Carbamic acid produced by the UV/EUV irradiation of interstellar ice analogs

¹ Department of Physics, National Central University, No. 300, Jhongda Rd, Jhongli City, Taoyuan County 32054, Taiwan e-mail: s9222002@cc.ncu.edu.tw; mnuevo@astro.ncu.edu.tw
² Graduate Institute of Astronomy, National Central University, No. 300, Jhongda Rd, Jhongli City, Taoyuan County 32049, Taiwan
³ National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan
⁴ Space Sciences Center and Department of Physics and Astronomy, University of Southern California, Los Angeles, CA 90089-1341, USA

Received: 24 October 2006
Accepted: 14 December 2006

Abstract

Context.Carbamic acid (NH₂COOH) is the smallest amino acid, smaller than the smallest proteinaceous amino acid glycine. This compound has never been observed in the interstellar medium (ISM). Previous experiments where ice mixtures containing H₂O, CO₂ and NH₃ were subjected to 1-MeV proton bombardment showed that carbamic acid is formed in a stable zwitterionic (NHCOO^-) form.

Aims.In the present work, we have carried out irradiations of ice mixtures containing H₂O, ¹²CO₂/¹³CO₂ and NH₃ with ultraviolet (UV)/extreme ultraviolet (EUV) photons provided by a synchrotron source in the 4–20 eV range, and compared the results with those obtained for energetic protons.

Methods.Infrared (IR) spectroscopy and mass spectrometry were used to identify the formed photo-products and monitor their evolution in the ices at 15 K and during the warming up to room temperature in the formed residues.

Results.We identified the IR absorption features of HNCO, OCN^-, CO, NH and NH₂CHO at low temperature in the ices, and features assigned to carbamic acid in the residues around 250 K. Finally, we conclude that under our experimental conditions, unlike what was obtained after bombardment with energetic protons, carbamic acid may be formed in the neutral form, and propose some photochemical pathways leading to its formation.