The van der Waals rovibrational states of the Ar–NH3 dimer

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Abstract
Starting from an ab initio anisotropic potential surface, we have calculated the bound rovibrational states of the Ar–NH3 dimer for J=0, 1, 2, and 3. There is good agreement with data available from molecular beam spectroscopy in the microwave and far-infrared regions. The nature of the states is discussed and they are correlated with free internal rotor states. In spite of the substantial barriers to internal rotation, Ar–NH3 resembles much more a free internal rotor complex than a nearly rigid dimer. Still, the van der Waals vibrations show characteristic differences from free internal rotations. In particular, there is strong mixing between the fundamental stretch vibration of the dimer and the lowest bending mode. We assign the band observed in the far-infrared region to the lowest, primarily bending, of these modes. Further transitions in the yet unexplored far-infrared regions are predicted.