Knowledge Bank

The methyl cation $CH3-$ is one of the most fundamental carbo-ions which plays an important role in many fields. According to ab-initio $calculations1,2,3$ and photoelectron $spectra4$, 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 $1400cm-1$. 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 $1-20cm-1$ 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 The perturbation treatment of the vibration-rotation hamiltonian is no longer applicable.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 $CH3+$ in the bending vibration frequency region at $7 \sim 8 \mu $m. The ion was produced in a $liq-N2$ cooled 6kHz ac glow discharge with gas mixture of 0.1:0.1:7.0 torr of $CH4:H2$: 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 $CH3+(B0=9.36cm-1$) 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 $CH3+$ through the 8.5 $\mu m$ window.