SIMPLE THEORY OF DIFFUSE STRUCTURE IN CONTINUOUS UV SPECTRA OF POLYATOMIC-MOLECULES .2. PHOTODISSOCIATION OF BENT SYMMETRICAL TRIATOMICS

We present two simple models for calculating photoabsorption spectra of bent symmetric triatomic molecules, one designed for light-heavy-light systems with an inactive bending mode and a more general one with no assumptions about atomic mass ratios and where bending motion may be active. Both models are extensions of an earlier model [R. T Pack, J. Chem. Phys. 65, 4765 (1976)] for the photodissociation of collinear symmetric triatomic molecules which showed that excitation to a single dissociative electronic state can give absorption spectra with rich structure due to bound degrees of freedom perpendicular to the dissociation coordinate. Similarly, the present models predict structure in the absorption spectra which is easily understood in terms of the vibrational frequencies and bond lengths, and the steepness and height of the excited state barrier. As a test case, we use the models to calculate the photoabsorption spectra of the X 1A1 --> A 1B1 transition in water which is known to involve a single dissociative excited state surface. We find that these models, which only require knowledge of the ground-state potential-energy surface near the minimum and the excited state potential-energy surface in the barrier region, are quite successful and may serve as simple tools to investigate photoabsorption spectra of other molecules.