Photodesorption of ices I: CO, N2, and CO2

Context. A longstanding problem in astrochemistry is how molecules can be maintained in the gas phase in dense inter- and circumstellar regions at temperatures well below their thermal desorption values. 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. This study aims to determine the UV photodesorption yields and to constrain the photodesorption mechanisms of three astro-chemically relevant ices: CO, N-2 and CO2. In addition, the possibility of co-desorption in mixed and layered CO: N-2 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 is monitored by reflection absorption infrared spectroscopy of the ice and simultaneous mass spectrometry of the desorbed molecules. Results. Both the UV photodesorption yield per incident photon and the photodesorption mechanism are highly molecule specific. The CO photodesorbs without dissociation from the surface layer of the ice, and N-2, which lacks a dipole allowed 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 N-2 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(+/- 1.3) x 10(-3) and <2 x 10(-4) molecules photon(-1) for pure CO and N-2, respectively. The CO2 photodesorption yield is 1.2(+/- 0.7) x 10(-3) x (1-e(-(x/2.9(+/- 1.1)))) + 1.1(+/- 0.7) x 10(-3) x (1-e(-(x/4.6(+/- 2.2)))) molecules photon-1, where x is the ice thickness in monolayers and the two parts of the expression represent a CO2 and a CO photodesorption pathway, respectively. At higher temperatures, the CO ice photodesorption yield decreases, while that of CO2 increases.