tokens: molecules can be due to their reaction to form NH 4 SH in the deposited ice. The most intense band of NH 4 SH and other ammonium salts that may be present is the transition located at 1470 cm −1 . It is Unfortunate that this band overlaps with the methanol band in the same region and complicates the estimation of the column density of . Figure 3. IR spectra of H 2 S–CH 3 OH–NH 3 –H 2 O (3:3:3:1) deposited on a cold substrate (50 K) before (blue) and after (red) electron irradiation with a 10 keV electron beam (fluence = 2 × 10 21 eV cm −2 ). Figure 4. Left: Gaussian deconvolution for the 2556 cm −1 absorption corresponding to H 2 S. Band A corresponds to CH 3 OH, bands B and C correspond to H 2 S (Jiménez-Escobar et al. 2014 ), and band D corresponds to H 2 O. Right: Gaussian deconvolution for the 3367 cm −1 absorption corresponding to NH 3 . The red curve in Figure 3 shows the post-irradiation spectrum of the “with H 2 S” mixture. We note a significant decrease in band intensity of the initial constituents, and particularly a total disappearance of the H 2 S band. This is interpreted as complete destruction of the H 2 S molecules by electron irradiation. Among products of irradiation we observe the same molecules produced after irradiation of the “without H 2 S” mixture. Additionally, in the “with H 2 S” mixture alone, after irradiation an intense band appears at 2040 cm −1 . This band is assigned to the C = O stretching mode of OCS, which is observed at 2019 cm −1 in pure OCS ice and shifted to 2046 cm −1 in OCS-H 2 O and OCS-CO 2 mixtures (Ferrante et al. 2008 ). Previous experimental studies reported the formation of OCS after irradiation of ices containing CO or CO 2 as the carbon source and H 2 S or SO 2 as sulfur source (Ferrante et al. 2008 ; Chen et al. 2015 ). Our assignment of the 2040 cm −1 band to OCS is supported by the absence of this band in the infrared spectrum of the irradiated “without H 2 S” mixture, where no sulfur was present. OCS can be a product of reactions between a CO (which is produced from the dissociation of methanol as explained above) and an S atom or HS radical, as illustrated here: The column density of OCS after irradiation at 50 K (before heating) is 1.7 × 10 16 molecules cm −2 . This corresponds to a ratio of N (OCS)/ N (H 2 S) = 8.5%. In Figure 3 it is noteworthy that the absorption band at 1477 cm −1 assigned to methanol is in the same position as the strong absorption band of () (Howett et al. 2007 ). This makes it difficult to detect any production of the ammonium salt during the irradiation and heating of the sample. Thus, to assess the chemistry, we have performed an irradiation experiment using 15 N labeled ammonia. Figure 5 compares the infrared spectra of labeled and unlabeled mixtures after irradiation. We observe a shift of 16 cm −1 of the OCN − band in agreement with the isotopic shift 17 cm −1 measured in the matrix-isolated molecule (Maki Decius 1959 ). This confirms our assignment of this
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