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Coming from this answer on the Worldbuilding stack, I'm trying to find a gas with a high refractive index in the visible light wavelengths. The highest index on this site, is chlorine at 1.000773. Is this the highest a gas can be? Or is there a fundamental limit on the refractive index of a gas?

As a note, I am not looking for the densest gas or the densest inert gas. Density does not seem to be correlated with refractive index in this case, as chlorine is not especially dense and the very dense perfluorobutane is not especially refractive.

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    $\begingroup$ I don't think there is a fundamental limit. Then again, benzene is not a gas at room temperature. $\endgroup$ – Ivan Neretin Nov 28 '17 at 20:10
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    $\begingroup$ Well, for a high refractive index (in a simple Lorentz oscillator type of model) both (1) lots of stuff, and (2) lots of electron-photon interactions per bit of stuff. Hard to do with a gas at STP. $\endgroup$ – Jon Custer Nov 28 '17 at 20:29
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    $\begingroup$ (1) For whatever gas it will surely depend on the temperature and pressure. So no real theory to predict it is a matter of looking RI for various gases at that temperature and pressure. (2) Would a supercritical fluid be within scope? For a gas there is not going to be a great density hence fairly small refractive index. With a supercritical fluid you'd have a lot more matter at the solid surface of a planet. $\endgroup$ – MaxW Nov 28 '17 at 21:44
  • $\begingroup$ Venus Gets Weirder: CO2 Oceans May Have Covered Surface $\endgroup$ – MaxW Nov 28 '17 at 22:13
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    $\begingroup$ For those who consider reopening, please see the relevant meta discussion. $\endgroup$ – Martin - マーチン♦ Dec 26 '17 at 12:06
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The question has to be more specific to get a specific answer.

Even in the table linked in the OP, bromine gas (1.001132) has a higher index of refraction than 1.000773.

At zero degrees C, 5462 Angstroms, and atmospheric pressure, The Refractive Index Dispersion and Polarization of Gases (1936) reports:

$\ce{Si2H6}$ 1.0016574

$\ce{Si2H6}$ , disilane, is a gas at room temperature and atmospheric pressure.

The index of refraction is extremely dependent upon pressure.

As an introduction, see INDEX OF REFRACTION AND DISPERSION OF SEVERAL GASES IN CERENKOV COUNTER USE where indexes of refraction of gases as high as 1.02963 (for 182.79 psig ethane) are measured.

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