Formation of the simplest amide in molecular clouds: formamide (NH${}_2$CHO) and its derivatives in H${}_2$O-rich and CO-rich interstellar ice analogs upon VUV irradiation

Abstract

The astronomical detection of formamide (NH${}_2$CHO) toward various star-forming regions and in cometary material implies that the simplest amide might have an early origin in dark molecular clouds at low temperatures. Laboratory studies have proven the efficient NH${}_2$CHO formation in interstellar CO:NH${}_3$ ice analogs upon energetic processing. However, it is still under debate, whether the proposed radical-radical recombination reactions forming complex organic molecules remain valid in an abundant H${}_2$O environment. The aim of this work was to investigate the formation of NH${}_2$CHO in H${}_2$O- and CO-rich ices under conditions prevailing in molecular clouds. Therefore, different ice mixtures composed of H${}_2$O:CO:NH${}_3$ (10:5:1), CO:NH${}_3$ (4:1), and CO:NH${}_3$ (0.6:1) were exposed to vacuum ultraviolet photons in an ultra-high vacuum chamber at 10 K. Fourier-transform infrared spectroscopy was utilized to monitor in situ the initial and newly formed species as a function of photon fluence. The infrared spectral identifications are complementarily secured by a temperature-programmed desorption experiment combined with a quadrupole mass spectrometer. The energetic processing of CO:NH${}_3$ ice mixtures mainly leads to the NH${}_2$CHO formation, along with its chemical derivatives such as isocyanic acid (HNCO) and cyanate ion (OCN${}^{-}$). The formation kinetics of NH${}_2$CHO shows an explicit dependency on ice ratios and compositions; the highest yield is found in H${}_2$O-rich ice. The astronomical relevance of the resulting reaction network is discussed.

16 pages, 6 figures, 2 tables

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