Measuring Complex Particulate Refractive Index Spectra

CHRISTOPHER M. SORENSEN, Prakash Gautam, Sami Labidi, Justin Maughan, Kurt Ehlers, Hans Moosmüller, Kansas State University

     Abstract Number: 580
     Working Group: Aerosol Physics

Abstract
We present a method to measure the real and imaginary parts of refractive indices of ensembles of particles. The foundation of this method is to suspend the particles in media of a variety of refractive indices and measure the spectral light scattering from and extinction through the suspension. When the real part of the particle refractive index is matched by the medium refractive index, both the scattering and extinction coefficients will pass through a minimum. This yields the real part of the particle refractive index spectra. The imaginary refractive index can then be obtained for a number of different situations as: (1) for particles small compared to the optical wavelength via Rayleigh scattering and extinction ratios, (2) for fractal aggregates of arbitrary size using the Rayleigh-Debye-Gans formalism when it holds, (3) for spherical particles using Mie theory, and (4) with less accuracy for particles of arbitrary size and shape, using modern scattering/extinction codes. If the real part of the refractive index for liquid media is not large enough (cost effective liquids go to n = 1.62, expensive ones to 1.87), extrapolations techniques may be viable for the Rayleigh regime and for spherical particles.