Theoretical study of the low-lying electronic spectrum of C2+2

Electronic states associated with the low-lying Born-Oppenheimer spectrum and crucial for the stability and experimental observability of the doubly positively charged carbon system C²⁺2 are studied by ab initio methods. This includes computation of potential energy curves and transition moments by multireference configuration interaction methods, and an investigation of the corresponding vibrational resonance levels and lifetimes. By analogy with the electronic ground state X³Š^-_g of the isoelectronic neutral molecule B₂, the lowest potential energy curve of C²⁺2 that supports quasibound vibronic motion belongs to the state 1 ³Š^-_g. In case of C²⁺2, however, this state is destabilized by a crossing with the repulsive potential energy curve of 1 ³Ý_u, and the induced electronic transitions represent the major decay channel of C²⁺2 (1 ³Š^-_g). Also the quintet state 1 ⁵Š^-_u is quasibound; whereas most of its vibronic levels are practically stable against dissociative tunneling, interactions with other electronic states furnish the principal decay mechanism for 1 ⁵Š^-_u. Additional bonding and stability properties of C²⁺2 are exposed by monitoring the behaviour of potential energy curves while rising the nuclear charge from the neutral to the doubly positively charged situation.