Knowledge Bank

The first reported spectrum of CH$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow><mrow\; data-mjx-texclass="ORD">+</mrow>}$\ was the $Ka=0$ series of the $\nu 3$ fundamental, recorded by our group in 1991.} \textbf{153}, 738 (1992).} Subsequently, the $Ka=1$ series was observed in 1993 and later analyzed by Jensen et al.} \textbf{172}, 194 (1995).} These spectra were recorded in liquid-nitrogen-cooled positive-column discharges of CH$\mathrm{<mrow\; data-mjx-texclass="ORD">4</mrow>}$, H$2$, and He. In the 1993 spectrum, absorption lines from CH$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow><mrow\; data-mjx-texclass="ORD">+</mrow>}$\ were discriminated from those of CH$\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow><mrow\; data-mjx-texclass="ORD">+</mrow>}$\ by recording the spectrum of a discharge without H$2$. In the absence of H$2$, the CH$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow><mrow\; data-mjx-texclass="ORD">+</mrow>}$\ lines grew stronger and the CH$\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow><mrow\; data-mjx-texclass="ORD">+</mrow>}$\ lines weakened. In the 1993 spectrum, weak lines attributed to CH$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow><mrow\; data-mjx-texclass="ORD">+</mrow>}$\ with $Ka=2$ were observed. The $Ka=2$ lines were significantly weaker than the $Ka=1$ lines because of a reduced Boltzman factor ($A\approx$ 67 \wn). The assignment of these lines was stalled because of their low intensity, the complicated spectral pattern of CH$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow><mrow\; data-mjx-texclass="ORD">+</mrow>}$\ (due to the combined effects of quasi-linearity and the Renner-Teller effect), and interfering absorption lines of CH$\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow><mrow\; data-mjx-texclass="ORD">+</mrow>}$, C$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$H$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow><mrow\; data-mjx-texclass="ORD">+</mrow>}$, and C$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$H$\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow><mrow\; data-mjx-texclass="ORD">+</mrow>}$. \vspace{2ex} Two new developments have led us to revisit the infrared spectrum of CH$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow><mrow\; data-mjx-texclass="ORD">+</mrow>}$, with the goal of assigning $Ka\ge 2$ lines. The first development is the observation of the $A~(0,3,0)3\leftarrow X~(0,0,0)2$ electronic band of CH$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow><mrow\; data-mjx-texclass="ORD">+</mrow>}$.} \textbf{121}, 11527 (2004).}$\mathrm{<mrow\; data-mjx-texclass="ORD">,</mrow>}$}^{+}$.''}\ This band provides combination differences that can be directly compared with the $\nu 3$ fundamental. The second development is the discovery of a new plasma chemistry --- allene and He in a water-cooled positive-column discharge --- that appears to be selective for CH$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow><mrow\; data-mjx-texclass="ORD">+</mrow>}$. This new chemistry allows us to confirm CH$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow><mrow\; data-mjx-texclass="ORD">+</mrow>}$\ as the carrier of previously observed lines. This paper will discuss progress made in the assignment of the infrared spectrum of CH$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow><mrow\; data-mjx-texclass="ORD">+</mrow>}$\ in light of these new developments.