Knowledge Bank

The $\nu 2$ (bending) fundamental of $CH2$ in its ground $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}B1$ state was first studied1 using the laser magnetic resonance technique; 10 different rotation-vibration transitions were detected. Later a tunable diode laser was used$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ to observe $\nu 2$ in $CD2$ and one further transition in $CH2$. We have now greatly extended the study of $CH2$ by measuring 53 new transitions in the $\nu 2$ band in the regions 800 to 911 and 1030 to $1173cm-1$. We use a diode laser spectrometer, Zeeman modulation, and a multiple reflection cell as in the earlier work,$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ but find somewhat different optimum conditions for $CH2$ production, namely a mild discharge through a flowing mixture of ketene (=0.2 torr) and helium (=2 torr). Discharge modulation was also used to detect the $CH2$ absorption, but the sensitivity was worse than for Zeeman modulation, partly because the $CH2$ exhibited a surprisingly long lifetime ($=$1 ms) following the cutoff of the discharge. The new transitions extend to higher values of N (10) and $Ka$ (3) than were observed previously. Most important are 10 P- and Q-branch transitions of the $Ka=2+3$ sub-band, since neither nor had been observed previously. A combined analysis of the available$\mathrm{<mrow\; data-mjx-texclass="ORD">1,2,3</mrow>}$ data results in improved $CH2$ parameters, notably for the spin-rotation interaction constant $caa$.