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://en.wikipedia.org/wiki/Standard_sea-level_conditions