Far-infrared vibration-rotation-tunneling spectroscopy of Ar-NH3: Intermolecular vibrations and effective angular potential energy surface

Two new intermolecular vibration-rotation-tunneling (VRT) bands of Ar-NH3 have been measured using tunable far infrared laser spectroscopy. We have unambiguously assigned these and a previously measured FIR band [ Gwo et al., Mol. Phys. 71,453 ( 1990) ] as II ( l,,n = 0) +B(O,,n = 0), 2( l,,n = 0) +-Z(O,,n = 0), and B(O,,n = 1) +-Z(O,,n = 0). The three upper states of these are found to be strongly mixed by anisotropy and Coriolis effects. A simultaneous least squares fit of all transitions has yielded vibrational frequencies, rotational and centrifugal distortion constants, and a Coriolis parameter as well as quadrupole hyperfine coupling constants for the upper states. An effective angular potential energy surface for Ar-NH, in its lowest stretching state has been determined from these data, after explicitly accounting for the effects of bend stretch interactions. Features of the surface include a global minimum at the near T-shaped configuration (0 = 90”)) a 30 cm ’ to 60 cm ’ barrier to rotation at 0 = 180” (or 0”)) and a very low barrier or possibly a secondary minimum at 8 = 0” (or 180”). Both attractive and repulsive interactions are shown to contribute significantly to the anisotropic forces in the complex. Comparison with ab initio calculations are presented.