  Complex organic molecules (COMs) have been identified in different
environments in star- forming regions. Laboratory studies show that COMs form
in the solid state, on icy grains, typically following a non-energetic
(atom-addition) or energetic (UV-photon absorption) trigger. So far, such
studies have been largely performed for single processes. Here, we present the
first work that quantitatively investigates both the relative importance and
the cumulative effect of (non-)energetic processing. We focus on astronomically
relevant CO:CH3OH = 4:1 ice analogues exposed to doses relevant for the
collapse stage of dense clouds. Hydrogenation experiments result in the
formation of methyl formate (MF HC(O)OCH3), glycolaldehyde (GA HC(O)CH2OH) and
ethylene glycol (EG H2C(OH)CH2OH) at 14 K. The absolute abundances and the
abundance fractions are found to be dependent on the H-atom/CO-CH3OH molecule
ratios and on the overall deposition rate. In the case that ices are exposed to
UV photons only, several different COMs are found. Typically, the abundance
fractions are 0.2 for MF, 0.3 for GA and 0.5 for EG as opposed to the values
found in pure hydrogenation experiments without UV in which MF is largely
absent: 0.0, 0.2-0.6 and 0.8-0.4, respectively. In experiments where both are
applied, overall COM abundances drop to about half of those found in the pure
UV irradiation experiments, but the composition fractions are very similar.
This implies COM ratios can be used as a diagnostic tool to derive the
processing history of an ice. Solid-state branching ratios derived here for GA
and EG compare well with observations, while the MF case cannot be explained by
solid-state conditions investigated here.
