Photodesorption of ices I : CO , N 2 and CO 2

Context. A long-standing problem in astrochemistry is how molecules can be maintained in the gas phase in dense interand circum-stellar regions. Photodesorption is a non-thermal desorption mechanism, which may explain the small amounts of observed cold gas in cloud cores and disk mid-planes. Aims. The aim is to determine the UV photodesorption yields and to constrain the photodesorption mechanisms 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. Methods. The UV photodesorption of ices is studied experimentally under ultra high vacuum conditions and at astrochemically relevant temperatures (15 – 60 K) using a hydrogen discharge lamp (7–10.5 eV). The ice desorption during irradiation is monitored by reflection absorption infrared spectroscopy of the ice and simultaneous mass spectrometry of the desorbed molecules. Results. Both the UV photodesorption yields per incident photon and photodesorption mechanisms are highly molecule specific. CO photodesorbs without dissociation from the surface layer of the ice. N2, which lacks an electronic transition in the wavelength range of the lamp, has a photodesorption yield that is more than an order of magnitude lower. This yield increases significantly due to co-desorption when N2 is mixed in with or layered on top of CO ice. CO2 photodesorbs through dissociation and subsequent recombination from the top 10 layers of the ice. At low temperatures (15 – 18 K) the derived photodesorption yields are 2.7 × 10 and < 2 × 10 molecules photon for pure CO and N2, respectively. The CO2 photodesorption yield is 1.2× 10 × (1− e)+ 1.1× 10 × (1− e) molecules photon, where x is the ice thickness in monolayers and the two parts of the expression represent a CO2 and CO photodesorption pathway.