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The abundance of H 2O on the grains is thus intrinsically linked to the amount of O 2 ice, which is itself regulated by non-thermal desorption mechanisms at low temperatures. O 2 hydrogenation has been proposed as a starting point for water formation on grains by Tielens & Hagen (1982) and proven experimentally to be effective by many studies (e.g., Miyauchi et al. Molecular oxygen plays a role in water formation in the gas phase after dissociation into atomic oxygen but also in the solid phase ( Hollenbach et al. In the gas phase, the N 2 derivative N is a major tracer of dense cold cores (e.g., Caselli et al. For instance, the presence of N 2 in the solid state leads to the formation of NH 3 (e.g., Aikawa et al. Two of these species are N 2 and O 2 because of their role in the formation of N- and O-bearing molecules in the gas phase and in the solid state. Precise measurements are especially important for abundant molecules that can play key roles in gas-grain chemical networks. Experimental measurements of the efficiencies of these processes are a prerequisite to understand which of these pathways must be considered to predict molecular gas abundances. Non-thermal desorption in the ISM is induced by cosmic-rays, exothermic reactions, shocks, electrons, and photon irradiation. 2010) and in protoplanetary disks (e.g., Willacy & Langer 2000 Dominik et al. 2012), in protostellar envelopes ( Kristensen et al. Non-thermal desorption has been proposed to take place in star-forming regions at every evolutionary stage to explain observations of cold gas: in prestellar cores ( Caselli et al. In regions where heating can be neglected, non-thermal desorption pathways are required to explain the presence of molecules in the gas phase that, without such mechanisms, should remain frozen. Constraining these desorption mechanisms is crucial since they bridge solid state and gas phase chemistry in space. Upon heating or non-thermal desorption, these molecules are released into the gas phase. In the cold and dense regions of the interstellar medium (ISM), molecules condense onto the surfaces of submicron-sized dust grains. Key words: astrochemistry / ISM: abundances / ISM: molecules / molecular data / molecular processes Rates vary between 10 -3 and 10 -2 photodesorbed molecules per incoming photon. Photodesorption rates of N 2 and O 2 integrated over the far-UV field from various star-forming environments are lower than for CO. In contrast, O 2 photodesorption in the 7−13.6 eV range occurs through dissociation and presents no vibrational structure.Ĭonclusions. The observed vibronic structure in the N 2 photodesorption spectrum, together with the absence of N 3 formation, supports that the photodesorption mechanism of N 2 is similar to CO, i.e., an indirect DIET (Desorption Induced by Electronic Transition) process without dissociation of the desorbing molecule.
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N 2 photodesorption mainly occurs through excitation of the b 1Π u state and subsequent desorption of surface molecules. Strategic N 2 and O 2 isotopolog mixtures are used to investigate the importance of dissociation upon irradiation. Absolute rates are calculated by using the well-calibrated CO photodesorption rates. Photodesorption of molecules is monitored through quadrupole mass spectrometry. Tunable radiation from the DESIRS beamline at the SOLEIL synchrotron in the astrophysically relevant 7 to 13.6 eV range is used to irradiate pure N 2 and O 2 thin ice films.
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Measurements of the wavelength-dependent photodesorption rates of these two infrared-inactive molecules provide astronomical and physical-chemical insights into the conditions required for their photodesorption. N 2 and O 2 are expected to play key roles in astrochemical reaction networks, both in the solid state and in the gas phase. This non-thermal desorption mechanism should be especially important where UV fluxes are high.Īims. Ultraviolet photodesorption of molecules from icy interstellar grains can explain observations of cold gas in regions where thermal desorption is negligible. Laboratoire de Chimie Physique, UMR 8000 CNRS-Universitéĭepartments of Chemistry and Astronomy, University ofĬontext. The de Physique Moléculaire pour l’Atmosphère et Sackler Laboratory for Astrophysics, Leiden Observatory, Leiden Astronomical objects: linking to databasesĮ.Including author names using non-Roman alphabets.Suggested resources for more tips on language editing in the sciences Punctuation and style concerns regarding equations, figures, tables, and footnotes