MICROWAVE ROTATIONAL SPECTRUM OF THE $Kr-CH_{4}$ VAN DER WAALS COMPLEX

The first pure rotational spectra of a rare gas-methane van der Waals complex, namely $K_{r}-CH_{4}$, were recorded, using a pulsed molecular beam Fourier transform microwave spectrometer. Five isotopomers, including $^{86}Kr-CH_{4}, ^{84}Kr-CH_{4}, ^{83}Kr-CH_{4}, ^{82}Kr-CH_{4}$, and $^{80}Kr-CH_{4}$, 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 $^{84}Kr$ and $^{86}Kr$ 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 $et al.^{b}$ 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.