Context. Carbamic acid (NH 2 COOH) 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 H 2 O, CO 2 and NH 3 were subjected to 1-MeV proton bombardment showed that carbamic acid is formed in a stable zwitterionic (NHAims. In the present work, we have carried out irradiations of ice mixtures containing H 2 O, 12 CO 2 / 13 CO 2 and NH 3 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 + 4 and NH 2 CHO 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.
The design, construction and commissioning of a beamline and spectrometer for inelastic soft X-ray scattering at high resolution in a highly efficient system are presented. Based on the energy-compensation principle of grating dispersion, the design of the monochromator-spectrometer system greatly enhances the efficiency of measurement of inelastic soft X-rays scattering. Comprising two bendable gratings, the set-up effectively diminishes the defocus and coma aberrations. At commissioning, this system showed results of spin-flip, d-d and charge-transfer excitations of NiO. These results are consistent with published results but exhibit improved spectral resolution and increased efficiency of measurement. The best energy resolution of the set-up in terms of full width at half-maximum is 108 meV at an incident photon energy tuned about the Ni L3-edge.
Carbonyl sulfide (OCS) is a key molecule in astrobiology that acts as a catalyst in peptide synthesis by coupling amino acids. Experimental studies suggest that hydrogen sulfide (H 2 S), a precursor of OCS, could be present in astrophysical environments. In the present study, we used a microwave-discharge hydrogen-flow lamp, simulating the interstellar UV field, and a monochromatic synchrotron light beam to irradiate CO:H 2 S and CO 2 :H 2 S ice mixtures at 14 K with vacuum ultraviolet (VUV) or extreme ultraviolet (EUV) photons in order to study the effect of the photon energy and carbon source on the formation mechanisms and production yields of S-containing products (CS 2 , OCS, SO 2 , etc.). Results show that (1) the photo-induced OCS production efficiency in CO:H 2 S ice mixtures is higher than that of CO 2 :H 2 S ice mixtures; (2) a lower concentration of H 2 S enhances the production efficiency of OCS in both ice mixtures; and (3) the formation pathways of CS 2 differ significantly upon VUV and EUV irradiations. Furthermore, CS 2 was produced only after VUV photoprocessing of CO:H 2 S ices, while the VUV-induced production of SO 2 occurred only in CO 2 :H 2 S ice mixtures. More generally, the production yields of OCS, H 2 S 2 , and CS 2 were studied as a function of the irradiation photon energy. Heavy S-bearing compounds were also observed using mass spectrometry during the warm-up of VUV/EUV-irradiated CO:H 2 S ice mixtures. The presence of S-polymers in dust grains may account for the missing sulfur in dense clouds and circumstellar environments.
By using a pump-probe technique, we have observed the nascent rotational population distribution of LiH (v=0) in the Li (2 2PJ) with a H2 reaction, which is endothermic by 1680 cm−1. The LiH (v=0) distribution yields a single rotational temperature at ∼770 K, but the population in the v=1 level is not detectable. According to the potential energy surface (PES) calculations, the insertion mechanism in (near) C2v collision geometry is favored. The Li (2 2PJ)–H2 collision is initially along the 2A′ surface in the entrance channel and then diabatically couples to the ground 1A′ surface, from which the products are formed. From the temperature dependence measurement, the activation energy is evaluated to be 1280±46 cm−1, indicating that the energy required for the occurrence of the reaction is approximately the endothermicity. As Li is excited to higher states (3 2S or 3 2P), we cannot detect any LiH product. From a theoretical point of view, the 4A′ surface, correlating with the Li 3 2S state, may feasibly couple to a repulsive 3A′ surface, from which the collision complex will rapidly break apart into Li (2 2PJ) and H2. The probability for further surface hopping to the 2A′ or 1A′ surfaces is negligible, since the 3A′ and 2A′ surfaces are too far separated to allow for an efficient coupling. The Li (3 2P) state is expected to behave similarly. The observation also provides indirect evidence that the harpoon mechanism is not applicable to this system.
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