THE $J=1\leftarrow 0$ ROTATIONAL TRANSITIONS OF $^{12}$CH$^+$, $^{13}$CH$^+$ and $^{12}$CD$^+$
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Date
2010
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Ohio State University
Abstract
The CH$^+$ ion is the first molecular ion identified in interstellar space. Dunham, \textbf{49},~26 (1937)} detected a couple of unidentified lines in near-UV, and later Douglas and Herzberg \textbf{94},~381 (1941)} identified them based on their laboratory observations. The electronic spectra have been investigated extensively. On the other hand, the pure rotational transitions are less extensively studied. Cernicharo {\em et al}, \textit{Astrophys. J.}, \textbf{483},~L65 (1997)} reported the interstellar detection of the $J$=2-1, 3-2, and 4-3 transitions in NGC 7027. Pearson and Drouin, \textbf{647},~L83 (2006)} reported the laboratory observation of the $J$=1-0 line of $^{12}$CH$^+$ at 835078.950 MHz and, based on this frequency, predicted the frequencies for $^{13}$CH$^+$ and CD$^+$. The predicted $^{13}$CH$^+$ frequency led to identification of the interstellar line, \textit{Astrophys. J.}, \textbf{634},~L49 (2005)}. In this talk, we present a new set of measurements of the $J$=1-0 lines for the normal species together with the $^{13}$C and D isotopic species. The overwhelming evidences obtained in our experiments support the new identifications. An extended negative glow discharge in a gas mixture of CH$_4$ ($\sim 0.5$ mTorr) diluted in He ($\sim 60$ mTorr) was used for production of CH$^+$ with the discharge current of about 15 mA. Axial magnetic filed up to 160 Gauss was applied. %The signal disappears with a couple of mTorr of H$_2$ or O$_2$. %The signal intensity increases about an order of magnitude when the cell temperature is lowered from -60 $^{circ}$C to liquid nitrogen temperture. The normal species line exhibited a surprisingly large Zeeman splitting for a $^1\Sigma$ molecule. The $^{13}$CH$^+$ line showed the spin-rotation hyperfine splitting, and at higher field of 150 Gauss an unresolved lineshape was exhibited due to combined hfs and Zeeman splittings. The spin-rotation splitting in the normal species was negligibly small. The CD$^+$ line showed much smaller Zeeman and spin-rotation splittings, as expected. Details of the mechanism to induce such Zeeman effect and the spin-rotation interaction will be presented. The transition frequencies for these $J=1-0$ lines are: 835137.498(20) MHz and 453521.847(20) MHz for $^{12}$CH$^+$ and CD$^+$, respectively. The transition frequencies for $^{13}$CH$^+$ are 830216.680(50) MHz ($F=3/2-1/2$) and 830214.961(50) MHz ($F=1/2-1/2$). %These frequencies are in good agreement with those derived from the optical spectra. The uncertainties reflect possible errors in correcting the Zeeman shifts.
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Author Institution: Department of Chemistry and Department of Physics and Astronomy, University of Waterloo, Waterloo, ON, Canada N2L 3G1;