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The first reported spectrum of CH$_{2}^{+}$\ was the $K_a=0$ series of the $\nu_3$ fundamental, recorded by our group in 1991.} \textbf{153}, 738 (1992).} Subsequently, the $K_a=1$ 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$_{4}$, H$_2$, and He. In the 1993 spectrum, absorption lines from CH$_{2}^{+}$\ were discriminated from those of CH$_{3}^{+}$\ by recording the spectrum of a discharge without H$_2$. In the absence of H$_2$, the CH$_{2}^{+}$\ lines grew stronger and the CH$_{3}^{+}$\ lines weakened. In the 1993 spectrum, weak lines attributed to CH$_{2}^{+}$\ with $K_a=2$ were observed. The $K_a=2$ lines were significantly weaker than the $K_a=1$ 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$_{2}^{+}$\ (due to the combined effects of quasi-linearity and the Renner-Teller effect), and interfering absorption lines of CH$_{3}^{+}$, C$_{2}$H$_{2}^{+}$, and C$_{2}$H$_{3}^{+}$. \vspace{2ex} Two new developments have led us to revisit the infrared spectrum of CH$_{2}^{+}$, with the goal of assigning $K_a\geq2$ lines. The first development is the observation of the $\tilde{A}(0,3,0)^3 \leftarrow \tilde{X}(0,0,0)^2$ electronic band of CH$_{2}^{+}$.} \textbf{121}, 11527 (2004).}$^{,}$}^{+}$.''}\ 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$_{2}^{+}$. This new chemistry allows us to confirm CH$_{2}^{+}$\ as the carrier of previously observed lines. This paper will discuss progress made in the assignment of the infrared spectrum of CH$_{2}^{+}$\ in light of these new developments.


{M. Rosslein, C. M. Gabrys, M.-F. Jagod, and T. Oka, \textit{J. Mol. Spectrosc.{P. Jensen, M. Brumm, W. P. Kraemer, and P. R. Bunker\textit{J. Mol. Spectrosc.{J. L. Gottfried and T. Oka, \textit{J. Chem. Phys.{See talk entitled ``The Near-Infrared Spectrum of CH$_{2
Author Institution: Department of Chemistry, Department of Astronomy \& Astrophysics,; and the Enrico Fermi Institute, The University of Chicago, Chicago; IL, 60637, USA