Slit-jet near-infrared spectroscopy and internal rotor dynamics of the ArH2O van der Waals complex: An angular potential-energy surface for internal H2O rotation

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Abstract
Near‐infrared vibration‐rotation spectra of jet‐cooled Ar‐H2O complexes are detected for the first time via direct absorption of tunable difference frequency infrared radiation in a slit supersonic expansion source. Transitions from both the lowest para and ortho complexes are observed which correlate to 000 and 101 rotational levels of free H2O, respectively, and permit spectroscopic characterization of the complex in both the ground (ν3=0) and asymmetric stretch excited (ν3=1) levels. From hot bands involving excited internal rotor states correlating with the 101 H2O rotational level, we determine the Σe(101)‐Πf(101) energy splitting to be 11.3333(3) cm−1 (J=1). In conjunction with far‐infrared measurements of the Σ(110)‐Π(101) and Π(110)‐Σ(101) energy splittings, this information permits determination of a two‐dimensional (2D) angular potential‐energy surface of the complex as a function of the H2O orientation. The barriers to internal rotation of an ArH2O differ for in‐plane (19 cm−1) and out‐of‐plane (33 cm−1 ) rotation of the H2O subunit. However, both barriers are only slightly higher than the ground‐state energies and, hence, the behavior of H2O in the complex is that of a nearly free rotor. Agreement with recent ab initio calculations by Chalasinski et al. [J. Chem. Phys. 9 4, 2807 (1991)], as well as with 3D fits solely to far‐infrared data by Cohen and Saykally [J. Chem. Phys. 9 4, 7991 (1991)], is remarkably good. Predictions based on this angular potential‐energy surface are made for the internal rotor states of ArHDO and ArD2O and compared with recent far‐infrared measurements by Suzuki et al. [J. Phys. Chem. 9 4, 824 (1991)].