The effect of anisotropy on cross section spectra of quartz, calcite and corundum spheres of moderate size

Höfer, Sonja; Mutschke, H.; Mayerhöfer, Th.G.

doi:10.1016/j.jqsrt.2020.107241

Cited by 1 publication

(3 citation statements)

References 32 publications

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“…Figures 2-4, presenting the results for the spheroidal particles originating from the three setups of the sphere, are organized as follows: the left panels show the spectra of the prolate particle (setups labeled "p"), the right panels show the spectra of the oblate particles (setups "o"), and below these are the spectra of the corundum sphere from Höfer et al (2020b) for comparison. The FDTD spectra, taking the material anisotropy fully into account, are presented in solid lines and the (sum) spectra of the fictive isotropic materials are given in dotted lines.…”

Section: Resultsmentioning

confidence: 99%

“…Bands 1-6 are vibrational resonances in the a-bplane and should thus dominate the blue and purple spectra, while bands 7-12 belong to the c-axis and should dominate the green spectra (however, see e.g., the appearance of the bulk modes 7 and 11 in setup 2p, o). Bands 2, 4, 7, 11 are bulk modes (Höfer et al 2020b), while bands 3, 6, 10, 12 are the lowest order surface modes (Fröhlich-modes). Modes 1, 5, 8 are overlapped bulk and Fröhlich modes; here the oscillator strengths are too low to generate distinct bulk modes.…”

Section: Resultsmentioning

confidence: 99%

“…In Höfer et al (2020b), we investigated the influence of the material anisotropy on the absorption spectra of a corundum sphere for three different setups and for the spheroidal particles, we extended the investigations to nine different setups derived from the geometries of the sphere, as depicted in Fig. 1.…”

Section: Simulation Setupsmentioning

confidence: 99%

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

Höfer

Mutschke

Mayerhöfer

2021

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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.

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Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%