Photodesorption of CO ice

TL;DR

This study provides the first laboratory measurement of CO ice photodesorption rates under ultra high vacuum, revealing a higher efficiency than previously estimated, which impacts understanding of molecular presence in star-forming regions.

Contribution

It presents the first experimental measurement of CO ice photodesorption rates, demonstrating a significantly higher efficiency than prior estimates and constraining the desorption mechanism.

Findings

Photodesorption rate of 3E-3 CO molecules per UV photon at 15 K.

Desorption rate is 100 to 100,000 times larger than previous estimates.

Single photon surface molecule desorption mechanism is supported.

Abstract

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 non-thermal 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 is presented, which gives a desorption rate of 3E-3 CO molecules per UV (7-10.5 eV) photon at 15 K. This rate is factors of 1E2-1E5 larger than previously estimated and is comparable to estimates of other non-thermal desorption rates. The experiments constrains 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.