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 formation of NH 2 CHO, 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.