Bridging the Modeling-Sampling Gap for Mid-Infrared Reflectance Spectroscopy of Small Particles, Films, and Fibers

The concentration dependency of the mid-infrared (MIR) absorbance of bacterial spores was determined. Linear relationships (R2 ≥ 0.99) were observed between the concentrations of spores and the baseline corrected peak absorbance for the amide absorbance regions, at coverages of less than 1 monolayer. Optical constants were calculated from these measurements, differing by a factor of 2-4 less than that in the literature. To test if the origin of the reduced cross-section was an electric field effect related to the surface selection rule, polystyrene spheres\u27 characteristics were studied using FT-IR reflection microspectroscopy; results show that absorption has a non-linear, stair-step-like dependence on particle coverage and a wavelength dependence that can be explained by electric field standing waves (EFSW) caused by the mirrored substrate. The same effect is found to cause progressive weakening of the observed intensity as a function of increasing wavelength in sub-monolayer coverage measurements. Thus differences seen between optical properties of particles calculated using the specular-reflection method versus those calculated using traditional aerosol methods are addressed herein given these data. A simple multilayer method for estimating particle absorption coefficients is demonstrated that compares well with reported bulk polystyrene ellipsometric values. Another simple method, based on sub-monolayer coverage spectra, is suitable for spectral classification analysis, but only semi-quantitatively determines absorption coefficients. An IR camera sensitized to spectral regions tuned for detection of crime-scene blood was developed. Adaption of an IR camera with a modulated light source and utilizing digital lock-in amplifier techniques permitted differentiated response due to diffuse reflection and thermal emission. Using the AC response generated by a modulated light source is advantageous over the conventional DC response via enhancing the chemical contrast in the image which enables detection of stains of interest. We also exploit a like-detects-like filtering approach to demonstrate a method of using a broadband thermal detector for chemical identification applications