Photodissociation of OBrO through the excited 22A″(12A2) electronic state: Theoretical prediction of the absorption cross-section

Extensive highly correlated multireference configuration interaction calculations were carried out to generate for the first time full three-dimensional (3D) potential energy surfaces of the 12A″(12B1) ground state and the most important 22A″(12A2) excited electronic state of OBrO. Because of the bound character of the 22A″ excited state, i.e. a barrier of about 0.75 eV, relative to the 22A″ potential minimum, must be overcome for dissociation into the BrO + O channel, only indirect dissociation is possible and a highly structured absorption spectrum is expected. The other possible dissociation channel of OBrO supplying Br + O2 should play only a minor role, because a large barrier of about 1.7 eV, relative to the 22A″ potential minimum hinders the dissociation. The calculated 12A″ and 22A″ 3D surfaces were used in three-dimensional (3D) time-dependent wave-packet calculations to obtain a first theoretical estimation of the absorption cross-section for the dissociation through the 22A″ excited state of OBrO. The calculated value of 21.7 × 10−18 cm2 is in reasonable agreement with experimental findings (13-20 × 10−18 cm2). Despite the close correspondence of theory and experiment, the calculations have shown a sensitive dependence on the potential energy data and the value of the transition dipole moment so that a reliable quantitative theoretical prediction of absorption cross-sections remains a difficult task.