Tunable far infrared laser spectroscopy of van der Waals bonds: Extended measurements on the lowest Sigma bend of ArHCl

A tunable far infrared laser system has been used to measure the vibration–rotation spectrum of the lowest Sigma bending state of ArHCl near 24 cm^−1 in a cw planar jet operating with a terminal jet temperature near 3 K. Over 60 transitions have been observed for both 35Cl and 37Cl isotopes with resolution of the quadrupole hyperfine structure. An improved set of molecular parameters was determined, including B, D, H, and eqQ for both upper and lower states. Very narrow linewidths (approximately 300 kHz) resulting in high resolution and sensitivity make this technique a powerful new method for the detailed investigation of intermolecular forces.