencies for the 3 2A′ state at the CASPT2/PVTZ level are in good agreement with those obtained experimentally by Hsu et al. The transition dipole moments from the ground state C2H to electronic excited states depend sensitively on the H–C–C bending angle and often peak at nonlinear configurations. Based on this and the dissociation behavior of the excited states, we predict A 1Πu and c 3Σu+ C2 fragments to be rich in population, the former of which is experimentally detected recently by Jackson et al. The ground X 1Σg+ and the a 3Πu state C2 are expected to be formed via the nonadiabatic process 3 2A′→2 2A′ or 4 2A′→3 2A′→2 2A′, which is in accord with the experimentally observed lifetime pattern by Hsu et al. No reverse barrier for CC–H dissociation was found on the X and A electronic states of C2H in the linear configuration. The 2 2A′ state, however, develops a distinct “barrier” (not a true saddle point) along dissociation coordinate when the H–C–C is significantly bent, due to the interaction with upper electronic state. Since the 2 2A′ state energetically prefers a linear dissociation, we suspect that the upper bound of the CC–H bond energy measured by Hsu et al. is not severally affected by this “barrier.”