Hibbitts, Charles - Relevance of UV Reflectance Spectroscopy to Inferring the Compositions of

Reflectance spectroscopy in the vacuum ultraviolet through visible wavelengths is sensitive to the abundance of opaque materials, especially transition metal cations.  Reflectance data were collected for nine glass samples, alumina, anorthite, and water frost. The glass samples are compositionally similar to those in the regolith on the Moon and Mercury, with the notable caveat that some of the iron is oxidized. It is known that the oxidation state of iron affects the visible-NIR region of the spectrum and may affect the UV as well [1,2,3]. Total iron in the glasses varies from 0 to ~21% when expressed as total FeO, with the fraction of iron as Fe3+ varying from 0 to almost 80%, but not correlated to total iron abundance. The abundances of other transition elements are small, with TiO2 being the most abundant and varying from 0 to about 1.5%. TiO2 con-tent does not correlate with FeO.
About 100 mg of glass samples ground to talc-like consistency (grain sizes <1 μm) was placed in a copper sample holder mounted in a vacuum chamber. Measurements were obtained with a McPherson 302 vacuum monochrometer with a deuterium source and a PMT detector mounted in front of a MgF2 window coated with a scintillation material. Each sample was maintained at room temperature and ~10-7–10-8 torr. Each sample was heated to ~ 80C overnight at this pressure to remove all adsorbed molecular water.  Thus, only hydroxyl, either internal or chemisorbed, remained present.
 The water ice spectrum is consistent with literature values for fine-grained frost [e.g. 4]. Both water ice and alumna are similar, staying bright into the VUV where single strong absorption feature occurs at ~200 nm for alumina and ~160 nm for water frost. Silicates behave differently due to the presence of cation and transition elements in their compositions (1,2). Both iron abundance and valence state control the position of the absorption near 300 nm, and are responsible for the NUV slope that has been often reported for these materials (3). As iron concentration decreases, the NUV drop off near 400 nm shifts to shorter wavelengths and the center of the absorption band also shifts shortward. The effects of iron oxidation in the VUV are small compared to the effects of iron abundance.  The position of the absorption band near 300 nm and the brightness of the VUV continuum near 225 nm are both sensitive to changes of a percent or less in iron abundance and may provide a an additional means for quantifying the iron abundance of low iron minerals and glasses. 
Acknowledgement: This work has been supported by NASA grants: NLSI NNA09DB31A, SSERVI NNA14AB02A, PGG NNX10AI58 and LASER NNX11AO54G.
[1] Sigel, G.H., (1974), J. Non-crystalline Solids, 13, 372-398. [2] Tippins, H.H., (1970), Phys. Rev. B, 1,1, 126-135. [3 Pieters, C.M & Englert, P.A., (1993), Univ. Cambridge Press.  [4] Hendrix, A.R. and C.J. Hansen, (2008), Icarus, 193, 323-333