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 H 2 O pressure from 1.9 × 10 −7 –9.5 × 10 −5 Torr using optical interference techniques. The refractive indices were measured for ice films deposited on a cooled, single‐crystal Al 2 O 3 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 ρ=0.93 g/cm³. Significantly lower refractive indices and densities were found at lower temperatures and higher H 2 O fluxes that were consistent with microporous ices. A refractive index of n=1.24 and density of ρ=0.68 g/cm³ were determined at 35 K and an H 2 O pressure of 4 × 10 −6 Torr. Above 35 K, the refractive index and density progressively increased with increasing temperature and decreasing H 2 O flux. The ice densities observed can be explained qualitatively using a ballistic deposition model. Estimates are also obtained for the surface diffusion of H 2 O on vapor‐deposited ice.