Determination of Complex Particulate Refractive Index Spectra Using Colloids

Justin Maughan, KURT EHLERS, Christopher M. Sorensen, Prakash Gautam, Raiya Ebini, Hans Moosmüller, Desert Research Institute

     Abstract Number: 97
     Working Group: Aerosol Physics

Abstract
We present analytical methods to infer the real and imaginary parts of refractive indices of particles suspended in a non-absorbing medium. The foundation of this method is to suspend the particles in non-absorbing media of a variety of refractive indices and measure the spectral light scattering from and extinction through the suspension. If the real part of the particle refractive index can be matched to that of the medium refractive index (cost effective liquids go to n = 1.62, expensive ones to 1.87), both the scattering and extinction coefficients will pass through a minimum, yielding the real part of the particle refractive index spectra. The imaginary part of the particle refractive index can then be obtained in certain situations using, for example, Rayleigh and Rayleigh-Debye-Gans formalisms (Moosmüller et al., 2025). When index matching is not possible, extrapolation methods and methods involving scattering/absorption measurements using multiple medium refractive indices may be used to infer particle complex refractive indices. We present several of these methods together with sensitivity analyses.

Reference: Moosmüller, H. J. B. Maughan, P. Gautam, and C. M. Sorensen (2025).  A Mixing Rule for Imaginary Parts of Refractive Indices of Aerosols or Colloids in the Rayleigh Regime.  J. Quant. Spectrosc. Radiat. Transfer, 331, 109254, doi.org/10.1016/j.jqsrt.2024.109254.