The desorption of H 2CO from interstellar grains analogues

J. A. Noble, P. Theule, F. Mispelaer, F. Duvernay, G. Danger, E. Congiu, F. Dulieu, T. Chiavassa

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60 Scopus citations

Abstract

Context. Much of the formaldehyde (H 2CO) is formed from the hydrogenation of CO on interstellar dust grains, and is released in the gas phase in hot core regions. Radio-astronomical observations in these regions are directly related to its desorption from grains. Aims. We study experimentally the thermal desorption of H 2CO from bare silicate surfaces, from water ice surfaces and from bulk water ice in order to model its desorption from interstellar grains. Methods. Temperature-programmed desorption experiments, monitored by mass spectrometry, and Fourier transform infrared spectroscopy are performed in the laboratory to determine the thermal desorption energies in: (i.) the multilayer regime where H 2CO is bound to other H 2CO molecules; (ii.) the submonolayer regime where H 2CO is bound on top of a water ice surface; (iii.) the mixed submonolayer regime where H 2CO is bound to a silicate surface; and (iv.) the multilayer regime in water ice, where H 2CO is embedded within a H 2O matrix. Results. In the submonolayer regime, we find the zeroth-order desorption kinetic parameters ν 0 = 10 28 mol cm -2 s -1 and E = 31.0 +-0.9 kJ mol -1 for desorption from an olivine surface. The zeroth-order desorption kinetic parameters are ν 0 = 10 28 mol cm -2 s -1 and E = 27.1 +-0.5 kJ mol -1 for desorption from a water ice surface in the submonolayer regime. In a H 2CO:H 2O mixture, the desorption is in competition with the H 2CO + H 2O reaction, which produces polyoxymethylene, the polymer of H 2CO. This polymerization reaction prevents the volcano desorption and co-desorption from happening. Conclusions. H 2CO is only desorbed from interstellar ices via a dominant sub-monolayer desorption process (E = 27.1 +-0.5 kJ mol -1). The H 2CO which has not desorbed during this sub-monolayer desorption polymerises upon reaction with H 2O, and does not desorb as H 2CO at higher temperature.

Original languageEnglish
Article numberA5
JournalAstronomy and Astrophysics
Volume543
DOIs
StatePublished - 2012
Externally publishedYes

Keywords

  • Astrochemistry
  • ISM: molecules
  • Molecular data
  • Molecular processes

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