Dipole moment analysis of excited van der Waals vibrational states of ArH35Cl

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Abstract
The far‐infrared laser electric resonance spectra of the prototypical atom–diatom complex ArH35Cl are analyzed using improved zero‐field molecular constants, yielding accurate permanent and transition dipole moments for the three lowest excited van der Waals vibrational states. The constants are obtained from a multistate fit to previous microwave, far‐infrared laser electric resonance, and far‐infrared tunable laser spectra, as well as new far‐infrared measurements of the Σ‐stretch state, which are reported here. The signs of the dipole moments and Coriolis coefficients establish the relative orientations of the HCl subunit in these states. The fit is found to converge only if these signs correspond to the HCl pointing in opposite directions along the a inertial axis in the Σ‐stretch and Σ‐bend states. A weak preference, near the experimental error limit, is found for the Ar–Cl–H average angle in the Π‐bend state to be greater than 90°, contrary to expectation. For the best fit, we obtain the a‐axis dipole moment components −0.5413(11) D (Σ bend), −0.263 45(29) D (Π bend), and 0.6754(36) D (Σ‐stretch); and the b‐axis components 0.365(12) D (Π and Σ‐bend) and −0.0465(43) D (Π and Σ stretches), where the signs of the Coriolis coefficients and μa for the Σ stretch have been arbitrarily fixed positive. For the expected Π‐bend configuration, with the Ar–Cl–H angle less than 90°, the magnitudes along the a axis change only slightly, but the b‐axis components become 0.149(12) and −0.1403(64) D for the Π–Σ‐bend and Π–Σ‐stretch interactions, respectively.