Bonding Model for Transition Metal and Rare Earth Monoxides and Laser Spectroscopy of Nickel-Oxide

We discovered that, for the transition metal and the rare earth monoxide series, the sum of the ionization potential of the metal, the energy of the lowest ( ...np) configuration of the metal ion and the thermochemical dissociation energy of the molecule adds up to a constant number. The correlation is particularly striking for the rare earth monoxides where the standard deviation is less than 1%. Based on this correlation we developed a new bonding scheme common for both the transition metal and rare earth monoxides. We propose that the bonding is invariant within the series and consists of an ionic and a covalent contribution. In our model a covalent contribution to the bonding of the inner-core d and f orbitals is negligible. This is in contrast to the current paradigm regarding the significant role of the d orbitals in the bonding in the first and second row transition metal oxides. Our model also appears to be in conflict with the M^{2+} O^{2-} ligand-field bonding model currently accepted for the rare earth monoxides. Based on the empirical correlation and the proposed bonding mechanism, however, we give a number of predictions regarding yet unmeasured fundamental quantities of some of the oxides such as permanent dipole moments, dissociation energies and equilibrium bond distances. We also present the results of the first high resolution laser spectroscopic study of the NiO molecule. Several bands in the green spectral region were found to originate from the ground state of NiO; their analysis allowed us to determine the following fundamental parameters: Ground state symmetry: ^3Sigma^-; Vibrational frequency: omega_{ rm e} = 8.39.1 cm^{ -1}; Equilibrium distance: r_ {rm e} = 1.627 A. With this work the determination of the ground state parameters for the first row transition metal oxides is now complete.