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The astronomical detection of formamide (NH$_2$CHO) toward variousstar-forming regions and in cometary material implies that the simplest amidemight have an early origin in dark molecular clouds at low temperatures.Laboratory studies have proven the efficient NH$_2$CHO formation ininterstellar CO:NH$_3$ ice analogs upon energetic processing. However, it isstill under debate, whether the proposed radical-radical recombinationreactions forming complex organic molecules remain valid in an abundant H$_2$Oenvironment. The aim of this work was to investigate the formation of NH$_2$CHOin 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 ultravioletphotons in an ultra-high vacuum chamber at 10 K. Fourier-transform infraredspectroscopy was utilized to monitor in situ the initial and newly formedspecies as a function of photon fluence. The infrared spectral identificationsare complementarily secured by a temperature-programmed desorption experimentcombined with a quadrupole mass spectrometer. The energetic processing ofCO:NH$_3$ ice mixtures mainly leads to the NH$_2$CHO formation, along with itschemical derivatives such as isocyanic acid (HNCO) and cyanate ion (OCN$^-$).The formation kinetics of NH$_2$CHO shows an explicit dependency on ice ratiosand compositions; the highest yield is found in H$_2$O-rich ice. Theastronomical relevance of the resulting reaction network is discussed.

astrophysical journal/the astrophysical journal

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