This proposal is devoted to the study of the solid phase counterparts
of the gas phase constituents in dense molecular clouds. Emphasis is
given on the detection of various solid state weak features which will
allow the identification of simple and presumably abundant molecules
expected to be present in the solid phase as interstellar grain mantles.
These molecules are essentially H2O as an isolated species (monomers,
dimers, trimers), O2, O3, N2, 13CO2, HCO, H2CO and CH4 as well as some
other minor species such as deuterated ones (HD, HDO, CH3D).
All these molecules can be viewed as key tests not only for grain
chemical models but also for the validity of laboratory simulations.
Most transitions have been chosen in totally opaque atmospheric windows
where ISO will provide an unique opportunity for many years.
In close connection with laboratory experiments, modelling studies
and parallel ground based observational work, the goal of this proposal
is a careful abundance determination of these solid state species and
a deeper understanding of the physico-chemical processes taking place
in interstellar grain mantles, in relation to the gas phase composition
usually studied from radioastronomy measurements.