  Major components of ices on interstellar grains in molecular clouds - water
and carbon oxides - occur at various optical depths. This implies that
selective desorption mechanisms are at work. An astrochemical model of a
contracting low-mass molecular cloud core is presented. Ice was treated as
consisting of the surface and three subsurface layers (sublayers).
Photodesorption, reactive desorption, and indirect reactive desorption were
investigated. The latter manifests itself through desorption from H+H reaction
on grains. Desorption of shallow subsurface species was included. Modeling
results suggest the existence of a "photon-dominated ice" during the early
phases of core contraction. Subsurface ice is chemically processed by
interstellar photons, which produces complex organic molecules. Desorption from
the subsurface layer results in high COM gas-phase abundances at Av =
2.4...10mag. This may contribute towards an explanation for COM observations in
dark cores. It was found that photodesorption mostly governs the onset of ice
accumulation onto grains. Reaction-specific reactive desorption is efficient
for small molecules that form via highly exothermic atom-addition reactions.
Higher reactive desorption efficiency results in lower gas-phase abundances of
COMs. Indirect reactive desorption allows to closely reproduce the observed
H2O:CO:CO2 ratio towards a number of background stars. Presumably this can be
done by any mechanism whose efficiency fits with the sequence CO > CO2 >> H2O.
After the freeze-out has ended, the three sublayers represent chemically
distinct parts of the mantle. 8...10.5mag is the likely AV threshold for the
appearance of CO ice. The lower value is supported by observations.
