Knowledge Bank

The Q branch of the pure rotational spectrum of $\mathrm{<mrow\; data-mjx-texclass="ORD">12</mrow>}CH4$ in the ground vibronic state has been studied from 8 to 20 GHz with a conventional Stark microwave spectrometer employing a signal averager. This spectrum is very weakly allowed through the centrifugal distortion electric dipole moment. The detection of four lines between 8 and 18 GHz using a backward-wave oscillator source has been previously $reported.1$ Three new lines have now been observed above 18 GHz with a klystron system. The seven microwave frequencies have been combined with three Q branch splittings available from double $resonance2$ and molecular $beam3$ work to determine the quartic, sextic and octic distortion constants of $\mathrm{<mrow\; data-mjx-texclass="ORD">12</mrow>}CH4$. The six constants obtained will be compared with those determined by other $workers,2,4$. The spectrometer used will be described briefly.