Knowledge Bank

The spectrum of the $CH2-$ ion is difficult to analyze because of the existence of the Renner-Teller effect and a low barrier to linearity. In our previous $study1$ of the $\nu 3$ band of $CH2+$, we could assign only 18 lines in the $Ka=0$ series of the $\alpha$-type parallel transition because of the irregularity of other series and because of the complexity of the overall spectrum due to other carbocations in the plasma. In order to maximize $CH2-$ signals as opposed to those of $CH3+$, we chose a plasma similar to the one in our initial scan but without $H2$ since. [ CH_{4} + He^{*} \stackrel{70%}{\rightarrow} CH_{2}^{+} \stackrel{H_{3}}{\rightarrow} CH_{3}^{-}. ] We found that a mixture of $CH4$/He=90mT/7 Torr in a liquid-$N2$ cooled positive column clearly distinguishes the object of our quest. An extensive scan around the band origin of $\nu 3=3131cm-1$ using such conditions allowed us to unambiguously assign the tow series for $Ka=1$ and some lines with $Ka=2$. The series are quite irregular because of the Renner-Teller perturbation in the $\nu 3$ state, but the ground state combination differences are fairly regular. Altogether, 45 transitions the been assigned, and 15 combination differences in the ground state have been fitted.