Ultraviolet optical constants of water and ammonia ices

Abstract

Real refractive indices for amorphous and hexagonal water ices were measured between 1610 and 3200 Å, and for amorphous ammonia ice between 1925 and 3200 Å. Upper limits on the absorption coefficients for these ices were established from extinction-coefficient measurements made at 1475 Å for water ices and at 1760 Å for amorphous ammonia ice. Amorphous ices were deposited from the gaseous phase onto a gold-coated sapphire substrate that was cooled to 77 K. Hexagonal water ice was formed by deposition at 155 K. The ice films were illuminated with monochromatic light at two angles of incidence. Interference-fringe periods, which were observed as the film thickness increased at a constant rate, were used to determine the real refractive index of the film. Damping of the fringe amplitude, as the film thickness increased, was used to establish the extinction coefficients.