Theoretical study of the spectroscopy of niobium nitride

State-averaged (SA) complete-active-space self-consistent-field (CASSCF) multireference configuration-interaction (MRCI) calculations are reported for the singlet and triplet states of NbN below about 20 000 cm⁻¹ and the quintet states below about 30 000 cm⁻¹. The theoretical spectroscopic constants for the four lowest triplet states, X³Δ, A³Σ⁻, B³Φ, and C³Π, are in excellent agreement with experiment. The calculations predict the ordering of states in the singlet manifold to be a¹Δ, b¹Σ⁺, c¹Γ, d¹Σ⁺, and e¹Π. These states have all been experimentally observed, except for the d¹Σ⁺ state near 13 000 cm⁻¹. The lowest quintet state, ⁵Π, is predicted to lie above 17 000 cm⁻¹. The calculations confirm the experimental conjecture, based on hyperfine splittings, that the X³Δ state is of predominantly 4dδ¹5s¹ character. Although the NbN molecule is very ionic, the ground-state dipole moment of 3.68 D is only moderately large, because of the polarization of the 5s electron away from nitrogen atom. Electronic transition moments are computed for all of the dipole-allowed transitions in the singlet and triplet manifolds. The radiative lifetimes for the v′ = 0 level are computed to be 37 ns, 35 ns, 1 μs, and 64 ns for the B³Φ, C³Π, d¹Σ⁺, and e¹Π states, respectively. For small v′, the largest Einstein coefficients all involve transitions with v′ = v″, because the transitions in NbN are nearly vertical. We constrast our results for NbN with similar calculations for the isoelectronic ZrO molecule. The considerable differences between the spectroscopy of ZrO and that of NbN are a result of the greater stability of the d orbitals for Nb, and the fact that Nb has a 4dⁿ⁺¹5s¹ ground state, whereas Zr has a 4dⁿ5s² ground state.