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 PI(1(0),n = 0) <-- SIGMA(0(0),n = 0), SIGMA(1(0),n = 0) <-- SIGMA(0(0),n = 0), and SIGMA(0(0),n = 1) <-- SIGMA(0(0),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-NH3 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 (theta = 90-degrees), a 30 cm-1 to 60 cm-1 barrier to rotation at theta = 180-degrees (or 0-degrees), and a very low barrier or possibly a secondary minimum at theta = 0-degrees (or 180-degrees). Both attractive and repulsive interactions are shown to contribute significantly to the anisotropic forces in the complex. Comparison with ab initio calculations are presented.