Molecular complexity is not a term people generally associate with the interstellar medium (ISM). However, close to 130 molecular species have so far been detected in the ISM. These range from the simplest and most abundant molecule, H2, to complex biologically relevant molecules, like the simple sugar Glycolaldehyde.

No efficient gas phase formation routes exists for a number of these molecules, such as molecular hydrogen and methanol. It is therefore generally accepted that these molecules must be formed on the surface of interstellar dust grains. Models show that the energy released in the formation of the most abundant molecule, molecular hydrogen, can have a huge impact on the thermal evolution of interstellar dust and molecular clouds where new stars are born.

Hence, to model star formation we need to know the processes and energy release mechanisms of molecular hydrogen formation. A detailed understanding of the formation process, including vital information on the energy release mechanism has, however, been missing.

This situation is rapidly changing as a number of groups worldwide have commenced laboratory experiments on surface reactions under interstellar conditions.