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I wonder if the refractive index of acetone, ethanol, methanol, heptane was measured in the range above 830 nm. The site refractiveindex.info provides it only for visible, and I couldn't find any other data so far.

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up vote 3 down vote accepted

I can't find any tabulation.

You can do a Sellmeier equation fit from visible data and extrapolate it to near-IR. BUT, the visible index is determined by the UV absorptions, so the extrapolation will only be reliable as long as you are much closer to the UV absorption bands than to the IR absorption bands. Since these solvents all have pretty strong high-frequency IR bands (CH stretch and OH stretch around 3um), you probably shouldn't extrapolate past 1um or so. (That's just a guess.)

I can't really say any more unless I know how accurate you need (what application) and what wavelength range you're interested in. You can figure everything out with an FTIR, but it can be a bit complicated.

ADDENDUM: If you calculate the index of refraction in the range 850nm-2.5um by extrapolating from the visible, you will overestimate it. The longer the wavelength, the more severe the overestimation. That's because the effect of a resonance, e.g. the CH stretch resonance, is to increase the refractive index for wavelengths longer than the resonance wavelength, and decrease the refractive index for wavelengths shorter than the resonance wavelength. Since your extrapolation from the visible would ignore the IR resonances, it would be an overestimate.

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Steve, thanks, that's the most detailed answer I was able to get so far. I have in the meantime checked a few references (CRC Handbook of Physics and Chemistry, pretty old Landolt-Boernstein, Handbook of Optics), and could only find the n_D values at best. Also the Industrial Solvents Handbook was checked by someone else from sci.optics for acetone and heptane, same result. I will check if your suggestion with resonances brings the simulation results closer to the experiment. –  texnic Jun 5 '11 at 8:09
This answer clearly deserves voting up but I don't have enough reputation for this yet. Sorry. Will come back once I've earned it. –  texnic Jun 5 '11 at 8:23
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