Electron-impact study of the SO radical using the R-matrix method

The SO radical with an even number of electrons belongs to an open-shell system due to its π2 ground-state electronic configuration. This configuration gives rise to three low-lying states X 3Σ-, a 1Δ, and b 1Σ+. The inclusion of these target states in a trial wave function of the entire scattering have important implications in the resonances that may be detected in this open-shell molecule. The R-matrix method is a well-established ab initio formalism which is employed to calculate elastic differential and integral cross sections, momentum-transfer cross sections, and inelastic cross sections. The Hartree-Fock ground-state configuration of SO is 1σ22σ23σ24σ21π45σ26σ27σ22π43π2. We have included 28 target states in the trial wave function of the scattering system. In our configuration-interaction (CI) model, we freeze 12 electrons in orbitals 1σ, 2σ, 3σ, 4σ, and 1π, the remaining 12 electrons are free to move among the eight orbitals 4σ, 5σ, 6σ, 7σ, 8σ, 9σ, 2π, and 3π. We have carried our scattering calculations in static-exchange, one-state with CI wave function, and 28-state models. We have detected a stable anionic bound state 2B1 of SO at various bond lengths of SO molecule. The vertical electron affinity value is 0.970 eV, which is comparable to the experimental value of 1.125 eV. We also detected two core-excited shape resonances, both of 2Π symmetry and with 1Δ and 1Σ+ as the parent states.