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

The hindered internal rotor states (n K = 00, 11, and 10) 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 = 11 and 10. 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 N≡C–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 H2-CN in the CN overtone region, which exhibits a rotationally resolved Σ ← Σ parallel band that is consistent with theoretical predictions for ortho-H2-CN