15min:

CHRISTOPHER F. NEESE AND TAKESHI OKA, Department of Chemistry, Department of Astronomy & Astrophysics, and the Enrico Fermi Institute, The University of Chicago, Chicago IL, 60637, USA.

The first reported spectrum of CH₂⁺ was the K_a=0 series of the ₃ fundamental, recorded by our group in 1991.\footnoteM. Rösslein, C. M. Gabrys, M.-F. Jagod, and T. Oka, J. Mol. Spectrosc. \textbf153, 738 (1992). Subsequently, the K_a=1 series was observed in 1993 and later analyzed by Jensen et al. These spectra were recorded in liquid-nitrogen-cooled positive-column discharges of CH₄, H₂, and He. In the 1993 spectrum, absorption lines from CH₂⁺ were discriminated from those of CH₃⁺ by recording the spectrum of a discharge without H₂. In the absence of H₂, the CH₂⁺ lines grew stronger and the CH₃⁺ lines weakened. In the 1993 spectrum, weak lines attributed to CH₂⁺ 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 ~~67~cm^-1). The assignment of these lines was stalled because of their low intensity, the complicated spectral pattern of CH₂⁺ (due to the combined effects of quasi-linearity and the Renner-Teller effect), and interfering absorption lines of CH₃⁺, C₂H₂⁺, and C₂H₃⁺.

Two new developments have led us to revisit the infrared spectrum of CH₂⁺, with the goal of assigning K_a\geq2 lines. The first development is the observation of the A(0,3,0)³ X(0,0,0)² electronic band of CH₂⁺.^, This band provides combination differences that can be directly compared with the 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 CH₂⁺. This new chemistry allows us to confirm CH₂⁺ as the carrier of previously observed lines. This paper will discuss progress made in the assignment of the infrared spectrum of CH₂⁺ in light of these new developments.