Studying comets is challenging because the main body of a comet is hidden within a coma — made up of gas and dust — produced by radiation from the Sun. Some gases are released directly from the nucleus through sublimation, and some are the product of photodissociation or reactions of those parent molecules. Hydrogen cyanide — HCN — (an acid) and ammonia — NH3 — (a base) have each been detected in cometary comae, and they are usually assumed to be parent molecules [e.g., 1, 2]. Ammonium cyanide — NH4CN — is easily formed on Earth by the acid-base reaction of HCN and NH3. In 2004, Gerakines et al. demonstrated that acid-base reactions can occur in low-temperature ices by the formation of NH4CN from HCN and NH3 at 18 K . Therefore, NH4CN may very likely be present on cometary nuclei, and may be involved in further chemical reactions both in nuclei and in comae, but there are a limited number of studies of NH4CN in relevant conditions. This study reports physical properties of pure crystalline NH4CN at 125 K. We have prepared pure crystalline NH4CN by vapor co-deposition of HCN with an excess of NH3 onto a cooled substrate held at 125 K in the center of a stainless-steel vacuum chamber. Transmission spectra of crystalline NH4CN in the near- and mid-infrared regions also were measured at 125 K (See Figure) to determine the band strengths and absorption coefficients of NH4+ and CN− infrared features. IR optical constants (n and k) at wavelengths from 2 to 20 μm (5000 to 500 cm−1) were also determined. We measured several physical properties of crystalline NH4CN at low temperatures (120-165 K), including visible refractive index (at a wavelength of 670 nm), mass density, vapor pressure, and enthalpy of sublimation. These properties are essential to experimental and theoretical studies, for example, in determining reaction efficiency or modeling IR spectra observed in space. Furthermore, this study is expected to contribute to the analyses of results from future missions to comets.
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