  Complex organic molecules (COMs) have been observed in comets, hot cores and
cold dense regions of the interstellar medium. It is generally accepted that
these COMs form on icy dust grain through the recombination reaction of
radicals triggered by either energetic UV- photon or non-energetic H-atom
addition processing. In this work, we present for the first time laboratory
studies that allow for quantitative comparison of hydrogenation and UV-induced
reactions as well as their cumulative effect in astronomically relevant
CO:CH3OH=4:1 ice analogues. The formation of glycolaldehyde (GA) and ethylene
glycol (EG) is confirmed in pure hydrogenation experiments at 14 K, except
methyl formate (MF), which is only clearly observed in photolysis. The
fractions for MF:GA:EG are 0 : (0.2-0.4) : (0.8-0.6) for pure hydrogenation,
and 0.2 : 0.3 : 0.5 for UV involving experiments and can offer a diagnostic
tool to derive the chemical origin of these species. The GA/EG ratios in the
laboratory (0.3-1.5) compare well with observations toward different objects.
