Experimental characterization of the weakly anisotropic CN X ²Σ⁺ + Ne potential from IR-UV double resonance studies of the CN-Ne complex

IR-UV double resonance spectroscopy has been used to characterize hindered internal rotor states (n^K = 0⁰, 1¹, and 1⁰) of the CN-Ne complex in its ground electronic state with various degrees of CN stretch (ν_CN) excitation. Rotationally resolved infrared overtone spectra of the CN-Ne complex exhibit perturbations arising from Coriolis coupling between the closely spaced hindered rotor states (1¹ and 1⁰) with two quanta of CN stretch (ν_CN = 2). A deperturbation analysis is used to obtain accurate rotational constants and associated average CN center-of-mass to Ne separation distances as well as the coupling strength. The energetic ordering and spacings of the hindered internal rotor states provide a direct reflection of the weakly anisotropic intermolecular potential between CN X ²Σ⁺ and Ne, with only an 8 cm^-1 barrier to CN internal rotation, from which radially averaged anisotropy parameters (V₁₀ and V₂₀) are extracted that are consistent for ν_CN = 0-3. Complementary ab initio calculation of the CN X ²Σ⁺ + Ne potential using MRCI+Q extrapolated to the complete one-electron basis set limit is compared with the experimentally derived anisotropy by optimizing the radial potential at each angle. Experiment and theory are in excellent accord, both indicating a bent minimum energy configuration and nearly free rotor behavior. Analogous experimental and theoretical studies of the CN-Ne complex upon electronic excitation to the CN B ²Σ⁺ state indicate a slightly more anisotropic potential with a linear CN-Ne minimum energy configuration. The results from these IR-UV double resonance studies are compared with prior electronic spectroscopy and theoretical studies of the CN-Ne system.