Photodissociation of HOBr. I. Ab initio potential energy surfaces for the three lowest electronic states and calculation of rotational-vibrational energy levels and wave functions

Potential energy surfaces are presented for the three lowest lying singlet electronic states of HOBr. The surfaces are computed using the recently dev eloped multireference averaged quadratic coupled clusters method and a TZ2P orbital basis set. They provide the basic data needed to compute the dynam ics of the HOBr + h nu --> OH + Br photodissociation process, which plays a key role in the bromine chemistry of the stratosphere. A pseudopotential i s used for the core electrons of the Br atom; this is shown not to introduc e any errors in the shape of the surfaces through direct comparison with sa mple all-electron calculations. Transition dipole moment surfaces for the t wo excitation processes (1 (1)A " <-- (X) over tilde (1)A' and 2 (1)A' <-- (X) over tilde (1)A') are also presented. These are computed using a multir eference singles and doubles configuration interaction method. A grid based method is developed to compute the vibrational-rotational states of the mo lecule and spectroscopic constants extracted from the computed molecular en ergy level spacings are compared with experimentally determined quantities. (C) 1999 American Institute of Physics. [S0021-9606(99)30514-6].