An investigation of three-body effects in intermolecular forces. II. Far-infrared vibration–rotation–tunneling laser spectroscopy of Ar2HCl

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Abstract
A second Ar2HCl intermolecular vibration–rotation band centered at 37.2 cm−1 has been measured and assigned as a b‐type transition originating from the ground state. Nuclear hyperfine splittings were resolved for both chlorine isotopes. The rotational constants determined from the data indicate coupling between an Ar–Ar stretching or bending coordinate and the Ar2 –HCl vibrational coordinates. As a result of this particular vibrational motion, Ar2H 35Cl undergoes an axis‐switching transition while the Ar2H 37Cl isotope does not. In addition, the measured hyperfine projections indicate the possibility of coupling between the Ar2 –HCl stretching and bending modes, preventing an absolute vibrational assignment. These results indicate that the ‘‘reversed adiabatic’’ approximation employed by Hutson, Beswick, and Halberstadt in their theoretical study of Ar2HCl [J. Chem. Phys. 9 0, 1337 (1989)] is not appropriate for the complicated intramolecular dynamics presently observed in this system.