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

[1] 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 199parts per thousand for CH3OD, 35parts per thousand for CD3OH, and 154parts per thousand 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.