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Optical Constants used in Radiative Transfer Models A look at calculated Qabs and Qscat Jeremy Yates UCL

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What do we use now Use MIE Theory n(a) = a -p n+ik for the specific material Geometry (spherical) of grains Produce Qabs and Qscat Solve RT equation and Energy Balance Gives T rad (x,y,z), SED, images

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MIE Theory Solves Maxwells equation for a plane wave interacting with a small particle Can produce n+ik if wanted Produces Scattering and Extinction Coeffs –Absorption Coeff deduced from these Produces an exact solution using spherical harmonic equations

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The Problems for Grains Current use of MIE theory assumes materials are homogeneous –No band gaps – creates features –No collective displacement of atoms – creates an electric field that can shift absorption bands Basically the n+ik data are too simple and dont have sufficient frequency resolution Poor or NO modellinf of features On Earth see lots of features, but no decent model Like Atomic Spectrocopy in the 1920s

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What do Real Mineralogists do Balan et al 2001, Am Min 86, 1321 Did ab initio QM calculations of n+ik and the absorption coeff of Kaolinite (Al 2 Si 2 O 5 (OH) 4 ) –Took into account the size and shape of particles and structure of the crystals –Band gaps and displacement –Calculated accurate absorption coeffs from 227 microns to 2.7 microns See a wealth of features.

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To Do UCL project – Yates and Bowey will –Compute absorption and scattering coeffs for sizes and shapes and materials of Astronomical interest –Quantify the feature temperature shift –Compare with IR Lab data –Compute RT using these to get accurate intensities for IR features and make predictions for new IR features to be observed with Spitzer and HSO

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