An Approach to the Calculation of Ω‐Splittings in Diatomic Molecules with Strongly Coupled Electronic States and its Application to NiH and NiD

Abstract

AbstractA matrix approach is proposed for calculating term energies and wavefunctions of rovibronic levels in diatomic molecules with strongly coupled electronic states. The interaction matrix is set up in a basis of uncoupled rovibronic functions of Hund's case (a) type. Its diagonalization yields directly term energies and the corresponding wavefunctions. This approach has been applied to calculate the rovibronic spectra of NiH and NiD, typical unsaturated transition metal compounds, which exhibit three low‐lying electronic states, 2Δ, 2∏, and 2Σ+. Spinorbit and rotational interactions have been taken into account in this case. Eigenvectors obtained from diagonalizing the J‐dependent interaction matrices exhibit strong mixing between 2∏3/2, νn+1 and 2Δ3/2, νn basis functions and between 2Σ+, νn and 2∏1/2, νn. The low‐lying states of NiH and NiD can therefore be regarded as belonging to Hund's case (c) coupling. For total angular momentum quantum numbers J up to 5.5 (7.5 is some cases) Ω‐splittings are presented. Very good agreement is observed with various measurements and with predictions from a super‐muliplet fit to experimental data, showing that the proposed method is applicable also in cases where the usual formulae for calculating Λ‐doublings derived from double‐perturbation theory fail.