Influence of surface coverage on the chemical desorption process
  In cold astrophysical environments, some molecules are observed in the gas
phase whereas they should have been depleted, frozen on dust grains. In order
to solve this problem, astrochemists have proposed that a fraction of molecules
synthesized on the surface of dust grains could desorb just after their
formation. Recently the chemical desorption process has been demonstrated
experimentally, but the key parameters at play have not yet been fully
understood. In this article we propose a new procedure to analyze the ratio of
di-oxygen and ozone synthesized after O atoms adsorption on oxidized graphite.
We demonstrate that the chemical desorption efficiency of the two reaction
paths (O+O and O+O$_2$) is different by one order of magnitude. We show the
importance of the surface coverage: for the O+O reaction, the chemical
desorption efficiency is close to 80 $\%$ at zero coverage and tends to zero at
one monolayer coverage. The coverage dependence of O+O chemical desorption is
proved by varying the amount of pre-adsorbed N$_2$ on the substrate from 0 to
1.5 ML. Finally, we discuss the relevance of the different physical parameters
that could play a role in the chemical desorption process: binding energy,
enthalpy of formation, and energy transfer from the new molecule to the surface
or to other adsorbates.
