Absorption cross sections and solar photodissociation rates of deuterated isotopomers of methanol

Photoabsorption cross sections of CH₃OH, CH₃OD, CD₃OH, and CD₃OD are determined in the spectral region 165-220 nm using synchrotron radiation as a light source. Excitation of methanol isotopomers in this wavelength region breaks mainly the O-H or O-D bond to form H or D, respectively. Significantly smaller absorption cross sections near the threshold region are observed for CH₃OD and CD₃OD. These data are used to calculate corresponding photodissociation rates by the solar flux at 1 AU. The rates for CH₃OD and CD₃OD are only about 80% and 82% of those for CH₃OH and CD₃OH, respectively. The effective photo-induced isotopic enrichment factors (per mil) are estimated to be 199‰ for CH₃OD, -35‰ for CD₃OH, and 154‰ for CD₃OD. Such large enrichment factors are expected to induce detectable H/D fractionation in comets or elsewhere if the photo-induced isotopic fractionation effect is important in controlling methanol chemistry