Laboratory Measurement of CH2DOH Line Intensities in the Millimeter-wave Region

Deuterium fractionation in molecules is known as one of the most powerful tools to study chemical processes during star and planet formation. Among various interstellar molecules, methanol often shows very high deuterium fractionation. It is the most abundant saturated organic molecule and is known as a parent species to form more complex organic molecules. However, deriving the abundance of deuterated methanol suffers from the uncertainty in the intrinsic line intensities (Sμ2) of CH3OH isotopologues. Due to their floppy nature, theoretical evaluation of the Sμ2 values is not straightforward, particularly for asymmetric-top asymmetric-frame isotopologues such as CH2DOH. In this study, we have measured the line frequencies and their intensities for CH2DOH in the millimeter-wave region from 216 to 264 GHz by using an emission-type millimeter and submillimeter-wave spectrometer. For the a-type J = 5 − 4 transition, the derived Sμ2 values are 13%–27% larger than those theoretically calculated, except for the transitions of Ka = 2 for e0 and Ka = 1 for e1 affected by avoided level crossing. For b-type transitions, significant systematic differences are found between theoretical and experimental Sμ2 values. The results of the present study enable us to accurately derive from observations the CH2DOH abundances, which are essential for understanding deuterium fractionation in various sources.