Transcript Optical Constants used in Radiative Transfer Models

Optical Constants used in
Radiative Transfer Models
A look at calculated Qabs and Qscat
Jeremy Yates UCL
What do we use now
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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
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
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
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.
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