Morphological study into the temperature dependence of solid ammonia under astrochemical conditions using vacuum ultraviolet and Fourier-transform infrared spectroscopy

Abstract

The authors present the results of a morphological study of solid ammonia using both Fourier-transform infrared and vacuum ultraviolet (VUV) spectroscopy. Dramatic changes in the VUV and infrared spectra at temperatures between 65 and 85 K provide a deeper insight into the structure of ammonia ice particularly with the observation of an exciton transition at 194 nm (6.39 eV) in the VUV spectrum, revealing a structure that is composed of crystallites. A complementary structure is observed in the IR spectrum at 1100 cm(-1) which is assigned to the symmetric deformation of ammonia molecules at the surfaces of the crystallites. Such spectral signatures may be used to identify the environment within which the ammonia ice is formed and provide a new route for obtaining information on the physical and chemical conditions occurring within the interstellar medium, on the surfaces of planetary bodies, and in Kuiper belt objects.

OPTICAL CONSTANTS OF NH3AND NH3:N2AMORPHOUS ICES IN THE NEAR-INFRARED AND MID-INFRARED REGIONS

“…Ices grown at T < 50 K, like in our experiments, are amorphous. When these ice samples are heated above the crystallization temperature (80 K), the ν 2 and ν 3 bands of NH 3 split into several peaks, which were associated with a polycrystalline sample with crystallites of random sizes and shapes by Dawes et al (2007). This effect is reproduced for our pure NH 3 ice samples when warmed to 85 K (see Figure 7, bottom panels, black line).…”

“…Nevertheless, the relative uncertainty between measurements is well below these values, which allows information on peak positions and band shapes to be extracted. It is interesting to appreciate the particular behavior of the band at ∼ 4479 cm −1 (ν 2 + ν 3 ) that develops a narrow peak in the NH 3 ice at 30 K, a feature that is not present in any other ices grown at 15 K. This narrow peak starts to appear for ice thicknesses above 1 μm and is believed to be related to morphology effects, as pointed out by other authors (Dawes et al 2007;see below). For the ice mixture, the main difference compared with pure ice is the appearance of some structure in the ∼ 5000 cm −1 band and a shift of the main peak of the ∼ 6515 cm −1 absorption.…”

“…The refractive index n 0 can be estimated from the intensity ratio between the maxima and the minima of the interference pattern collected by the photodiode (Goodman 1978). For amorphous NH 3 ice, there are some n 0 values given in the literature that range from 1.37 (Wood & Roux 1982; 20 K) to 1.44 (Dawes et al 2007;25 K). From our experiments for pure NH 3 ices at 15 K and 30 K, we estimated n 0 values of 1.36 ± 0.01 and 1.38 ± 0.01, respectively, and 1.26 ± 0.01 for the NH 3 :N 2 (1:1.7) mixture at 15 K. As far as we know, no previous n 0 values have been published for NH 3 :N 2 mixtures.…”

“…It is well established that an amorphous phase of ammonia is obtained when samples are prepared below 50 K and that a crystalline phase (polycrystalline) is obtained when heating above 80 K. Dawes et al (2007) conducted a detailed study on the morphology and phases of solid ammonia films based on vacuum ultraviolet and IR spectroscopy and concluded that the IR spectrum of a polycrystalline ice film is largely influenced by the size and shape of the crystalline domains obtained, which in turn depend strongly on the generation conditions. Ices grown at T < 50 K, like in our experiments, are amorphous.…”

“…The first study was conducted by Reding & Hornig (1951), who recorded and assigned the MIR spectrum of crystalline ammonia at 83 K. In the 1970s and 1980s, other works reported IR spectra of the different phases of ammonia (Robertson et al 1975;Pipes et al 1978). Some discrepancies in the assignments of the IR spectra to different phases are still open to debate, as revealed in the works of Holt et al (2004), Zheng & Kaiser (2007), and Dawes et al (2007).…”