Effect of binding energies on the encounter desorption
  The abundance of interstellar ice constituents is usually expressed with
respect to the water ice because, in denser regions, a significant portion of
the interstellar grain surface would be covered by water ice. The binding
energy (BE), or adsorption energy of the interstellar species regulates the
chemical complexity of the interstellar grain mantle. Due to the high abundance
of water ice, the BE of surface species with the water is usually provided and
widely used in astrochemical modeling. However, the hydrogen molecules would
cover some part of the grain mantle in the denser and colder part of the
interstellar medium. Even at around ~ 10K, few atoms and simple molecules with
lower adsorption energies can migrate through the surface. The BE of the
surface species with H2 substrate would be very different from that of a water
substrate. However, adequate information regarding these differences is
lacking. Here, we employ the quantum chemical calculation to provide the BE of
95 interstellar species with H2 substrate. These are representative of the BEs
of species to a H2 overlayer on a grain surface. On average, we notice that the
BE with the H2 monomer substrate is almost ten times lower than the BE of these
species reported earlier with the H2 O c-tetramer configuration. The encounter
desorption of H and H2 was introduced (with ED (H, H2 ) =45 K and ED (H2 , H2 )
=23 K) to have a realistic estimation of the abundances of the surface species
in the colder and denser region. Our quantum chemical calculations yield higher
adsorption energy of H2 than that of H (ED (H, H2 ) = 23 - 25 K and ED (H2, H2
) =67 - 79 K). We further implement an astrochemical model to study the effect
of encounter desorption with the resent realistic estimation. The encounter
desorption of the N atom (calculations yield ED (N, H2 ) =83 K) is introduced
to study the differences with its inclusion.
