Theoretical study of the electronic structure of iridium monoxide

Potential energy curves of 25 low-lying electronic states of IrO are calculated with the multi-state complete active space second order perturbation theory (MS-CASPT2) method. The lowest state calculated without spin–orbit interactions has the 2Π symmetry. However, when spin–orbit coupling effect is considered, the ground state is the X4Δ7/2 and the 2Π3/2 is located 633 cm−1 above. The excited A2Δ state splits into A2Δ3/2 and A2Δ1/2, which are 8233 and 9741 cm−1 higher than the X4Δ2/7 state respectively. Electronic transitions from the two excited states H2ΔΩ and D2ΠΩ to the A2ΔΩ state produce the rotationally resolved bands observed experimentally.