  (Abridged) Gas-phase complex organic molecules are commonly detected in the
warm inner regions of protostellar envelopes. Recent models show that
photochemistry in ices followed by desorption may explain the observed
abundances. This study aims to experimentally quantify the broad-band
UV-induced production rates of complex organics in CH3OH-rich ices at 20-70 K
under ultra-high vacuum conditions. The reaction products are mainly identified
by RAIRS and TPD experiments. Complex organics are readily formed in all
experiments, both during irradiation and during a slow warm-up of the ices to
200 K after the UV lamp is turned off. The relative abundances of photoproducts
depend on the UV fluence, the ice temperature, and whether pure CH3OH ice or
CH3OH:CH4/CO ice mixtures are used. C2H6, CH3CHO, CH3CH2OH, CH3OCH3, HCOOCH3,
HOCH2CHO and (CH2OH)2 are all detected in at least one experiment. The derived
product-formation yields and their dependences on different experimental
parameters, such as the initial ice composition, are used to estimate the CH3OH
photodissociation branching ratios in ice and the relative diffusion barriers
of the formed radicals. The experiments show that ice photochemistry in CH3OH
ices is efficient enough to explain the observed abundances of complex organics
around protostars and that ratios of complex molecules can be used to constrain
their formation pathway.
