APPROXIMATE THEORETICAL MODEL FOR THE FIVE ELECTRONIC STATES ($\Omega$ = 5/2, 3/2, 3/2, 1/2, 1/2) ARISING FROM THE GROUND $3d^{9}$ CONFIGURATION IN NICKEL HALIDE MOLECULES AND FOR ROTATIONAL LEVELS OF THE TWO $\Omega$ = 1/2 STATES IN THAT MANIFOLD

Abstract

An effective Hamiltonian for a non-rotating diatomic molecule containing only crystal-field and spin-orbit operators has been set up to describe the energies of the five spin-orbit components that arise in the ground electronic configuration of the nickel monohalides. The model assumes that bonding in the nickel halides has the approximate form Ni$^{+}$X$^{-}$, with an electronic $3d^{9}$ configuration plus closed shells on the Ni$^{+}$ moiety and a closed shell configuration on the X$^{-}$ moiety. Least-squares fits of the observed five spin-orbit components of the three lowest electronic states in NiF and NiCl are then carried out in terms of the three crystal field parameters $C_{0}, C_{2}, C_{4}$ and the spin-orbit coupling constant $A$. Following this, the usual effective Hamiltonian $B(\textbf{J-L-S})^2$ for a rotating diatomic molecule is used to derive expressions for the unusually large $\Omega$-type doubling parameter $p$ in the two $\Omega$ = 1/2 states in the $3d^{9}$ manifold. These expressions show (for certain sign conventions) that the sum of the two $p$ values should be $-2B$, but that their difference can vary between $-10B$ and $+10B$. The theoretical magnitudes for $p$ are in good agreement with the two observed $p$ values for both NiF and NiCl, but the signs are not. The experimental signs can be brought into agreement with the theoretical signs by a fairly massive change in +/- parity assignments in the NiF and NiCl literature. The last part of the talk will focus on the theoretical and experimental implications of these parity changes.