Calculation of the fine structure and intensity of the singlet-triplet transitions in the imidogen radical

The singlet-triplet transition moments are calculated for the NH radical by multiconfiguration self-consistent field (MCSCF) method with a quadratic response (QR) technique. The band systems in the visible region (b1Σ+→X3Σ− and a1Δ→X3Σ−) of the NH radical are analyzed in comparison with previous ab initio treatments and with the recent experimental data in attempt to solve some discrepancies. The b1Σ+→X3ΣΩ− transition moments ratio for the two spin sublevels Ω = 1 and Ω=0 of the ground state is well reproduced and the radiative lifetime of the b1Σ+ state (τb=58 ms) is obtained in a good agreement with the experimental value τb=53(−13+17) ms. The A3Π←a1Δ transition probability is calculated for the first time and found to be in an excellent agreement with the recent optical pumping measurements of the NH radical in a molecular beam, where population transfer from the metastable a1Δ state to the ground X3Σ− state is achieved. For the a1Δ→X3Σ− transition some improvement is achieved in comparison with the previous ab initio results, but the calculated radiative lifetime (τa=3.9 s) is still much lower than the recent measurement provides (τa=12.5 s). The zero field splitting and spin-rotation coupling constants are calculated for the ground state by different methods and advantage of the density functional theory is stressed.