Fixed-angle reflectance spectroscopy using a commercial Fourier transform infrared (FTIR) spectrometer was employed to derive the optical constants n and k of several uranium compounds. This technique relies upon measurement of the quantitative reflectance R(ν) spectra from a polished surface across a broad spectral range (in this case, the mid- and far-IR covering ca. 7500 to 50 cm-1 ) followed by application of the Kramers-Kronig transformation (KKT). Near-normal fixed-angle measurements as used in this technique require continuous reflectance spectra to as low a wavenumber value as possible. Here, we discuss some of the many challenges in measuring the far-IR and very far-IR (terahertz) spectra using an interferometric instrument, particularly those stemming from small sample sizes, typically just millimeters on a face for crystalline samples, as well as limitations due to optical components and diffraction. We apply this method to single-crystal UO2 and its mineralogical form uraninite, as well as other Ubearing minerals such as autunite [Ca(UO2)2(PO4)2·8-12H2O] and the dehydrated form of autunite, meta-autunite. In addition to the specular reflectance spectra, x-ray diffractometry was used as a confirmatory technique to analyze the surface composition of the species. Deriving the infrared optical constants for such U-bearing species (as well as other solids) will enable nondestructive detection under a variety of environmental and compositional conditions.
The morphology, size, and media surrounding a solid material can all influence its optical properties, such as scattering, absorption, and reflection of light. While it is possible to measure the optical properties of hundreds of individual surface or particle configurations, it is impractical. Conversely, knowledge of the bulk optical constants n and k of a pure solid facilitates computation of arbitrarily sized particles, shapes, surrounding media, etc. As we describe here, single-angle reflectance spectroscopy is one such method used to obtain the bulk optical constants of solids. In particular, solid crystalline materials typically have responses in the mid- and far-infrared arising from phenomena such as lattice (phonon) vibrations as well as stretching or bending vibrations, among others. These vibrational modes in the mid- and far-infrared often present unique experimental challenges as the wavelength of light across such a wide spectral range varies greatly and can dimensionally approach the magnitude of specimens and even optical apertures used to limit the illumination area. Here we describe challenges and solutions, with an emphasis on optical instrumentation and far-infrared spectra, related to measuring realistic-sized mineralogical samples (down to ca. 2 mm) where sample purity, exposed surface area, cost, and rarity can all play important roles in obtaining the optical constants n and k.
Other than water, pure bulk liquids and solids are rarely encountered in the environment, but more commonly exist as layers on various substrates, e.g. concrete, metals, glass, etc. Unlike gas-phase transmission spectra, condensed-phase reflectance spectra depend not only on absorption, but also on the material’s refraction and reflection at interfaces. Providing reference spectra to account for the plethora of morphological conditions (e.g. substrate, layer thickness, particle or droplet size) that may be encountered under different scenarios is a daunting challenge. An alternative approach is to derive the complex optical constants, n and k, which can be used to model the optical phenomena in media and at interfaces, minimizing the need for a vast number of laboratory measurements. The current status of obtaining such optical constants for both solids and liquids is briefly summarized.
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