A Study of Electric Field Standing Waves on Reflection Microspectroscopy of Polystyrene Particles

Abstract

We have been investigating the mid-infrared (MIR) reflection spectrum of microparticles on mirrored substrates. Gold-coated porous alumina filters were used as a substrate to layer the particles and provide consistent reflection spectra. Polystyrene spheres with measured diameters of 0.42 μm were studied using Fourier transform infrared (FT-IR) reflection microspectroscopy, and spectra are shown for coverages in the range 0.5–6 monolayers (ML). Results show that absorption has a nonlinear, stairstep-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 spectra as a function of increasing wavelength in sub-monolayer coverage measurements. Scattering effects in the spectra are consistent with surface scattering at the antinodes of the EFSW. These observations provide explanations for differences seen between optical properties of particles calculated using the specular-reflection method versus those calculated using traditional aerosol methods. A simple multilayer method for estimating particle absorption coefficients is demonstrated that compares well with values reported using ellipsometry for bulk polystyrene. Another simple method based on sub-monolayer coverage spectra provides spectra suitable for classification analysis but is only semi-quantitative at determining absorption coefficients.