  This review paper summarizes the state-of-the-art in laboratory based
interstellar ice chemistry. The focus is on atom addition reactions,
illustrating how water, carbon dioxide and methanol can form in the solid state
at astronomically relevant temperatures, and also the formation of more complex
species such as hydroxylamine, an important prebiotic molecule, and
glycolaldehyde, the smallest sugar, is discussed. These reactions are
particularly relevant during the dark ages of star and planet formation, i.e.,
when the role of UV light is restricted. A quantitative characterization of
such processes is only possible through dedicated laboratory studies, i.e.,
under full control of a large set of parameters such as temperature, atom-flux,
and ice morphology. The resulting numbers, physical and chemical constants,
e.g., barrier heights, reaction rates and branching ratios, provide information
on the molecular processes at work and are needed as input for astrochemical
models, in order to bridge the timescales typical for a laboratory setting to
those needed to understand the evolutionary stages of the interstellar medium.
Details of the experiments as well as the astrochemical impact of the results
are discussed.
