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.