Knowledge Bank

The pure rotational transition frequencies of H$2$D$+$, $212\leftarrow 111,202\leftarrow 101,211\leftarrow 110$, and $313\leftarrow 212$, have been measured in the laboratory precisely by using a tunable far-infrared spectrometer. Among them, the $212\leftarrow 111$ line was recently detected in space toward Sgr B2 by Cernicharo {\it et al.}. Their identification was made based on a calculated line frequency estimated from the spectroscopic data of Amano and Hirao. It has been found that our measured frequency of this line, 2363242.82(69) MHz, is lower by about 20 MHz than the estimated value. All the available THz lines and known millimeter- and submillimeter-wave lines together with the combination differences derived from the infrared transitions are fitted to the Watson effective Hamiltonian. A set of improved molecular constants are obtained. \ ----------------------------------------\ J.Cernicharo, E. Polehampton, and J.R. Goicoechea. {\it Astrophys. J.}, {\bf 657}, L21-L24 (2007).\ T. Amano and T. Hirao, {\it J. Mol. Spectrosc.}, {\bf 233}, 7-14 (2005).\