15min:
DETECTION OF THE NEAR-INFRARED SPECTRUM OF CH2+.

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 Piu 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 nu3 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 sim100. A least-squares fit of 57 transitions from the A(0,3,0)1 leftarrowX(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 leftarrowX(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 nuo-c=\hbox-53 cm-1 for the A(0,3,0)1 leftarrowX(0,0,0)0 band and \triangle nuo-c=-18~cm-1 for the A(0,4,0)0 leftarrowX(0,0,0)1 band). Assignment of the A(0,3,0)2 leftarrowX(0,0,0)1 and A(0,3,0)3 leftarrowX(0,0,0)2 bands is underway.