Knowledge Bank

The first pure rotational spectra of a rare gas-methane van der Waals complex, namely $Kr-CH4$, were recorded, using a pulsed molecular beam Fourier transform microwave spectrometer. Five isotopomers, including $\mathrm{<mrow\; data-mjx-texclass="ORD">86</mrow>}Kr-CH4,84Kr-CH4,83Kr-CH4,82Kr-CH4$, and $\mathrm{<mrow\; data-mjx-texclass="ORD">80</mrow>}Kr-CH4$, were studied. Two sets of transitions were measured in the range of 4-18 GHz: one with $K=0$, $J=0$ to $J=4$, the other with $K=1$, $J=1$ to $J=4$. These transitions were assigned to occur within the A, $K=0$ and F, $K=1$ states, respectively. Of the latter set, two transitions of the $\mathrm{<mrow\; data-mjx-texclass="ORD">84</mrow>}Kr$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">86</mrow>}Kr$ containing isotopomers were first measured at $NIST,a$ Rotational constants and centrifugal distortion constants were fitted separately for both states. The determined rotational constants agree well with the values determined in the previous IR study by Pak $etal.b$ with a difference less than one MHz for the $A$, $K=0$ state, and a somewhat larger difference of 24 MHz for the $F$, $K=1$ state. This larger difference might indicate a misassignment in the IR region for the F, $K=1$ state. The search for the $E$-state transitions is still ongoing.