Refractive Index of Gaseous and Liquid Hydrogen

The refractive index of gaseous and liquid hydrogen has been measured by an interferometric method at temperatures between 15° and 298.15°K and at pressures up to 230 atm. The measurements have been analyzed in terms of the density and temperature dependence of the Lorentz–Lorenz function, L–L ≡ [(nλ2 − 1) / (nλ2 + 2)]ρ−1, where nλ is the refractive index at λ = 5462 Å and ρ is the fluid density in grams per cubic centimeter. The precision and reproducibility of L–L is better than 0.05% in most cases. L–L for gaseous parahydrogen first increases with increasing density to a maximum and then decreases to values below the low-density limit. L–L is also slightly temperature dependent; the low-density limit increases with increasing temperature; the maximum on the L–L isotherms decreases with increasing temperature. L–L for saturated liquid parahydrogen decreases with increasing density by about 0.1% at temperatures between 15° and 32°K. The difference in L–L for normal and parahydrogen is consistent with previous theoretical and experimental estimates of the molecular-polarizability difference.