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Abstract: A longstanding problem in astrochemistry is how molecules can be maintainedin the gas phase in dense inter- and circumstellar regions. Photodesorption isa non-thermal desorption mechanism, which may explain the small amounts ofobserved cold gas in cloud cores and disk mid-planes. This paper aims todetermine the UV photodesorption yields and to constrain the photodesorptionmechanisms of three astrochemically relevant ices: CO, N2 and CO2. In addition,the possibility of co-desorption in mixed and layered CO:N2 ices is explored.The ice photodesorption is studied experimentally under ultra high vacuumconditions and at 15-60 K using a hydrogen discharge lamp 7-10.5 eV. The icedesorption during irradiation is monitored by reflection absorption infraredspectroscopy of the ice and simultaneous mass spectrometry of the desorbedmolecules. Both the UV photodesorption yields per incident photon and thephotodesorption mechanisms are molecule specific. CO photodesorbs withoutdissociation from the surface layer of the ice. N2, which lacks an electronictransition in this wavelength range, has a photodesorption yield that is morethan an order of magnitude lower. This yield increases significantly due toco-desorption when N2 is mixed in with or layered on top of CO ice. CO2photodesorbs through dissociation and subsequent recombination from the top 10layers of the ice. At low temperatures 15-18 K the derived photodesorptionyields are 2.7x10^-3 and <2x10^-4 molecules photon-1 for pure CO and N2,respectively. The CO2 photodesorption yield is 1.2x10^-3x1-e^-X-2.9 +1.1x10^-3x1-e^-X-4.6 molecules photon-1, where X is the ice thickness inmonolayers and the two parts of the expression represent a CO2 and COphotodesorption pathway.



Autor: Karin I. Oberg, Ewine F. van Dishoeck, Harold Linnartz

Fuente: https://arxiv.org/







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