Experimental characterization of the CN X ²Σ ⁺ + Ar and H ₂ potentials via infrared-ultraviolet double resonance spectroscopy

The hindered internal rotor states (n ^K 0 ⁰, 1 ¹, and 1 ⁰) of the CN-Ar complex with two quanta of CN stretch (v _CN 2), along with its ground state (v _CN 0), have been characterized by IR-UV double resonance and UV spectroscopy. Analysis of rotationally structured bands enable n ^K assignments and reveal perturbations due to Coriolis coupling between two closely spaced hindered rotor states, n ^K 1 ¹ and 1 ⁰. A deperturbation analysis is carried out to derive accurate rotational constants and their associated CN center-of-mass to Ar bond lengths as well as the magnitude of the coupling. The energetic ordering and spacings of the CN-Ar hindered rotor states provide a direct experimental probe of the angular dependence of the CN X ²Σ ⁺ + Ar potential and permit radially averaged anisotropy parameters (V ₁₀ 5.2 cm ^-1 and V ₂₀ 3.2 cm ^-1) to be determined. This analysis indicates a relatively flat potential about a linear NC-Ar configuration with a barrier to CN internal rotation of only ∼12 cm ^-1. The angular potentials determined from experiment and ab initio theory are in good accord, although theory predicts a higher barrier to CN internal rotation. A similar approach yields the infrared spectrum of H ₂-CN in the CN overtone region, which exhibits a rotationally resolved Σ ← Σ parallel band that is consistent with theoretical predictions for ortho-H ₂-CN.