Tunable far-infrared laser spectroscopy of van der Waals bonds: thejkc= 10← 00Σ bending vibration of Ar-14NH3

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Abstract
Hyperfine resolved spectra have been measured for the Ar-NH3 complex over the range 21–28 cm-1 using a tunable far-infrared laser in combination with a continuous planar supersonic jet. Twenty-three transitions are assigned to the lowest Σ-bending vibration (v 0 = 26·470633(77)cm-1) from the (asymmetric) inversion level of the Σ, j k c = 00 internal rotor state of NH3 to the Σ, (10) symmetric inversion level. A nearly free-rotor model is used to deduce the zeroth-order intermolecular vibrational energy-level diagram and corresponding selection rules, which are used in assigning the spectra. All evidence obtained in this study supports the contention of Nelson et al. that NH3 is an inverting nearly free rotor in this T-shaped complex.