Reflectance from solids and solid particles: the need for the optical constants n and k and far-IR measurement challenges

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.