PHOTODESORPTION OF SOLID CO2BY LYα

Abstract

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 Lyα (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 ∼10 17 photons cm −2 , it is dominated by CO with smaller amount of O2, C, and CO2, with the presence of O2 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 ∼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.