DIRECT MEASUREMENTS OF THE FUNDAMENTAL ROTATIONAL TRANSITIONS OF CD AND $^{13}$CH $(X^2\Pi_r)$

Author Institution: University of Arizona, Steward Observatory, Department of Chemistry Arizona Radio Observatory, Tucson, AZ 85721; Jet Propulsion Laboratory, Pasadena, CA 91109The $N=1\rightarrow1$ and $ N=1\rightarrow2$ rotational transitions of CD, and the $N=1 \rightarrow 1$ lines of $^{13}$CH have been measured in their $^2\Pi_r$ ground electronic states using sub-mm direct absorption spectroscopy. The measurements below 600 GHz (CD: $N=1\rightarrow1$ and $^{13}$CH: $N=1\rightarrow1$) were carried out at Arizona, while those in the 900 GHz range were conducted at JPL (CD: $N=1\rightarrow1$). The two radical species were created in an electrical discharge of either $^{13}$CH$_4$ or CD$_4$. Both lambda-doubling and hyperfine splitttings were resolved in the spectra. The data were analyzed with a case(b) effective Hamiltonian, resulting in an improvement in the lambda-doubling and deuterium, proton, and $^{13}$C hyperfine constants. Highly accurate rest frequencies are now available for astronomical searches for these species. CH is an abundant and widespread interstellar molecule, and thus CD and $^{13}$CH should be of astrophysical interest