Spectroscopy of the Diatomic 3d Transition Metal Oxides

The diatomic oxides of the 3d transition metals have astonishingly com­ plicated spectra which even now are by no means fully understood. The interest in them stems from their importance in astrophysics, high tem­ perature chemistry, and in theoretical understanding of the chemical bond­ ing in simple metal systems. The 3d transition metals have a special place in astrophysics because of the great stability of the nuclei. 56Fe, for example, has the lowest mass per nucleon of any nucleus, so that it is the final product in the thermonuclear processes that fuel stars. The nuclei surrounding iron in the 3d transition series are almost as stable, which makes for a local maximum in the cosmic abundances of these elements ( 1 ). Because of the high cosmic abundance of oxygen and the large dissociation energies of the diatomic oxides of the earlier 3d metals, the band systems of compounds such as TiO and VO completely dominate the spectra of cooler (M-type) stars if they contain metal-rich recycled supernova material (2). Low-temperature astrophysics is high-temperature terrestrial chemistry and, not surprisingly, it is found that the thermodynamically stable high temperature forms of the metal oxides are diatomic vapors. However, by the standards of room tem­ perature chemistry the 3d monoxides are highly refractory materials, so that the difficulty of getting them into the gas phase under nonequilibrium low temperature conditions has held back progress with their spectra. The real problems with their spectra come from the many unpaired electrons, which produce huge numbers of low-lying electronic states,