Recent debate over the source of surficial OH at non-polar latitudes on the Moon has not been adequately resolved. A common qualitative hypothesis is that OH is formed by solar wind H+ becoming embedded in the lunar regolith and subsequently bonding to oxygen [1, 2], but experiments more closely simulating lunar conditions  have shown insufficient hydroxyl creation to account for the strength of the reported 3-14% absorption signal [4-6]. With the goal of determining a precise formation rate of OH due to H+ implantation in lunar soils, we are performing a series of experiments on both terrestrial analogs and lunar soils. The resulting changes in Si-OH absorption depth are measured using transmission FTIR spectroscopy.
All experiments are done in a UHV chamber with a base pressure of <1x10-9 Torr (mostly H2 remaining). The samples were first baked to ~400 °C for 4 hrs to remove surface contaminates due to atmospheric exposure, then directly transferred to the analysis position without being exposed to laboratory air. Performing the sample preparation and analysis entirely under vacuum conditions ensures that no water will adsorb/diffuse into the material or alter bonding between implanted hydrogen and structural atoms. Transmission FTIR spectroscopy is used to determine the pre-irradiated water content, and the sample is then irradiated with 1 - 5 keV H+ ions to a fluence of 5x1017 H+/cm2. The increase in Si-OH content due to irradiation is determined by subtracting the unirradiated spectra from the irradiated spectra. Measuring the band area of the resulting increase and using reported values of molar absorption coefficients [7,8] we can find the concentration in terms of column density (OH/cm2).
From our analyses, we find that irradiation of amorphous SiO2 and olivine thin sections produces an increase in the OH absorption band centered near 2.8μm. The band increases with fluence and plateaus at fluences of ~2x1017 ions/cm2. The size of the band increase corresponds to column densities of 2.2x1016 OH/cm2 in SiO2 and 2.3x1015 OH/cm2 in olivine. Further results using lunar soils will be presented.
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- D.R. Bell, et al., Hydroxide in olivine: A quantitative determination of the absolute amount and calibration of the IR spectrum. Journal of Geophysical Research 108 (2003)
- K.M. Davis, et al., Quantitative infrared spectroscopic measurement of hydroxyl concentrations in silica glass. Journal of Non-Crystalline Solids 203 (1996)