Effects of anisotropy on absorption cross-section spectra of medium-sized spheroidal corundum particles

Abstract

Context. It has been widely accepted that corundum particles condense in the atmospheres of oxygen-rich asymptotic giant branch stars and effectively produce an infrared emission feature at 13 μm. Laboratory experiments have predicted that these particles have the shape of oblate spheroids. Aims. We investigate the influence of the material anisotropy of uniaxial corundum on absorption cross section spectra of medium sized spheroidal particles in the infrared spectral region. Methods. We compared absorption cross-section spectra of the anisotropic corundum particles gained by finite-difference time-domain simulations to spectra calculated by a weighted sum approximation of the according fictive isotropic materials, with one material having the dielectric function of the a–b-plane and the other having the dielectric function of the c-axis of corundum. We carried out investigations for different axes ratios of the spheroids, particles volumes, and different geometries of the dielectric axes to the particle axes as well as to the polarization and propagation direction of the incident light. Results. We observed several effects attributed to anisotropy that are non-additive, so that they cannot be reproduced with the combined spectra of the isotropic materials. Conclusions. Care should be taken when calculating the corundum infrared spectrum with simpler approaches. When particle sizes above 1 μm are to be considered, the T-matrix formalism delivers correct band shifts and bulk modes for many, but not all bands. This remains true in orientation-averaged spectra and for particles in the 0.1 μm size range.

О Решении Проблемы Рассеяния Света Сфероидами Для TM- И Те-Мод При Использовании Сфероидального Базиса

“…Представление несферических рассеивателей сфероидами является подходом, применяемым в разных областях науки, например, в нанооптике [1], медицине [2], лабораторном анализе [3], микроволновых экспериментах [4] и т. д. Подход часто использовался в оптике атмосферы [5][6][7][8] и находит широкое применение в современной астрофизике из-за недостаточной пока информации о форме и структуре космических пылинок [9][10][11][12][13].…”

On solution of the light scattering problem for a spheroid in the TM and TE modes when using spheroidal bases

“…The representation of actual scatterer by spheroids is an approach widely used in various fields of science: for example, in atmospheric optics [1], astronomy [2], medicine [3], nanooptics [4], laboratory analysis [5][6][7][8], etc. The spheroidal model is especially often used in modern astrophysics due to the lack of information about the shape of non-spherical cosmic dust particles [9][10][11][12][13].…”

О Связи T-Матриц, Возникающих В Осесимметричной Задаче Рассеяния Света Сфероидом

“…Представление реальных рассеивателей сфероидами является подходом, широко применяемым в разных областях науки: например, в оптике атмосферы [1][2][3], астрономии [4], медицине [5], нанооптике [6], лабораторном анализе [7] и т. д. Особенно часто используется сфероидальная модель в современной астрофизике из-за отсутствия информации о форме несферических космических пылинок [8][9][10][11][12].…”