Comparison of dielectric and refractive virial coefficients and collision induced absorption bands

The second dielectric virial coefficient minus the second refractive virial coefficient is equal to the Kramers–Kroning contributions arising from the far-infrared and infrared pressure induced absorption bands due to bimolecular interactions. The usefulness in comparing such results is illustrated for CO2, C2H4, CH4, and SF6. It is shown that for SF6 there may be as yet unmeasured collision induced bands and for C2H4 the far-infrared collision induced absorption is anomalously large.