The Singlet-Triplet Interaction of 3 Π and 1 Σ + States in Linear Triatomic Molecules

A two-electron model that describes the singlet-triplet interaction of the 1 Σ + and 3 Π states in linear triatomic molecules is proposed in this study. The analysis is based on the consideration of the spin-orbital coupling in an electronic Hamiltonian and the use of its symmetry properties. It is shown that the symmetry operators of the electronic Hamiltonian include both spatial (which affect the electronic coordinates) and matrix (which affect the electronic spins) operations. This study takes into account only the deformation π modes, and the resulting 7 × 7 vibronic matrix in fact describes the relativistic pseudo-Renner effect ( 3 Π + 1 Σ + ) × π. The eigenvalues of the vibronic matrix (i.e., potential energy surfaces) have axial symmetry. The vibronic matrix includes nine parameters. Three of them have an electrostatic origin, and six parameters are determined by the spin-orbital coupling.