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[46P.05] Laboratory IR Studies of Ion-Irradiated Ices Containing H₂O, CO₂ and NH₃

M. H. Moore (NASA's Goddard Space Flight Center), R. K. Khanna (University of Maryland)

Acid-base reactions in irradiated ices containing molecules such as H₂O, CO₂, and NH₃ are one scheme we are examining for the formation of species whose spectra could match the unidentified 6.8 micron interstellar ice feature in NGC 7548 IRS9 and RAFGL 7009S. Similar reactions could be important on the icy surfaces of some satellites. We have examined the reaction of NH₃ with a layer of carbonic acid, H₂CO₃, and identified the formation of carbamic acid. In this experiment, a layer of NH₃ is condensed at 18K and covered with a layer of H₂O + CO₂ ice. During irradiation, H₂CO₃ is formed in the H₂O + CO₂ ice layer (JGR 96, E2, 17,541, 1991). During slow warming the NH₃ diffuses through and reacts with the carbonic acid. The spectrum evolves into a stable set of features near 250 K. The features are identified as carbamic acid, NH₃COO (Spectrochimica Acta, accepted, 1998). The spectrum of carbamic acid is a good match with the 6.8 micron band, but since carbamic acid also contains a strong 7.7 micron band, it is not a good candidate for the interstellar feature. Additional ice experiments in which NH₃ is intimately mixed with H₂O and CO₂ result in a weaker 7.7 micron component. In progress are studies where similar reactions are done on a magnesium silicate surface.