Investigation of orientation effects in films of nitric acid trihydrate

Abstract

An investigation of orientation effects in films of nitric acid trihydrate (NAT) is presented, based on a systematic study of reflection–absorption infrared spectra of samples of varying thickness. The samples are prepared by vapor deposition on an Al substrate at 90 K and annealing at 175 K to produce crystalline α-NAT films. Spectra were recorded at a grazing angle of 75°, with polarized radiation. The observed spectra are compared with predictions of a classical Fresnel model, in order to test the available optical indices of NAT, which are of great importance for the accurate interpretation of data from remote sensing measurements. Whereas a similar procedure yielded satisfactory results for water ice, it is found that the agreement is not acceptable for NAT crystals. An explanation is suggested in terms of anisotropic or birefringent properties of the crystals, or as due to a preferential alignment of the films. The infrared activity of a peak at ≈1140 cm⁻¹, which had previously been associated to changes in the crystal structure upon heating, is here explained in terms of a rearrangement of the polycrystals in the sample.