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Title: Indirect ultraviolet photodesorption from CO:N{sub 2} binary ices — an efficient grain-gas process

Ultraviolet (UV) ice photodesorption is an important non-thermal desorption pathway in many interstellar environments that has been invoked to explain observations of cold molecules in disks, clouds, and cloud cores. Systematic laboratory studies of the photodesorption rates, between 7 and 14 eV, from CO:N{sub 2} binary ices, have been performed at the DESIRS vacuum UV beamline of the synchrotron facility SOLEIL. The photodesorption spectral analysis demonstrates that the photodesorption process is indirect, i.e., the desorption is induced by a photon absorption in sub-surface molecular layers, while only surface molecules are actually desorbing. The photodesorption spectra of CO and N{sub 2} in binary ices therefore depend on the absorption spectra of the dominant species in the sub-surface ice layer, which implies that the photodesorption efficiency and energy dependence are dramatically different for mixed and layered ices compared with pure ices. In particular, a thin (1-2 ML) N{sub 2} ice layer on top of CO will effectively quench CO photodesorption, while enhancing N{sub 2} photodesorption by a factor of a few (compared with the pure ices) when the ice is exposed to a typical dark cloud UV field, which may help to explain the different distributions of CO and N{sub 2}H{sup +}more » in molecular cloud cores. This indirect photodesorption mechanism may also explain observations of small amounts of complex organics in cold interstellar environments.« less
Authors:
; ; ; ; ;  [1] ; ;  [2] ;  [3] ;  [4]
  1. Laboratoire de Physique Moléculaire pour l'Atmosphère et l'Astrophysique (LPMAA), CNRS UMR 7092, UPMC Univ. Paris 6, F-75252 Paris (France)
  2. Sackler Laboratory for Astrophysics, Leiden Observatory, Leiden University, P.O. Box 9513, NL-2300-RA Leiden (Netherlands)
  3. Laboratoire de Chimie Physique (LCP), CNRS UMR 8000, Univ. Paris Sud 11, F-91400 Orsay (France)
  4. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States)
Publication Date:
OSTI Identifier:
22348456
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 779; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ABSORPTION; ABSORPTION SPECTRA; ABUNDANCE; CARBON MONOXIDE; COMPARATIVE EVALUATIONS; DESORPTION; DISTRIBUTION; EFFICIENCY; ENERGY DEPENDENCE; EV RANGE; ICE; LAYERS; MOLECULES; NITROGEN; PHOTONS; SURFACES; ULTRAVIOLET RADIATION