Optical Dispersion and Molar Refraction at Zero Frequency for Compressed Nitrogen, Argon, and Carbon Dioxide Measured as Functions of Density

Abstract

Data obtained by displacement interferometry have been reduced to give the variation of refractive index at infinite wave-length with density for pure nitrogen up to 0.01656 gram per cc. Precise data are also presented for dispersion in pure argon, and the Cauchy constants under NTP conditions are evaluated as follows for this gas: ${A}_{0}\ensuremath{-}1=0.0002771\ifmmode\pm\else\textpm\fi{}0.0000008$, ${B}_{0}=1.477\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}$. It is also noted that the Lorentz-Lorenz function, and hence the molar refraction at zero frequency, are independent of density for these two gases; the density range for argon extending up to 0.03342 gram per cc. The values of molar refraction are found to be 4.369\ifmmode\pm\else\textpm\fi{}0.003 cc for nitrogen and 4.138\ifmmode\pm\else\textpm\fi{}0.012 cc for argon. Preliminary work on carbon dioxide indicates a slight negative deviation from linearity for ($A\ensuremath{-}1$) with density, and a marked negative variation of the Lorentz-Lorenz function with density. The latter means a decrease in the molar refraction at zero frequency, and suggests a decrease in the molar polarization with density. This is not in accord with the findings of Keyes and Oncley who report an increase of molar polarization with density for this gas. The NTP values of the Cauchy constants for C${\mathrm{O}}_{2}$ are also evaluated as follows: $({A}_{0}\ensuremath{-}1)=0.0004419\ifmmode\pm\else\textpm\fi{}0.0000036$, and ${B}_{0}=2.791\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}$. The extrapolated value of the molar refraction at zero frequency is found to be approximately 6.65 cc.