Dispersion Analysis of Infrared Spectra of Randomly-Oriented Uniaxial Materials

• Applicant: Privatdozent Dr. Thomas Mayerhöfer
• Subject Area: Analytical Chemistry ; Physical Chemistry of Solids and Surfaces, Material Characterisation
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 554544354

To determine infrared optical properties, single crystals are generally advantageous. However, since preparing single crystals is often complicated, polycrystalline samples are typically used instead. For materials with symmetry lower than cubic, polycrystalline materials exhibit a crystallite size effect. In micro-homogeneous samples, a scalar dielectric function results, while in micro-heterogeneous samples, light detects the individual crystallites and non-zero cross-polarization terms result. Consequently, the shapes, relative intensities, and peak positions of bands change. Neither micro-homogeneous nor micro-heterogeneous samples alone can be used for dispersion analysis. However, if both types of samples are available, it should be possible to determine the two principal components of the dielectric tensor functions for optically uniaxial materials. Therefore, the proposed project aims to develop a new kind of dispersion analysis for polycrystalline uniaxial materials, allowing the determination of oscillator parameters and optical constants from two principal spectra recorded from micro-homogeneous and micro-heterogeneous samples. This includes investigating situations where one of these two principal spectra is substituted by a spectrum of an oriented polycrystalline sample with the c-axis aligned perpendicular to the surface. Additionally, the project will explore the relevance of the crystallite size effect in molecular infrared spectroscopy, especially for organic and biological materials with typically small oscillator strengths. The findings from this project are expected to have direct implications for the broader infrared spectroscopy community, including those involved in band fitting and the analysis of organic and biological materials.

• DFG Programme: Research Grants