  A longstanding problem in astrochemistry is how molecules can be maintained
in the gas phase in dense inter- and circumstellar 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. This paper aims 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.
The ice photodesorption is studied experimentally under ultra high vacuum
conditions and at 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. Both the UV photodesorption yields per incident photon and the
photodesorption mechanisms are molecule specific. CO photodesorbs without
dissociation from the surface layer of the ice. N2, which lacks an electronic
transition in this wavelength range, 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 2.7x10^-3 and <2x10^-4 molecules photon-1 for pure CO and N2,
respectively. The CO2 photodesorption yield is 1.2x10^-3x(1-e^(-X/2.9)) +
1.1x10^-3x(1-e^(-X/4.6)) molecules photon-1, where X is the ice thickness in
monolayers and the two parts of the expression represent a CO2 and CO
photodesorption pathway.
