Tunable far infrared laser spectroscopy of van der Waals bonds: Ar-NH3

Hyperfine resolved vibration-rotation-tunneling spectra of Ar--NH3 and (NH3)2, generated in a planar supersonic jet, have been measured with the Berkeley tunable far infrared laser spectrometer. Among the seven rotationally assigned bands, one band belongs to Ar--NH3, and the other six belong to (NH3)2. To facilitate the intermolecular vibrational assignment for Ar--NH3, a dynamics study aided by a permutation-inversion group theoretical treatment is performed on the rovibrational levels. The rovibrational quantum number correlation between the free internal rotor limit and the semi-rigid limit is established to provide a basic physical picture of the evolution of intermolecular vibrational component states. An anomalous vibronically allowed unique Q branch vibrational band structure is predicted to exist for a near prolate binary complex containing an inverting subunit. According to the model developed in this work, the observed band of Ar--NH3 centered at 26.470633(17) cm-1 can correlate only to either the fundamental dimeric stretching band for the A2 states with the NH3 inversional quantum number vi = 1, or the Ka = 0 left-arrow 0 subband of the lowest internal-rotation-inversion difference band. Although the estimated nuclear quadrupole coupling constant favors a tentative assignment in terms of the first possibility, a definitive assignment will require far infrared data and a dynamical model incorporating a potential surface