Carbamic acid and carbamate formation in NH$_{\sf 3}$:CO$_{\sf 2}$ ices – UV irradiation versus thermal processes

Abstract

Context. We study carbamic acid [NH 2 COOH] and ammonium carbamate [NH 2 COO - ][NH4 + ] formation in interstellar ice analogs. Aims. We demonstrate how carbamic acid [NH 2 COOH] and ammonium carbamate [NH 2 COO - ][NH 4 + ] can be formed from both thermal reactions and energetic photons in an NH 3 :CO 2 ice mixture. Methods. Infrared and mass spectroscopy are used to monitor NH 3 :CO 2 ice mixture evolution during both warming and VUV photon irradiation. Results. Carbamic acid and ammonium carbamate can be produced thermally in a 1:1 ratio from NH 3 and CO 2 above 80 K. They can be also formed in a 28:1 ratio by less efficient processes such as energetic photons. Our study and its results provide fresh insight into carbamic acid formation in interstellar ices. Conclusions. We demonstrate that care is required to separate irradiation-induced reactivity from purely thermal reactivity in ices in which ammonia and carbon dioxide are both present. From an interstellar chemistry point of view, carbamic acid and ammonium carbamate are readily produced from the ice mantle of a typical interstellar grain and should therefore be a detectable species in molecular clouds.