Probing the electronic structure of the nickel monohalides: Spectroscopy of the low-lying electronic states of NiBr and NiCl

Laser induced fluorescence and single vibronic level emission spectroscopy has been used to probe five low-lying electronic states (X²Π_3/2, A²Δ_5/2, X²Π_1/2, A²Δ_3/2 and B²) of NiBr and NiCl that arise from the 3d⁹ configuration of Ni⁺. Of these, for NiBr only the X²Π_3/2 and A²Δ_5/2 states have been previously observed experimentally, and a complete vibrational analysis of the low-lying states has not been reported for either molecule. In this work, term energies and a complete set of vibrational parameters were derived for all five electronic states for both NiBr and NiCl, and these are compared with recent high level ab initio calculations. In contrast to NiI, there are very few perturbations observed in the vibronic structure of these levels for either NiBr or NiCl. The data set derived in this work affords a detailed analysis of periodic trends in the Nickel monohalide series.