UV-induced photodesorption and photochemistry of O2 ice

In dense inter- and circumstellar clouds, molecules are maintained in the gas phase at temperatures well below their thermal accretion value, and the UV photodesorption of ice has been proposed as an efficient non-thermal desorption mechanism explaining this observation. The present work aims at studying the UV photodesorption yield of solid O2, a molecule considered as a starting point in the formation of water. For the infrared inactive O2 the standard spectroscopic procedure does not work, and a new experimental procedure is introduced that links quadrupole mass spectrometry of photodesorbed oxygen ice in the gas phase to a RAIR spectrum of a polar reference molecule in the solid state (in our case CO). The resulting photodesorption rate of pure O2 ice is found to be (6 ± 2) × 10⁻⁴ molecules photon⁻¹ upon irradiation with broad-band Lyα light. The main photodesorption product is O2; a smaller part photodesorbs as O-atoms. Within the accessible range, the photodesorption rate does not vary with ice temperature. Upon UV irradiation also ozone ice is formed and the present work allows in parallel to determine the photodesorption rate for O3 as (3 ± 1) × 10⁻⁴ and (5 ± 2) × 10⁻⁴ molecules photon⁻¹ for 14 and 52 K, respectively. Ozone is found not to desorb directly, but via its fragments O2 and O; UV induced dissociation is concluded to be much more efficient than direct photodesorption.