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Refractive index of Mercury analog particles from light scattering measurements

Presentation #116.08 in the session Mercury (Poster + Lightning Talk)

Published onOct 23, 2023
Refractive index of Mercury analog particles from light scattering measurements

An inverse light scattering method for deriving the complex refractive index of a mm-sized single particle in a specific wavelength using laboratory measurements is presented. Laboratory measurements were done using the 4π scatterometer, which measures Mueller matrix elements of a particle suspended in air using acoustic levitation as a function of scattering angle. The measured samples were fragments of silicate glass, created as Mercury surface analogs by Carli et al. (Icarus 266, 267, 2016). To obtain the complex refractive index of a glass sample, measurements were compared to simulations from a SIRIS4 Fixed Orientation (SIRIS4 FO) geometric optics simulation. The 4π scatterometer is a unique instrument which measures Mueller matrix elements from a particle using linear polarizers and a detector rotating around the particle on a rotational stage. The scatterometer uses an acoustic levitator as a sample holder, which provides nondestructive measurements and full orientation control of the sample. To compare the measurement results to simulations, the SIRIS4 single-particle geometric optics code was modified to handle particles in a fixed orientation. The modified SIRIS4 FO calculates the Mueller matrix elements over the full solid angle around the particle, as functions of the two angles, which give the direction of observation of the scattered wave compared to the direction of the incident wave. A 3D model of the shape of the measured particle was constructed using X-ray microtomography, and was transported to SIRIS4 FO. The complex refractive index was obtained with a nonlinear least squares analysis by minimizing the sum of squared residuals between the measurements and simulations with varying refractive index values. Finally, confidence regions were constrained for the results, by estimating the computed residuals between simulations and measurements as the random errors in the nonlinear model. The obtained real part of the refractive index was n = 1.59 ± 0.02 and the imaginary part k = 2.05 ± 0.46 · 10−5 . Previous values of the refractive index derived by the manufacturers of the glass fit into our confidence regions.

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