JENNIFER L. GOTTFRIED AND TAKESHI OKA, Department of Chemistry, Department of Astronomy & Astrophysics, and the Enrico Fermi Institute, University of Chicago, Chicago, IL 60637.
The molecular ion CH2+ is of special interest because of its quasilinearity (resulting from a small barrier to linearity, \hbox1089 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 u state that splits into X2A1 and A2B1 states as the molecule bends. Although in the ground state the molecule is a near-prolate asymmetric rotor (\angleHCH=140o), the excited state equilibrium geometry is linear. Both the initial detection of the infrared spectrum of the 3 band,\footnoteP. Jensen, M. Brumm, W.P. Kraemer, and P.R. Bunker, J. Mol. Spectrosc. \underline\textbf172, 194 (1995) and a more recent PFI-ZEKE study\footnoteS. Willitsch and F. Merkt, J. Chem. Phys. \underline\textbf118, 2235 (2003) confirm the bent nature of CH2+ in the ground state. This work presents the first experimental spectroscopic characterization of the A2B1 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/CH4 and He/CH4/H2 liquid-nitrogen cooled positive column discharges was used to identify these transitions, the strongest of which had a signal-to-noise ratio of 100. A least-squares fit of 57 transitions from the A(0,3,0)1 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 A(0,4,0)0 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\footnoteP.R. Bunker, M.C. Chan, W.P. Kraemer, and P. Jensen, Chem. Phys. Lett. \underline\textbf341, 358 (2001) of the rovibronic spectra of CH2+ show good agreement with the observed spectrum (\triangle o-c=\hbox-53 cm-1 for the A(0,3,0)1 X(0,0,0)0 band and \triangle o-c=-18~cm-1 for the A(0,4,0)0 X(0,0,0)1 band). Assignment of the A(0,3,0)2 X(0,0,0)1 and A(0,3,0)3 X(0,0,0)2 bands is underway.