Chemical desorption versus energy dissipation: insights from ab-initio
  molecular dynamics of HCO formation
  Molecular clouds are the cold regions of the Milky Way where stars form. They
are enriched by rather complex molecules. Many of these molecules are believed
to be synthesized on the icy surfaces of the interstellar submicron-sized dust
grains that permeate the Galaxy. At 10 K thermal desorption is ineffcient and,
therefore, why these molecules are found in the cold gas has tantalized
astronomers for years. The assumption of the current models, called chemical
desorption, is that the molecule formation energy released by the chemical
reaction at the grain surface is partially absorbed by the grain and the
remaining one causes the ejection of the newly formed molecule into the gas.
Here we report an accurate ab-initio molecular dynamics simulations aimed to
study the fate of the energy released by the first reaction of the H addition
chain on CO, CO + H $\rightarrow$ HCO, occurring on a crystalline ice surface
model. We show that about 90% of the HCO formation energy is injected towards
the ice in the first picosecond, leaving HCO with an energy content (10-15 kJ
mol$^{-1}$) more than a factor two lower than its adsorption energy (30 kJ
mol$^{-1}$). As a result, in agreement with laboratory experiments, we conclude
that chemical desorption is ineffcient for this specific system, namely H + CO
on crystalline ice. We suspect this behavior to be quite general when dealing
with hydrogen bonds, which are responsible of both the cohesive energy of the
ice mantle and the interaction with adsorbates, as the HCO radical, even though
ad hoc simulations are needed to draw specific conclusions on other systems.
