Photodesorption of CO Ice

At the high densities and low temperatures found in star-forming regions, all molecules other than H2 should stick on dust grains on timescales shorter than the cloud lifetimes. Yet these clouds are detected in the millimeter lines of gaseous CO. At these temperatures, thermal desorption is negligible, and hence a nonthermal desorption mechanism is necessary to maintain molecules in the gas phase. Here the first laboratory study of the photodesorption of pure CO ice under ultra-high vacuum conditions is presented, which gives a desorption rate of CO molecules per UV (7-10.5 eV) photon at 15 K. This rate is factors of larger than previously estimated and is comparable to estimates of other nonthermal desorption rates. The experiments constrain the mechanism to a single photon desorption process of ice surface molecules. The measured efficiency of this process shows that the role of CO photodesorption in preventing total removal of molecules in the gas has been underestimated.