Knowledge Bank

The lowest $\Pi$ bending state in ArHCl has been more fully characterized using far infrared laser/microwave double resonance spectroscopy. This analysis includes a reassignment of the far infrared spectrum of $ArH35Cl$ previously reported by Ray et al. and Marshall et al., as well as an analysis of the $ArH37Cl$ spectrum. The following new molecular constants have been determined for the $\Pi$ state in $ArH35Cl:\nu =1018.6861(3)GHz,B=1740.9(1)MHz,q1=-49.1(3)MHz,\nu =.2603(2)D,eqQaa=5.6(2.7)MHz$, and $eqQbb-eqQcc=-73.2(2.7)MHz$; and for $ArH37Cl:\nu =1018.4700(3)MHz,B=1691.4(2)MHz,q1=-47.0(3)MHz,\nu =.2606(1)D,eqQaa=4.0(1.8)MHz,andeqQbb-eqQcc=-57.3(1.8)MHz$. The new value of the rotational constant gives much better agreement with the value calculated by Hutson1 from the M5 double minimum potential surface than the previous value. Thus with the recently reported spectrum of the I bending vibration of $ArHCl2$ this analysis provides further strong support for a double minimum character in the van der Waals surface. In addition, the powerful combination of the double resonance technique, to assign complicated spectra, with the sensitive intracavity spectroscopic technique has been demonstrated