Knowledge Bank

We report the first spectroscopic observation of the $CH2+$ molecular ion in the $X~2A1$ ground electronic state as part of our continuing search for carboions in glow discharges. This ion has been known from the early days of mass spectroscopy and previous experimental works on $CH2+$ have reported mobility and reactivities. These studies show that $CH2+$ is one of the principal products of the Penning ionization of $CH4$ but that it reacts with $H2$ with a large Langevin cross-section to form $CH3+$. Our refinement of the previous discharge cell allowed us to place more primary ionization products into the infrared path. The spectrum was found as part of a comprehensive spectroscopic search with a plasma mixture of $CH4/H2/He=0.12/0.10/7$ Torr. Not surprisingly, turning off the $H2$ increased signals of this particular ion. As measured in the C$H4/H2$/He plasma, the strongest $CH2+$ lines have a signal to noise of 30. The $v3$ band origin falls at $3141.37cm-1$. The ground state is the lower component of a Renner-Teller doublet, and ab initio theory indicates that the barrier to linearity is small ( $\le$ $1000cm-1$). This introduces complications to the rotational structure. So far only the $Ka=0$ series has been definitely $identified.1$ More analysis is in progress.