Knowledge Bank

We have developed a technique in which we use a cw planar supersonic jet expansion and a tunable far infrared (FIR) laser to investigate the low energy vibrations and tunneling motions of weakly bound van der Waals molecules. The planar jet expansion provides a high density of clusters as well as a relatively long pathlength (1.5 inches) for direct absorption spectroscopy. We also observed a factor of 5-10 reduction in the linewidth over a conventional circular nozzle, resulting in an increase in resolution. The tunable far infrared radiation is the product of non-linear mixing in a GaAs Schottky barrier diode of line tunable FIR radiation and tunable (2-75 GHz) microwave radiation. This laser provices much broader tunability and a simplified spectrum over more conventional Stark or Zeeman tuned FIR spectrometers. The first system studied using this technique was the previously studied $\Sigma$ bend of ArHC1. We greatly extended the existing data set and improved both ground and excited state constants as well as adding higher order centrifugal distortion constants. We have also used this technique to study more complicated dimers, including $ArH2O$ and $(HC1)2$.