Study of the photon-induced formation and subsequent desorption of CH3 OH and H2 CO in interstellar ice analogs

Methanol and formaldehyde are two simple organic molecules that are ubiquitously detected in the interstellar medium. An origin in the solid phase and a subsequent nonthermal desorption into the gas phase is often invoked to explain their abundances in some of the environments where they are found. Experimental simulations under astrophysically relevant conditions have been carried out to find a suitable mechanism for that process. We explore the in situ formation and subsequent photon-induced desorption of these species, studying the UV photoprocessing of pure ethanol ice, and a more realistic binary H2O:CH4 ice analog. Ice samples deposited onto an infrared transparent window at 8 K were UV-irradiated using a microwave-discharged hydrogen flow lamp. Evidence of photochemical production of these two species and subsequent UV-photon-induced desorption into the gas phase were searched for by means of a Fourier transform infrared spectrometer and a quadrupole mass spectrometer, respectively. Formation of CH3OH was only observed during photoprocessing of the H2O:CH4 ice analog, but no photon-induced desorption was detected. Photochemical production of H2CO was observed in both series of experiments. Photochemidesorption of formaldehyde, i.e., photon-induced formation on the ice surface and inmediate desorption, was observed, with a yield of 4.4 x 10-4 (molecules/incident photon) when the H2O:CH4 ice analogs were photoprocessed. While certain C-bearing species, in particular H2CO, were found to desorb upon irradiation, nonthermal desorption of CH3OH was not observed. So far, there is no experimental evidence of any efficient CH3 OH desorption induced by UV photons.