UV photoprocessing of CO₂ ice: a complete quantification of photochemistry and photon-induced desorption processes^⋆

¹ Centro de Astrobiología (INTA-CSIC), Ctra. de Ajalvir, km 4, Torrejón de Ardoz, 28850 Madrid, Spain
e-mail: rmartin@cab.inta-csic.es
² Sackler Laboratory for Astrophysics, Leiden Observatory, University of Leiden, PO Box 9513, NL 2300 RA Leiden, The Netherlands
³ Space Sciences Center and Department of Physics and Astronomy, University of Southern California, Los Angeles, CA90089-1341, USA
⁴ Department of Physics, National Central University, Jhongli City, 32054 Taoyuan County, Taiwan
⁵ Instituto de Estructura de la Materia-Consejo Superior de Investigaciones Científicas (IEM-CSIC), 28006 Madrid, Spain

Received: 2 March 2015
Accepted: 7 September 2015

Abstract

Context. Ice mantles that formed on top of dust grains are photoprocessed by the secondary ultraviolet (UV) field in cold and dense molecular clouds. UV photons induce photochemistry and desorption of ice molecules. Experimental simulations dedicated to ice analogs under astrophysically relevant conditions are needed to understand these processes.

Aims. We present UV-irradiation experiments of a pure CO₂ ice analog. Calibration of the quadrupole mass spectrometer allowed us to quantify the photodesorption of molecules to the gas phase. This information was added to the data provided by the Fourier transform infrared spectrometer on the solid phase to obtain a complete quantitative study of the UV photoprocessing of an ice analog.

Methods. Experimental simulations were performed in an ultra-high vacuum chamber. Ice samples were deposited onto an infrared transparent window at 8K and were subsequently irradiated with a microwave-discharged hydrogen flow lamp. After irradiation, ice samples were warmed up until complete sublimation was attained.

Results. Photolysis of CO₂ molecules initiates a network of photon-induced chemical reactions leading to the formation of CO, CO₃, O₂, and O₃. During irradiation, photon-induced desorption of CO and, to a lesser extent, O₂ and CO₂ took place through a process called indirect desorption induced by electronic transitions, with maximum photodesorption yields (Y_pd) of ~1.2 × 10^-2 molecules incident photon^-1, ~9.3 × 10^-4 molecules incident photon^-1, and ~1.1 × 10^-4 molecules incident photon^-1, respectively.

Conclusions. Calibration of mass spectrometers allows a direct quantification of photodesorption yields instead of the indirect values that were obtained from infrared spectra in most previous works. Supplementary information provided by infrared spectroscopy leads to a complete quantification, and therefore a better understanding, of the processes taking place in UV-irradiated ice mantles.