Formation of interstellar methanol ice prior to the heavy CO freeze-out
  stage
  The formation of methanol (CH3OH) on icy grain mantles during the star
formation cycle is mainly associated with the CO freeze-out stage. Yet there
are reasons to believe that CH3OH also can form at an earlier period of
interstellar ice evolution in CO-poor and H2O-rich ices. This work focuses on
CH3OH formation in a H2O-rich interstellar ice environment following the
OH-mediated H-abstraction in the reaction, CH4 + OH. Experimental conditions
are systematically varied to constrain the CH3OH formation yield at
astronomically relevant temperatures. CH4, O2, and hydrogen atoms are
co-deposited in an ultrahigh vacuum chamber at 10-20 K. OH radicals are
generated by the H + O2 surface reaction. Temperature programmed desorption -
quadrupole mass spectrometry (TPD-QMS) is used to characterize CH3OH formation,
and is complemented with reflection absorption infrared spectroscopy (RAIRS)
for CH3OH characterization and quantitation. CH3OH formation is shown to be
possible by the sequential surface reaction chain, CH4 + OH -> CH3 + H2O and
CH3 + OH -> CH3OH at 10-20 K. This reaction is enhanced by tunneling, as noted
in a recent theoretical investigation (Lamberts et al. 2017). The CH3OH
formation yield via the CH4 + OH route versus the CO + H route is approximately
20 times smaller for the laboratory settings studied. The astronomical
relevance of the new formation channel investigated here is discussed.
