VUV Processing of Nitrile Ice: Direct Comparison of Branching in Ice and TPD Spectra

The interplay between radiation chemistry and sublimation dynamics of condensed organic compounds on cold grains is fundamental to describe observed gas- and ice-phase molecular abundances in the interstellar medium (ISM). Infrared measurements are generally used to identify molecules synthesized in irradiated ices in laboratory experiments, while mass spectrometric techniques have been used to monitor the products following temperature-programmed desorption (TPD). The IR measurements are often used quantitatively to monitor the chemical transformation of ices during the course of irradiation, but the gas-phase methods applied with TPD generally do not permit quantitative branching determination. Here, we combine reflection–absorption infrared spectroscopy (RAIRS) of ices with broadband rotational spectroscopy of the sublimed products to study the branching of photoproducts produced by the VUV (120–160 nm) irradiation of condensed CH₃CN (methyl cyanide) and CH₃CH₂CN (ethyl cyanide) ices. This permits direct comparison between the ice-phase and gas-phase branching following temperature-programmed desorption (TPD). This comparison is analogous to astronomical observations of ices in protostellar disks, such as by the James Webb Space Telescope employed in conjunction with ALMA observations in the corresponding warm-up regions of the same objects. In the condensed CH₃CN VUV-processed ices, we quantified the HCN, CH₃NC (methyl isocyanide), H₂CCNH (ketenimine), CH₃NH₂ (methylamine), and CH₄ (methane) abundances. The CH₃CH₂CN ices also readily produced the corresponding isocyanide and HCN in addition to a significant yield of CH₂CHCN (vinyl cyanide). The ethyl cyanide ice produced (CH₃)HCCNH (methyl ketenimine) rather than CH₃NH₂, and no CH₄ formation was observed. In the gas phase, we detected the isocyanides, HCN, and CH₂CHCN. The relative abundances of photoproducts are normalized to the abundance of the isocyanide formed in both ices. Following irradiation of CH₃CN, the HCN:CH₃NC ratio was found to be 0.5 ± 0.1 and 1.5 ± 0.1 in the ice and gas phase, respectively. The HCN:CH₃CH₂NC ratios were 1.7 ± 0.2 in the ice phase and 5.7 ± 0.4 in the gas phase for CH₃CH₂CN. The CH₂CHCN:CH₃CH₂NC ratio for ice and gas phases was found to be 0.9 ± 0.1 and 2.5 ± 0.5, respectively. The ice- and gas-phase relative abundances could all be brought into agreement if the unknown IR band strength of the isocyanide C–N stretch is assumed to be ∼3 times larger than that of the cyanides.