Charge transfer in lunar materials: Interpretation of ultraviolet-visible spectral properties of the moon

The electronic spectra of lunar materials in the ultraviolet-visible region are dominated by charge transfer bands involving transitions between oxygen and cations of titanium, iron, and chromium. Energy level calculations, using the SCF-X_α scattered wave molecular orbital method, on the isolated coordination polyhedral clusters Fe^IIIO₆^9-, Fe^IIO₆^10-, Ti^IVO₆⁸-, Ti^IIIO₆^9-, Cr^IIIO₆^9-, and Fe^IIO₄^6- suggest that oxygen → metal charge transfer energies increase in the order: oct. Fe³⁺ < tet. Fe²⁺ oct. <. Ti⁴⁺ < oct. Fe²⁺ < oct. Ti³⁺ < oct. Cr³⁺. These results correlate with spectral data for lunar materials which show that absorption edges occur at lower energies for samples with high Ti abundances and low Ti³⁺/Ti⁴⁺ ratios. The calculations support, therefore, the use of remote sensing reflectance spectral measurements to map titanium-rich areas of the moon.