INFRARED SPECTRUM OF THE $\nu_{3}$ BAND OF $CH_{3}^{+}$

$^{1}$ Mark W. Crofton et al., J. Chem. Phys., 83, 3702 (1985). Address of Crofton, Jagod, Rehfuss, and Oka: Department of Chemistry and Department of Astronomy and Astrophysics, The University of Chicago, Chicago, Illinois 60637. Address of Kreiner: Department of Physical Chemistry, University of Ulm, Ulm, Federal Republic of Germany.Author Institution:We have $observed^{1}$ the vibration-rotation spectrum of the $\nu_{3}$ band of $CH_{3}^{+}$. This ion was produced both in air-cooled and water-cooled discharge cells with a gas mixture of $He:H_{2}:CH_{4}$ (700:20:1) with a total pressure of $\approx 7$ Torr. The spectrum was observed using a difference frequency laser system combined with velocity modulation. The spectrum appeared in the same region as that of $C_{2}H_{3}^{+}$, but intensity variation with chemistry made it possible to discriminate lines of the two species. About 150 lines have been measured and assigned. An extensive analysis was done using the symmetric rotor Hamiltonian with l-resonance and higher order effects, $i.e. A_{1} - A_{2}$ splitting. The spectral pattern clearly indicates that the geometrical structure of $CH_{3}^{+}$ has $D_{3h}$ symmetry as anticipated by theory. The following preliminary fundamental vibration-rotation constants have been obtained (in $cm^{-1}$): $\begin{array}{llllllllll}\nu_{0} +\frac{3}{4}\eta k=3106.53995(980)&B^{0} &=& 9.36208(26)& C^{0}&=& 4.61252\\& B^{\prime} &=& 9.27448(63) &C^{\prime} &=&4.56959 &C^{\prime}_{\zeta_{33}} &=& 0.52025\\\end{array}$ In separating $C^{\prime}_{\zeta_{33}}$ from $C^{\prime}$, we used a calculated inertial defect of $\Delta_{o}= 0.0535$ $amu{\AA}^{2}$