2010A&A...522A.108M

We present the novel InterStellar Astrochemistry Chamber (ISAC), designed for studying solids (ice mantles, organics, and silicates) in interstellar and circumstellar environments: characterizing their physico-chemical properties and monitoring their evolution as caused by (i) vacuum-UV irradiation; (ii) cosmic ray irradiation; and (iii) thermal processing. Experimental study of thermal and photodesorption of the CO ice reported here simulates the freeze-out and desorption of CO on grains, providing new information on these processes. ISAC is an UHV set-up, with base pressure down to P=2.5x10^–11mbar, where an ice layer is deposited at 7K and can be UV-irradiated. The evolution of the solid sample was monitored by in situ transmittance FTIR spectroscopy, while the volatile species were monitored by QMS. The UHV conditions of ISAC allow experiments under extremely clean conditions. Transmittance FTIR spectroscopy coupled to QMS proved to be ideal for in situ monitoring of ice processes that include radiation and thermal annealing. Thermal desorption of CO starting at 15K, induced by the release of H₂ from the CO ice, was observed. We measured the photodesorption yield of CO ice per incident photon at 7, 8, and 15K, respectively yielding 6.4±0.5x10^–2, 5.4±0.5x10^–2, and 3.5±0.5x10^–2 CO molecules photon (7.3-10.5eV)^–1. Our value of the photodesorption yield of CO ice at 15K is about one order of magnitude higher than the previous estimate. We confirmed that the photodesorption yield is constant during irradiation and independent of the ice thickness. Only below ∼5 monolayers ice thickness the photodesorption rate decreases, which suggests that only the UV photons absorbed in the top 5 monolayers led to photodesorption. The measured CO photodesorption quantum yield at 7K per absorbed photon in the top 5 monolayers is 3.4 molecules/photon. Experimental values were used as input for a simple model of a quiescent cloud interior. Photodesorption seems to explain the observations of CO in the gas phase for densities below 3-7x10⁴cm-3. For the same density of a cloud, 3x10⁴cm-3, thermal desorption of CO is not triggered until T=14.5K. This has important implications for CO ice mantle build up in dark clouds.