Zeeman spectroscopy and deperturbation of the low-lying states of NiH

Abstract

High resolution laser spectroscopy of the NiH molecule in a magnetic field has revealed strong homogeneous and heterogeneous perturbations among all of the low-lying electronic states. Fully resolved Zeeman splitting patterns from transitions between NiH magnetic sublevels were recorded with the technique of Zeeman optical–optical double resonance (ZOODR) spectroscopy. Using only the zero-field rotational energy levels as input to an electronic structure model, we have calculated Zeeman splittings (g values) for 19 rotational levels, and the predicted splittings are in very good agreement with observed Zeeman spectra. A group of 10 NiH molecular electronic states is seen to form a supermultiplet of levels originating from the Ni+ (3d9)2D atomic multiplet. We describe an effective Hamiltonian matrix that contains explicit terms coupling low-lying states through spin–orbit, vibrational, and rotational interactions. Supermultiplet eigenvectors graphically illustrate the profound mixing hidden beneath the apparent regularity of term value plots for the low-lying states of NiH. The success of the supermultiplet model for this simplest case (a single hole in a highly contracted 3d subshell), namely the successful prediction of strongly J dependent g values, makes us confident that this model will be applicable to other transition metal monohydrides.