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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.