Effects of H2 coating of grains on depletion of molecular species
  Physical conditions in dense and cold regions of interstellar clouds favour
the formation of ice mantles on the surfaces of interstellar grains. It is
predicted that most of the gaseous species heavier than H2 or He will adsorb
onto the grains and will disappear from the gas-phase, changing its chemistry,
within 10^9/n_H years. Nonetheless, many molecules in molecular clouds are not
completely depleted in timescales of 10^5 yr. Several speculative mechanisms
have been proposed to explain why molecules stay in the gas phase, but up to
now none are fully convincing. At the same time, these mechanisms are not
mutually exclusive and we can still explore the effects of other possible
processes. We speculate on the consequences of H2 coating of grains on the
evaporation rates of adsorbed species. More experiments and simulations are
needed to calculate the evaporation rate Eevap(X-H2).
