VIBRATIONAL STARK SPECTROSCOPY .1. BASIC THEORY AND APPLICATION TO THE CO STRETCH

The effects of uniform electric fields on equilibrium interatomic separations, vibrational energy levels, and infrared transition moments are summarized for diatomic molecules in a matrix in which random orientation is maintained in both ground and excited states. The dependence of each property on field strength F can be expressed as a function of a linear and a quadratic term in the field strength. For vibrational energy levels, these terms are conventionally but imprecisely described as the vibrational dipole moment change and the vibrational polarizability change, respectively, accompanying excitation to a given vibrational excited state; for the dipole moment gradient, which determines the intensity, the analogous terms correspond to the transition polarizability and transition hyperpolarizability, respectively. Parameters describing the electric field response can be defined in terms of the quadratic and cubic force constants, and the first, second, and third gradients of the dipole moment, polarizability, and first hyperpolarizability of the molecule considered. This develops the earlier work of Hush and Williams (1974), Gready, Bacskay and Hush (1977-8), Lambert (1983-91), and the more recent work of Bishop et al. (1993-). The electric field response can, in principle, be measured in a manner analogous to that of electronic transitions in electroabsorption spectroscopy. The general form of this, based on the analysis of Liptay for the Stark effect on electronic transitions, is outlined for the corresponding vibrational case, and computational strategies are discussed. Stark vibrational spectral parameters for CO calculated at different levels of quantum calculation and by different strategies for analysis of the electric field perturbation data are presented.