A Theoretical Study of the Stark Effect in Triatomic Molecules: Application to H2O

Abstract

We report here the development of a theoretical method to calculate Stark splittings and state-dependent dipole moments for a triatomic molecule based on the MORBID approach [P. Jensen, J. Mol. Spectrosc.128, 478-501 (1988)]. The Stark parameters are obtained directly from the potential energy surface and dipole moment function of the molecule. The transformation of the dipole moment from molecule-fixed axes to space-fixed axes is carried out by means of irreducible tenser formalism. We apply the new method to H2O, since for this molecule we have an accurate potential energy surface [P.Jensen, J. Mol. Spectrosc.133, 438-460 (1989)] obtained by fitting to experimental data, and a high-quality ab initio dipole moment surface [U. G. Jørgensen and P. Jensen, J. Mol. Spectrosc.161, 219-242 (1993)]. The results of calculations on the basis of these input data are in good agreement with experiment. However, we have improved this agreement further by refining the dipole moment surface in least-squares fits of experimental Stark data, in particular, of the very accurate MBER measurements by S. Shostak, W. Ebenstein, and J. S. Muenter [J. Chem. Phys.94, 5875-5882 (1991)]. With the refined dipole moment surface, we have calculated state-dependent dipole moment values. The variation of these values with vibrational excitation is in close agreement with that found in the experimentally derived values of Shostak et al.