Knowledge Bank

A high resolution ($0.06cm-1$) infrared absorption spectrum, obtained using a multi-pass absorption cell and a scanning Ebert spectrometer, is presented for the region in which the combination band $\nu 2+2\nu 3$ of methane is expected to appear. A well-defined Q- and R-branch is observed. The R-branch consists of fine structure components in groups separated by about 10 to $11cm-1$. An analysis of the R-branch is made, assuming it belongs to $\nu 2+2\nu 2$, and an explanation based on a successful analysis of other methane bands$\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}$ is given for the unexpectedly regular structure. The fine structure components arising from tetrahedral vibration-rotation interactions are examined and compared with other bands of methane.