Infrared reflectivity of uniaxial microcrystalline powders

The Fresnel reflection coefficients for an absorbing uniaxial crystal are derived in a convienient form for the case in which the optic axis has a general orientation with respect to the plane of incidence. In the region of a strongly absorbing vibrational mode the dominant reflection mechanism of a microcrystalline powder is assumed to be a composite regular (mirror) reflection from the properly aligned faces of crystalline fragments in which the orientations of the optic axes are random. The angle-of-incidence dependence of the transverse-magnetic reflectivity from the ${\nu }_3$ mode in microcrystalline sodium nitrate is modeled and compared with experimental data.