ORDER AND COMPLEXITY IN THE ELECTRONIC SPECTRA OF DIATOMIC TRANSITION METAL COMPOUNDS

Author Institution: Department of Chemistry, University of Britis ColumbiaDiatomic transition metal compounds, such as oxides and nitrides, have two types of low-lying excited electronic states; in one type electrons are permuted among the mental orbitals and, in the other type, the charge-transfer states, electrons are promoted from the light atom to the metal. The relative energies of these two types of states depend on the ionization potential of the metal. For the early transition metal compounds, where the I.P. of the metal is much lower than that of the light atom, the charge transfer states lie fairly high, so that the electronic spectra consist mainly of fairly simple well-separated transitions that are easily understood in the single configuration approximation. The later transition metal compounds have much lower-lying charge-transfer states. There are many more of these states than there are of the intra-metal states, and they interact strongly with each other and with the intra-metal states. The result is that the spectra degenerate into long overlapping and irregular vibrational progressions of extreme complexity. In any given region there are usually one or two ``bright"" states which carry the oscillator strength, and a host of perturbing ``dark"" states which pick up a small amount of intensity by a variety of mechanisms. Severe rotational perturbations occur in every observable vibrational level, and electronic assignments become virtually impossible. The most extreme examples of this effect studied so far occur in FeO and CoO. Nuclear hyperfine structure, for metals with odd atomic number such as V, Mn and Co, adds an additional dimension of complexity