TUNABLE FAR INFRARED-LASER SPECTROSCOPY OF A TERNARY VANDERWAALS CLUSTER AR2HCL - A SENSITIVE PROBE OF 3-BODY FORCES

The first far infrared intermolecular vibration-rotation spectrum of a ternary van der Waals cluster has been measured near 39.5 cm-1 and assigned to an a-type SIGMA bending vibration of Ar2HCl. Spectra of both chlorine isotopes were observed and nuclear quadrupole hyperfine structure was resolved. Values of the fitted constants (rotational constants, hyperfine projections) evidence large amplitude out-of-plane motion, and demonstrate the sensitivity of spectroscopic observables to the three body forces operative in the the Ar2HCl system. Spectroscopic predictions calculated by Hutson et al. from pairwise-additive and "three-body" corrected potential energy surfaces [J. Chem. Phys. 90, 1337 (1989)] are compared to experimental results.