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 rovibrational energy levels of water taking into account the anomalous centrifugal distortion displayed by this molecule is presented. In agreement with a previous investigation (L. H. Coudert, J. Mol. Spectrosc. 154, 427-442, 1992), a four-dimensional vibration-rotation Hamiltonian is constructed in which the large-amplitude bending nu2 Mode and the three degrees 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 satisfactory behavior than that used in the previous investigation. After introducing phenomenological distortion terms, this Hamiltonian is used to carry out three analyses of already published data and to determine this potential energy function. In the first analysis, the rotational levels belonging to the ground vibrational state (J.-M. Flaud et al., Mol. Phys. 32, 499-521, 1976) are fitted with a unitless standard deviation 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 the last analysis, microwave and infrared data corresponding to rotational transitions within the ground vibrational state are fitted with a standard deviation of 1.1 using 36 parameters. (C) 1994 Academic Press, Inc.