2012ApJ...759...90M

The 4.62 µm infrared (2164.5/cm) 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 H₂ O 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 ν₃OCN^– band appears as a broad feature peaking at 4.64 µm with a secondary maximum at 4.54 µm 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 7.66 µm (2ν₂) and 8.20 µm (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.