Quartet states of the acetylene cation: electronic structure calculations and spin-orbit coupling terms

Highly correlated ab initio methods have been used to generate one-dimensional cuts of the six-dimensional potential energy surfaces of the quartet and lowest doublet states for the HCCH(+) ion along the CH, CC, and cis and trans bending coordinates. Transition dipole moments and spin-orbit matrix elements are deduced. For the lowest 1 (4)Sigma(u) (+) state, the calculations predict a possible photon emission through the 1 (4)Pi(g)<--1 (4)Sigma(u) (+) transition competing with internal conversion and predissociation processes. The potential surfaces are used together with spin-orbit matrix elements to discuss the metastability and the predissociation processes forming the C(2)HC(2)H(+)+H(+)H products. Multistep spin-orbit induced predissociation pathways are suggested.