PHOTODESORPTION OF SOLID CO2 BY LYα

We measured desorption of atoms and molecules from films of solid carbon dioxide in an ultrahigh vacuum from 6 to 60 K under irradiation with Ly alpha (121.6 nm, 10.2 eV) photons, an important process in the balance between gas phase and condensed molecules in the interstellar medium. The measurements use microgravimetry and mass spectrometry during irradiation and temperature programmed desorption after irradiation. At low photon fluences, the desorption flux consists mainly of O atoms and, after similar to 10(17) photons cm(-2), it is dominated by CO with smaller amount of O-2, C, and CO2, with the presence of O-2 indicating solid-state chemical reactions. At high fluences (up to 10(18) photons cm(-2)), the desorption yields saturate at values much higher than in previous studies. The yields (molecules/photon), derived assuming stoichiometric desorption, reach 0.014 at 6 K, growing to similar to 0.2 at 50 and 60 K. Warming the films during irradiation gives rise to pressure spikes that suggest desorption of trapped species in pores or at defects, possibly assisted by radical-induced reactions. Such an effect could be significant for radiation-processed CO2-coated interstellar grains that are heated by, i.e., cosmic ray impacts or grain-grain collisions. We discuss the experiments considering photochemical mechanisms and compare them to the results of ion irradiation.