Refractive index and density of vapor-deposited ice

The refractive indices of vapor-deposited ice films were measured versus substrate temperature from 35-140 K and H2O pressure from 1.9 x 10(-7) - 9.5 x 10(-5) Torr using optical interference techniques. The refractive indices were measured for ice films deposited on a cooled, single-crystal Al2O3 substrate in vacuum. Ice densities were calculated from these refractive indices using the Lorentz-Lorenz relationship. Above 120 K, the ice films had a refractive index of n=1.31 and a density of rho=0.93 g/cm(3). Significantly lower refractive indices and densities were found at lower temperatures and higher H2O fluxes that were consistent with microporous ices. A refractive index of n=1.24 and density of rho=0.68 g/cm(3) were determined at 35 K and an H2O pressure of 4 x 10(-6) Torr. Above 35 K, the refractive index and density progressively increased with increasing temperature and decreasing H2O flux. The ice densities observed can be explained qualitatively using a ballistic deposition model. Estimates are also obtained for the surface diffusion of H2O on vapor-deposited ice.