TERAHERTZ SPECTROSCOPY OF DEUTERATED METHYLENE BI-RADICAL, CD2

setlength{belowdisplayskip}{3pt} setlength{abovedisplayskip}{2pt} Methylene, the parent of the carbene compounds, plays a crucial role in many chemical reactions. This bi-radical is a known interstellar molecule that has been detected towards hot cores in dense interstellar clouds. CH$_2$ is also thought to be present in cometary atmospheres. In the gas phase chemical models of both dense and diffuse molecular clouds, CH$_2$ is a key intermediate in interstellar carbon chemistry which is produced primarily by dissociative recombination of the methyl ion, CH$^+_3$. Recently tentative detection of the mono-deuterated methyl ion, CH$_2$D$^+$ has been reported toward an infrared source in the vicinity of Orion.footnote{D. C. Lis, P. F. Goldsmith, E. A. Bergin et al. 2009, in Submillimeter Astrophysics and Technology, ASP Conf. Ser., 417, 23.} Deuterated methylene CHD and CD$_2$ can be produced from this ion or its counterpart CHD$^+_2$ by dissociative recombination with an electron: begin{equation} mathrm{CH_{2}D^{+} + e^{-} rightarrow CHD+ H ~or~{CH_{2} + D}}, end{equation} begin{equation} mathrm{CHD_{2}^{+} ~~+ e^{-} rightarrow CHD+ D ~or~CD_{2} + H}. end{equation} Thus, both CHD and CD$_2$ can be observed in warm interstellar clouds, where the deuterium fractionation process is important. Precise laboratory reference data are desirable for radioastronomical observation of these molecules. Here we report on our high-resolution spectroscopic investigation on the deuterated methylene radical, CD$_2$ ($X$ $^3$B$_1$) up to 1.45 THz. At present time, eleven out of the twelve fine-structure components of four pure rotational transitions have been newly recorded, and these measurements double the number of previously observed transitions.footnote{H. Ozeki and S. Saito J. Chem. Phys. 1996, 104, 2167.} CD$_2$ was generated in a discharge in CD$_2$CO which was obtained from the flash pyrolysis of acetic anhydride-d6 ((CD$_3$CO)$_2$O). Effort is currently made to measure the astronomically important $1_{11} - 0_{00}$ transition whose fine-structure components are predicted to occur at 1.224,1.228 and 1.234 THz