Modelling IR spectra of polycrystalline materials in the large crystallites limit—quantitative determination of orientation

Abstract

The aim of this paper is twofold. First, average reflectance and transmittance theory (ARTT) is extended to allow for materials consisting of bianisotropic domains. This extension is based on the use of a modified 4 ? 4 matrix formalism (Berreman 1972?J. Opt. Soc. Am.?62 502) using partial modal fields instead of total fields, following a suggestion of Lin-Chung and Teitler (1984?J. Opt. Soc. Am. A?1 703). This formalism is adjusted to the needs of ARTT by the treatment of singularities arising either from degenerate eigenvalues or by singular forms of the dynamical matrix. Second, ARTT is generalized to permit the treatment of preferred orientation. The theory is then applied to model experimental reflectance spectra of fresnoite glass ceramics oriented by the method of electrochemically induced nucleation. The excellent agreement between experimental and modelled spectra suggests that it is possible in principle to distinguish between different orientation distributions having the same average orientation with the help of the cross-polarization terms. In addition, for a non-normal incidence of the probing light wave, measurements of these terms allow us to determine not only the magnitudes of the orientation angles but also their signs. These findings show that orientation can be determined by optical measurements of one-photon processes to a considerably higher degree than assumed by linear dichroism theory.