Which gas at 15 °C and 1 atmosphere of pressure has the highest refractive index in visible light?

For my own curiosity's sake, I am attempting to find the gas that at 15 °C and 1 atmosphere of pressure in pure form has the highest refractive index on average and overall, for visible light. Assume visible light means having a wavelength in the range of 380 to 750 nanometers.


Please keep in mind that I am not looking for the densest gas whether active or inert. Density does not necessarily correlate with a materials refractive index.


If for some reason volume needs to be specified, please assume a 1x1x1 kilometer region.


I am being very specific in what I am seeking and have specified wavelengths, pressure, temperature, purity, and even volume.


If I must for whatever reason, I must be any more specific then please assume that the gravity is the same as on Earth at sea level on the equator though I do not actually require that level of precision.


Sea level conditions are commonly used as a common standard for predicting the behaviors of different materials here on Earth and a common standard of that nature was exactly what I needed.


The reasons for the described conditions specificity is to provide a decent set of common standard conditions for measurement and avoid the mistakes of a similar previous question, see:
[What gas has the highest refractive index?](https://chemistry.stackexchange.com/questions/86545/what-gas-has-the-highest-refractive-index)
<https://en.wikipedia.org/wiki/Standard_sea-level_conditions>




[The Engineering Toolbox](https://www.engineeringtoolbox.com/refractive-index-d_1264.html) gives indices of refraction for various gases, presumably at one atmosphere pressure(and thus requiring temperatures above 15°C in many cases). Among substances that are actually gases at 15°C and one atmosphere, (di)methyl ether at 1.000891 tops this list. This makes dimethyl ether about three times as strongly refractive against a vacuum as air.


But that list is not exhaustive. The refractive index of **uranium hexafluoride** as a function of temperature and pressure is given in Ref. [1](https://inis.iaea.org/search/searchsinglerecord.aspx?recordsFor=SingleRecord&RN=14731108) yields 1.001455 at 632.8 nm and 1.001496 at 457.9 nm, both at 15°C and 760 torr (1 atmosphere). Uranium hexafluoride is thus over 60% more refractive against a vacuum than dimethyl ether, and more than five times as refractive as air.


**Reference**


1. Wright, S.P. (Aug 1982). Index-of-refraction measurement of uranium hexafluoride vapor at 6328 and 4579 nanometers (K/AIS--5003). United States


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