On the relevance of the H2 + O reaction pathway for the surface
  formation of interstellar water - A combined experimental and modeling study
  The formation of interstellar water has been commonly accepted to occur on
the surfaces of icy dust grains in dark molecular clouds at low temperatures
(10-20 K), involving hydrogenation reactions of oxygen allotropes. As a result
of the large abundances of molecular hydrogen and atomic oxygen in these
regions, the reaction H2 + O has been proposed to contribute significantly to
the formation of water as well. However, gas phase experiments and
calculations, as well as solid-phase experimental work contradict this
hypothesis. Here, we use precisely executed temperature programmed desorption
(TPD) experiments in an ultra-high vacuum setup combined with kinetic Monte
Carlo simulations to establish an upper limit of the water production starting
from H2 and O. These reactants are brought together in a matrix of CO2 in a
series of (control) experiments at different temperatures and with different
isotopological compositions. The amount of water detected with the quadrupole
mass spectrometer upon TPD is found to originate mainly from contamination in
the chamber itself. However, if water is produced in small quantities on the
surface through H2 + O, this can only be explained by a combined classical and
tunneled reaction mechanism. An absolutely conservative upper limit for the
reaction rate is derived with a microscopic kinetic Monte Carlo model that
converts the upper limit into a maximal possible reaction rate. Incorporating
this rate into simulations run for astrochemically relevant parameters, shows
that the upper limit to the contribution of the reaction H2 + O in OH, and
hence water formation, is 11% in dense interstellar clouds. Our combined
experimental and theoretical results indicate however, that this contribution
is likely to be much lower.
