  This study focuses on the formation of two molecules of astrobiological
importance - glycolaldehyde (HC(O)CH2OH) and ethylene glycol (H2C(OH)CH2OH) -
by surface hydrogenation of CO molecules. Our experiments aim at simulating the
CO freeze-out stage in interstellar dark cloud regions, well before thermal and
energetic processing become dominant. It is shown that along with the formation
of H2CO and CH3OH - two well established products of CO hydrogenation - also
molecules with more than one carbon atom form. The key step in this process is
believed to be the recombination of two HCO radicals followed by the formation
of a C-C bond. The experimentally established reaction pathways are implemented
into a continuous-time random-walk Monte Carlo model, previously used to model
the formation of CH3OH on astrochemical time-scales, to study their impact on
the solid-state abundances in dense interstellar clouds of glycolaldehyde and
ethylene glycol.
