Laboratory Measurement of Millimeter-wave Transitions of 13CH2DOH for Astronomical Use

Methanol (CH _3 OH) is an abundant interstellar species and is known to be an important precursor of various interstellar complex organic molecules. Among the methanol isotopologues, CH _2 DOH is one of the most abundant isotopologues and it is often used to study the deuterium fractionation of CH _3 OH in interstellar medium. However, the emission lines of CH _2 DOH can sometimes be optically thick, making the derivation of its abundance unreliable. Therefore, observations of its presumably optically thin ^13 C substituted species, ^13 CH _2 DOH, are essential to overcome this issue. In this study, the rotational transitions of ^13 CH _2 DOH have been measured in the millimeter-wave region from 216 GHz to 264 GHz with an emission-type millimeter- and submillimeter-wave spectrometer by using a deuterium and ^13 C enriched sample. The frequency accuracy of measured ^13 CH _2 DOH is less than a few kHz, and the relative line intensity error is less than 10% in most of the frequency range by taking advantage of the wide simultaneous frequency-coverage of the emission-type spectrometer. These results offer a good opportunity to detect ^13 CH _2 DOH in space, which will allow us to study the deuterium fractionation of CH _3 OH in various sources through accurate determination of the CH _2 DOH abundance.