Laser spectroscopy of the low‐lying electronic states of NbN: Electron spin and hyperfine effects in the states from the configurations σδ and δπ

Rotational and hyperfine analyses have been carried out for the (0,0) bands of the C ³Π–X ³Δ, e ¹Π–X ³Δ, and f ¹Φ–a ¹Δ transitions of gaseous NbN from laser excitation spectra taken at sub‐Doppler resolution. The δπ C ³Π and e ¹Π states lie only 102 cm⁻¹ apart in zero order but the spin–orbit matrix element between them, which is the sum of the spin–orbit constants for the δ and π electrons, is 698 cm⁻¹; as a result the ³Π₁ spin component lies below both the ³Π₀ and ³Π₂ components, and its hyperfine structure is highly irregular. This irregularity is an extreme example of how cross terms between the spin–orbit interaction and the Fermi contact hyperfine operator alter the apparent value of the hyperfine a constant, the coefficient of I⋅L in the magnetic hyperfine Hamiltonian. Molecular parameters for the C ³Π and e ¹Π states have been obtained from a combined fit to the two of them. Including data for the B ³Φ state recorded earlier [Azuma et al., J. Chem. Phys. 91, 1 (1989)], detailed information is now available for all six of the electronic states from the electron configurations σδ and δπ. It has been verified that the spin–orbit/Fermi contact cross terms cause roughly equal and opposite shifts in the hyperfine a constants for the singlet states and the Σ=0 components of the triplet states. After allowing for this effect, it has been possible to interpret the hyperfine a constants in terms of one‐electron parameters for the δ and π electrons, in similar fashion to spin–orbit parameters. Wavelength resolved fluorescence, following selective laser excitation of the C ³Π, e ¹Π, and f ¹Φ states, has led to the discovery of three new electronic states, δ² c ¹Γ, δ² A ³Σ⁻, and σ² b ¹Σ⁺, besides giving the absolute position of a ¹Δ. Strong configuration interaction mixing is found to occur between the σ² b ¹Σ⁺ and δ² d ¹Σ⁺ states. The low‐lying electronic states of NbN are now well understood.