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Article Dans Une Revue Monthly Notices of the Royal Astronomical Society Année : 2015

Segregation of O2 and CO on the surface of dust grains determines the desorption energy of O2

Résumé

Selective depletion towards pre-stellar cores is still not understood. The exchangebetween the solid and gas phases is central to this mystery. The aim of this paper is to showthat the thermal desorption of O2 and CO from a submonolayer mixture is greatly affected bythe composition of the initial surface population. We have performed thermally programmeddesorption (TPD) experiments on various submonolayer mixtures of O2 and CO. Pure O2 andCO exhibit almost the same desorption behaviour, but their desorption differs strongly whenmixed. Pure O2 is slightly less volatile than CO, while in mixtures, O2 desorbs earlier thanCO. We analyse our data using a desorption law linking competition for binding sites withdesorption, based on the assumption that the binding energy distribution of both molecules isthe same. We apply Fermi–Dirac statistics in order to calculate the adsorption site populationdistribution, and derive the desorbing fluxes. Despite its simplicity, the model reproduces theobserved desorption profiles, indicating that competition for adsorption sites is the reason forlower temperature O2 desorption. CO molecules push-out or ‘dislodge’ O2 molecules from themost favourable binding sites, ultimately forcing their early desorption. It is crucial to considerthe surface coverage of dust grains in any description of desorption. Competition for accessto binding sites results in some important discrepancies between similar kinds of molecules,such as CO and O2. This is an important phenomenon to be investigated in order to develop abetter understanding of the apparently selective depletion observed in dark molecular clouds.
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hal-01948057 , version 1 (11-01-2019)

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J. Noble, S. Diana, F. Dulieu. Segregation of O2 and CO on the surface of dust grains determines the desorption energy of O2. Monthly Notices of the Royal Astronomical Society, 2015, 454 (3), pp.2636-2646. ⟨10.1093/mnras/stv2157⟩. ⟨hal-01948057⟩
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