Ab initio configuration-interaction study of the ground and low-lying electronic states of NiI.

For the first time, we have studied the potential-energy curves, spectroscopic terms, vibrational levels, and the spectroscopic constants of the ground and low-lying excited states of NiI by employing the complete active space self-consistent-field method with relativistic effective core potentials followed by multireference configuration-interaction calculations. We have identified six low-lying electronic states of NiI with doublet spin multiplicities, including three states of Δ symmetry and three states of Π symmetry of the molecule within 15 000 cm^-1. The lowest ²Δ state is identified as the ground state of NiI, and the lowest ²Π state is found at 2174.56 cm^-1 above it. These results fully support the previous conclusion of the observed spectra although our computational energy separation of the two states is obviously larger than that of the experimental values. The present calculations show that the low-lying excited states [13.9] ²Π and [14.6] ²Δ are 3 ²Π and 3 ²Δ electronic states of NiI, respectively. Our computed spectroscopic terms, vibrational levels, and spectroscopic constants for them are in good agreement with the experimental data available at present. In the present work we have not only suggested assignments for the observed states but also computed more electronic states that are yet to be observed experimentally. [ABSTRACT FROM AUTHOR]