Carbo-ion spectroscopy with tunable diode lasers: The bending vibrations of $CH_{3}^{+}$

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1994

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Ohio State University

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The methyl cation $CH_{3}^{-}$ is one of the most fundamental carbo-ions which plays an important role in many fields. According to ab-initio $calculations^{1,2,3}$ and photoelectron $spectra^{4}$, the band origins of its two bending vibrations. $\nu_{2}$(the out of plane bending vibration) and $\nu_{4}$ (degenerate bending vibration) have been predicted to be between 1350 and $1400 cm^{-1}$. The theoretical studies show that the vibrational frequencies of $\nu_{2}$ (parallel band) and $\nu_{4}$ (perpendicular band) are very close to each other with a difference of $1-20 cm^{-1}$ and a strong Coriolis interaction between the two vibrational states are expected. The perturbation treatment of the vibration-rotation hamiltonian is no longer applicable. We present the first high resolution spectra of $CH_{3}^{+}$ in the bending vibration frequency region at $7 \sim 8 \mu $m. The ion was produced in a $liq-N_{2}$ cooled 6kHz ac glow discharge with gas mixture of 0.1:0.1:7.0 torr of $CH_{4}: H_{2}$: He. Velocity modulation is used in order to discriminate ion signal. 22 spectral lines have been observed. The limitation of mode coverage, large rotational constant of $CH_{3}^{+} (B_{0}=9.36 cm^{-1}$) and strong Coriolis interaction make the analysis difficult. If successful, the analysis will provide better understanding of vibration-rotation interaction in the molecule. Our observation will also provide a vehicle for the astronomical observation of $CH_{3}^{+}$ through the 8.5 $\mu m$ window.

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1. D.J DeFrees and A.D. McLean. J. Chem. Phys. 82, 333 (1985) 2. P. Botsehwina, in Ion and Cluster Ion Spectroscopy and Structure, edited by J.P. Maier (Elvier, Amsterdam, 1989) 3. W.P. Kraemer and V. Spirko, J. Molec. Spec. 149, 235 (1991) 4. J. Dyke, N. Jonathan, E. Lee and A. Morris, J. Chem. Soc. Faraday Trans. 2 72, 1385 (1976) 1a. Feodor Lynen Fellow (Alexander von Humbolt Foundation).
Author Institution: Department of Chemistry and Department of Astronomy and Astrophysics, The University of Chicago

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