AstrophysicsOrigin of the 6.85 µm band near young stellar objects:The ammonium ion (NH matches two absorption features that are observed towards embedded young stellar objects, i.e., the strong 6.85 µm feature and the 3.26 µm feature. The characteristic redshift with temperature of the interstellar 6.85 µm feature is well reproduced. The abundance of NH + 4 in interstellar ices would be typically 10% relative to H 2 O. The experiments show that the counterions produce little distinct spectral signature but rather a pseudo-continuum if a variety of them is present in a H 2 O dominated environment. The anions could therefore go undetected in IR spectra of interstellar ice. In the ISM, where additional mechanisms such as surface chemistry and additional elements such as sulfur are available many acids and an even wider variety of anions could be produced. These components may be detectable once the ices sublime, e.g., in hot cores.

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The effects of proton irradiation on mixed H,O + COz (1:l) ices at 20 K were investigated by infrared and mass spectroscopy. Infrared bands due to several radical (HCO, CO,) and molecular (CO) product species were identified. In addition, several new broad and complex i.r. features were observed. On slow warming, the broad features evolved into a 215-250 K residual film whose absorptions have been tentatively assigned to carbonic acid. This identification agrees with the spectral data for irradiated H,O+ r3COz ice and the results of an approximate normal coordinate analysis.