Determination of physical and infrared absorption properties of crystalline NH4CN at low temperatures relevant to comets

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 [3]. 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.

1: W. F. Huebner, L. E. Snyder, D. Buhl, 1974, Icarus 23: 580-584 ; 2: S. Tegler, S. Wyckoff, 1989, The Astrophysical Journal 343: 445-449 ; 3: P. A. Gerakines, M. H. Moore., R. L. Hudson, 2004, Icarus 170: 202-213