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[57.02] IR Spectra of Isocyanic Acid (HNCO) and its Reaction Product with Ammonia: Possible Identification of the 4.6 and 6.8 Micron Absorption Features of Several Protostellar Objects

M.S. Lowenthal, R.K. Khanna (Univ. of Maryland, College Park), M.H. Moore (NASA/Goddard Space Flight Center)

Strong absorption features are found at 4.6 microns (2170 cm^-1) and 6.8 microns (1470 cm^-1) in the spectra of several protostellar objects (e.g. W33A, NGC 7538 IRS9, RAFGL 7009S). Ices with similar features may have been present in the early solar system. Realistic candidates responsible for the 6.8 micron band include CH₃OH, and/or ions such as CO₃⁼, HCO₃^-, and NH₄⁺. The 4.6 micron feature is due to the absorption of the cyanate ion (OCN^-). We have synthesized and obtained the infrared spectrum of pure, solid HNCO and measured its intrinsic band strength. We have also investigated the IR spectrum of HNCO covered with a layer of H₂O and NH₃ at low temperatures (~10K). During warm-up to ~100K, an acid-base reaction occurs and new peaks develop around 1470 cm^-1 and 2170 cm^-1 which are due to NH₄⁺ and OCN^- ions, respectively. Irradiation of solid HNCO shows evidence of OCN^- formation at low temperatures. The conditions under which NH₄OCN converts into urea ((NH₂)₂CO) will be discussed. Ammonium cyanate is a good candidate for the source of the 4.6 and 6.8 micron interstellar features and may also exist in cometary ices.