Optical Constants used in Radiative Transfer Models

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

2. 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 Trad (x,y,z), SED, images

3. 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

4. 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 don’t 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

5. 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 (Al2Si2O5(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.

6. 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