DETECTION OF THE NEAR-INFRARED SPECTRUM OF $CH_{2}{^{+}}$

$^{a}$M. R\""{o}sslein, C.M. Gabrys, M.-F. Jagod, and T. Oka, J. Mol. Spectrosc. 153, 738 (1992) $^{b}$P. Jensen, M. Brumm, W.P. Kraemer, and P.R. Bunker, J. Mol. Spectrosc. 172, 194 (1995) $^{c}$S. Willitsch and F. Merkt, J. Chem. Phys. 118, 2235 (2003) $^{d}$P.R. Bunker, M.C. Chan, W.P. Kraemer, and P. Jensen, Chem. Phys. Lett. 341, 358 (2001)Author Institution: Department of Chemistry, University of Chicago; Department of Astronomy \& Astrophysics, and the Enrico Fermi Institute, University of ChicagoThe molecular ion $CH_{2}{^{+}}$ is of special interest because of its quasilinearity (resulting from a small barrier to linearity, $1089 cm^{-1}$) and the strong interaction of its ground and first excited states due to the Renner-Teller effect. At linearity the ground state is a doubly degenerate ${^{2}}\Pi_{u}$ state that splits into $\widetilde{X}^{2}A_{1}$ and $\widetilde{A}^{2}B_{1}$ states as the molecule bends. Although in the ground state the molecule is a near-prolate asymmetric rotor $(\angle HCH=140^{\circ})$, the excited state equilibrium geometry is linear. Both the initial detection of the infrared spectrum of the $\nu_{3}$ $band^{a,b}$ and a more recent PFI-ZEKE $study^{c}$ confirm the bent nature of $CH_{2}{^{+}}$ in the ground state. This work presents the first experimental spectroscopic characterization of the $\widetilde{A}^{2}B_{1}$ state. Using a Ti:sapphire laser, we have observed four new absorption bands in the near-infrared region $(11,000-13,000 cm^{-1})$. The comparison between spectra recorded in $He/CH_{4}$ and $He/CH_{4}/H_{2}$ liquid-nitrogen cooled positive column discharges was used to identify these transitions, the strongest of which had a signal-to-noise ratio of ${\sim} 100$. A least-squares fit of 57 transitions from the $\widetilde{A}(0,3,0)^{1} \leftarrow \widetilde{X}(0,0,0)^{0}$ band has given preliminary values for the excited state molecular constants $B=7.140(22)$ and $D_{N}=-0.00034(16)$. The fit of 46 lines from the $\widetilde{A}(0,4,0)^{0} \leftarrow \widetilde{X}(0,0,0)^{1}$ band resulted in the following excited state constants: $B=6.796(56)$ and $D_{N}=-0.00170(42)$. Recent ab initio $predictions^{d}$ of the rovibronic spectra of $CH_{2}{^{+}}$ show good agreement with the observed spectrum ($\Delta \nu_{o-c}=-53 cm^{-1}$ for the $\widetilde{A}(0,3,0)^{1}\leftarrow \widetilde{X}(0,0,0)^{0}$ band and $\Delta \nu_{o-c}=- 18 cm^{-1}$ for the $\widetilde{A}(0,4,0)^{0}\leftarrow \widetilde{X}(0,0,0)^{1}$ band). Assignment of the $\widetilde{A}(0,3,0)^{2}\leftarrow \widetilde{X}(0,0,0)^{1}$ and $\widetilde{A}(0,3,0)^{3}\leftarrow \widetilde{X}(0,0,0)^{2}$ bands is underway