Cyanate ion in compact amorphous water ice

Abstract

The 4.62μm infrared (2164.5cm-1) absorption band, observed in ice mantels toward many young stellar objects, has been mostly attributed to the ν3 (CN stretch) band of OCN- ions. We present in this work a spectroscopic study of OCN- ions embedded in compact amorphous ice in a range of concentrations and temperatures relevant to astronomical observations together with quantum mechanical calculations of the ν3 band of OCN- in various H2O environments. The ice samples containing the ions are prepared through hyperquenching of liquid droplets of K+OCN- solutions on a substrate at 14 K. The ν3 OCN- band appears as a broad feature peaking at 4.64μm with a secondary maximum at 4.54μm and is much weaker than the corresponding peak in the liquid solution or in the solid salt. A similar weakening is observed for other OCN- absorption peaks at 7.66μm (2ν2) and 8.20μm (ν1). The theoretical calculations for the ν3 vibration lead to a range of frequencies spanning the experimentally observed width. This frequency spread could help explain the pronounced drop in the band intensity in the ice. The OCN - ν3 band in the present compact ices is also broader and much weaker than that reported in the literature for OCN- ions obtained by variously processing porous ice samples containing suitable neutral precursors. The results of this study indicate that the astronomical detection of OCN- in ice mantels could be significantly impaired if the ion is embedded in a compact water network. © 2012. The American Astronomical Society. All rights reserved.