ANALYSIS OF THE ROTATIONAL LEVELS OF WATER AND DETERMINATION OF THE POTENTIAL-ENERGY FUNCTION FOR THE BENDING NU(2) MODE

A theoretical approach suitable for the calculation of the rovibration al energy levels of water taking into account the anomalous centrifuga l distortion displayed by this molecule is presented. In agreement wit h a previous investigation (L. H. Coudert, J. Mol. Spectrosc. 154, 427 -442, 1992), a four-dimensional vibration-rotation Hamiltonian is cons tructed in which the large-amplitude bending nu2 Mode and the three de grees of freedom corresponding to the overall rotation of the molecule are considered simultaneously. However, the potential energy function adopted in the present approach displays a physically much more satis factory behavior than that used in the previous investigation. After i ntroducing phenomenological distortion terms, this Hamiltonian is used to carry out three analyses of already published data and to determin e this potential energy function. In the first analysis, the rotationa l levels belonging to the ground vibrational state (J.-M. Flaud et al. , Mol. Phys. 32, 499-521, 1976) are fitted with a unitless standard de viation of 2.4 using 36 model parameters. In the second analysis, the rotational levels belonging to the upsilon2 = 1 state (C. Camy-Peyret et al., Mol. Phys. 33, 1641-1650, 1977) are fitted for J less-than-or- equal-to 24 with a standard deviation of 2.3 using 31 parameters. In t he last analysis, microwave and infrared data corresponding to rotatio nal transitions within the ground vibrational state are fitted with a standard deviation of 1.1 using 36 parameters. (C) 1994 Academic Press , Inc.