Experimental and computational study of light scattering by irregular particles with extreme refractive indices: hematite and rutile

Abstract

We present measurements of the complete scattering matrix as a function of the scattering angle of randomly oriented irregular hematite and rutile particles. The measurements were made at a wavelength of 632.8 nm in the scattering angle range from 5-174 degrees. Apart from their astronomical interest, these two samples are extremely interesting from a theoretical point of view, because they both have high real parts of the refractive index (about 3.0 for the hematite and 2.73 for the rutile). In addition, the hematite sample has a high imaginary part of the refractive index k, with values between 10 −1 and 10 −2 , whereas rutile is a non-absorbing material (k ≈ 0) at the studied wavelength. The scattering patterns of these mineral particles are quite similar to each other but show remarkable differences when compared to the results obtained for irregular mineral particles with moderate real parts of the refractive index. The measured results for both samples were compared with results of Mie calculations for projected surface equivalent spheres and T-matrix calculations for various spheroidal and cylindrical shapes. Both the experimental and theoretical results presented in this work seem to indicate that the scattering behavior of irregular mineral particles that have a high real part of the refractive index is not very dependent on the shape of the particles. In this case, Mie theory may give reasonable results despite the irregular shapes of the particles.

Scattered light mapping of protoplanetary disks

“…The scattering phase function of dust grains depends on several parameters including their size, shape, composition, and internal structure (e.g., Bohren & Huffman 1983;Muñoz et al 2006;Min et al 2012Min et al , 2016. To gain insight into the scattering properties of the dust grains residing in the HD 100546 disk surface, we obtained the phase function from the unscaled Q φ polarized intensity images.…”

Polarized scattered light from self-luminous exoplanets

“…The refractive index affects both the single scattering albedo and polarization efficiency. For example, the single scattering albedo is close to unity for silicates and the single scattering polarization is approximately bell-shaped (e.g., Volten et al 2001), whereas carbon-rich material has a stronger absorbing efficiency and a polarization curve that is overall lower and deviating from being bell-shaped (Muñoz et al 2006). In this study, we used a simplified, parameterized thermal structure and implementation of the clouds, whereas the formation or cloud condensates and photochemical hazes is a complex process which is controlled by many aspects of the atmosphere such as the thermal structure, atmospheric dynamics, and (non-equilibrium) chemistry (see e.g., Helling et al 2008).…”

Q-Space Analysis of the Light Scattering Phase Function of Particles with Any Shape

“…Q-space plots of the S 11 matrix element (scattered intensity for unpolarized incident light) for light scattering from some of the Amsterdam-Granada data set [14]. (a) Feldspar, Redy clay, Quartz, Loess, Sahara [18], Allende [19], Green clay, and Fly ash [20] measured at λ = 441.6 nm; (b) Hematite [21,22], Rutile [22], Martian analog (palagonite) [23], and Sahara sand (Libya) [15] measured at λ = 632.8 nm; (c) Volcanic ash (Redoubt A, Redoubt B, Spurr Ashton, Spurr Anchorage, Spurr Gunsight, Spurr Stop 33) [24] measured at λ = 632.8 nm; and (d) Olivine S, Olivine M, Olivine L, and Olivine XL [19] measured at λ = 441.6 nm. In all graphs the lines are power law fits to the data and the number to the right of the plot is the power law exponent.…”