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T. AMANO, Department of Chemistry and Department of Physics and Astronomy, University of Waterloo, Waterloo, ON, Canada N2L 3G1 .
In interstellar carbon chemistry, CH3+ is thought to be an important and abundant molecular ion. However, as it is a symmetric planar molecule and, as a result, it has no permanent dipole moment, it is almost impossible to detect this species by radio astronomical observations. Its deuterated species, CH2D+ and CHD2+, possess the dipole moment, so the rotational lines should be observable. Rösslein et al. \footnoteM. Rösslein et al. , Astrophys. J. \textbf382, L51 (1991) and Jagod et al. \footnoteM.-F. Jagod et al. , J. Mol. Spectrosc. \textbf153,~666 (1992) observed the infrared spectra of these deuterated species. Demuynck and coworkers tried to observe CH2D+ rotational lines in an extended negative glow discharge with no success. More recently Lis et al. \footnoteD. C. Lis et al. , in Submillimeter Astrophysics and Technology, ASP Conference Series , \textbf417,~23 (2009) reported tentative identification of CH2D+ toward Ori IRc2.
The molecular constants and the predicted rotational transition frequencies given by Röslein et al. a were a good starting point in searching for the rotational lines. A very weak feature was found almost exactly at the calculated frequency for the 212-111 transition. Eventually the line appeared stronger enough for precise frequency measurements, after adjusting the reaction conditions. The optimum gas mixture was found to be CH4 (
3 mTorr), CD4 (
1 mTorr), H2 (
2 mTorr), and He (
35 mTorr). It is interesting to note that helium is essential to produce CH2D+. No signals were detectable with Ar buffer.
Although the signal was seen without H2, it appears to play a subtle role in the formation, resulting in about a factor 2 increase in intensity. Adding D2 instead of CD4 resulted in no signal. The observations were made with about 16 mA discharge current with liquid nitrogen cooling. As this ion is a light molecule and the signal was only weakly observed, four transitions were detected so far in the 280-890 GHz region. All observed transition frequencies agree within 1MHz of the predicted frequencies. These laboratory transition frequencies strongly support the tentative astronomical identification by Lis et al. d