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.
