Carbamic acid and carbamate formation in NH:CO ices – UV irradiation versus thermal processes

Université de Provence, Laboratoire de Physique des Interactions Ioniques et Moléculaires Centre de St-Jérôme, Avenue Escadrille Normandie-Niémen, 13397 Marseille, France e-mail: fabrice.duvernay@univ-provence.fr

Received: 7 July 2008
Accepted: 3 October 2008

Abstract

Context. We study carbamic acid [ NH₂COOH] and ammonium carbamate [ NH₂COO^-] [ NH₄⁺] formation in interstellar ice analogs.

Aims. We demonstrate how carbamic acid [ NH₂COOH] and ammonium carbamate [ NH₂COO^-] [ NH₄⁺] can be formed from both thermal reactions and energetic photons in an NH₃:CO₂ ice mixture.

Methods. Infrared and mass spectroscopy are used to monitor NH₃:CO₂ 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 NH₃ and CO₂ 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.