A COUPLED-CHANNELS POTENTIAL FIT DESCRIBING THE LOW-LYING X2∆, 2Π AND 2Σ+ STATES OF NiH TO EXPERIMENTAL ACCURACY
I. Havalyova, P. Crozet, Amanda J. Ross, A. Pashov, I Bozhinova · Proceedings of the 2021 International Symposium on Molecular Spectroscopy · 2021
A direct potential fit to Hund's case (a) potential curves for the low-lying `supermultiplet' states (${X{}^2\\Delta}$, ${{}^2\\Pi}$, ${}^2\\Sigma^{+}$) of NiH and to R-dependent spin-orbit and rotational coupling functions has reproduced the experimental term values of $^{58}$NiH, $^{60}$NiH, $^{62}$NiH up to 6500 cm$^{-1}$, with a root mean square deviation very close to the estimated uncertainty of 0.01 cm$^{-1}$. Second-order Born-Oppenheimer breakdown corrections to the rotational Hamiltonian had to be included to achieve this result. The spin-orbit interaction A$_{so}$(R) associated with Ni$^{+}$ is large compared to the energy separations between the three electronic states, so that most of the observed rovibrational states are strong mixtures of the $\\Omega$-components of the multiplet. This made the fitting procedure particularly difficult, because there were no perturbation-free data to determine the starting values for the model functions. Potential curves were optimized from RKR turning-points generated from earlier work using an effective Hamiltonian approach\\footnote{M. Abbasi $et~al$, J. Mol. Spectrosc. \\underline{349} 49-59 (2018)}, and the spin-orbit and rotational coupling functions from theoretical predictions\\footnote{C. Marian, J. Chem. Phys., \\underline{93(2)} 1176-1186 (1990)}. We believe that this model may be reliably extrapolated to higher rotational states, with potential applications in the simulation of high temperature spectra, for example in the context of stellar atmospheres.