Singlet-triplet interaction of the adiabatic terms (X)over-tilde1Σ+ and (a)over-tilde3Σ+ in the linear triatomic molecules

The paper introduces the 4x4 vibronic matrix that describes singlet-triplet interaction of adiabatic terms (1)Sigma(+) and (3)Sigma(+) via deformation z-modes of the linear triatomic system. Our analysis takes into account spin-orbital coupling in ab initio Breit-Pauli form in the framework of two-electronic model. It is shown that the symmetry operators of electronic Hamiltonian include both space operations (acting on electronic coordinates) and matrix operations (acting on electronic spins). Eigenvalues of vibronic matrix (i.e. potential energy surfaces) are invariants of the molecular symmetry group C-infinity nu. The 4x4 vibronic matrix includes 4 electrostatic parameters and 3 parameters caused by spin-orbital interaction.