Ab initio study of the singlet-triplet transitions in hypobromous acid

The absorption spectra of the HOBr molecule in the near-ultraviolet and in the visible regions produced by excitation to the dissociative triplet and singlet states have been studied by the multi-configuration self-consistent field method with linear and quadratic response techniques. A large basis set and the complete Breit-Pauli form of the spin-orbit coupling operator a re used. It is shown that the singlet-triplet (S-T) transition to the lowes t triplet state a(3)A " <-- X(1)A' is responsible for a weak absorption in the visible region (440-650 nm) recorded recently by Barnes et al. (J. Phys . Chem. 100(1996) 453). The photodissociation of the HOBr molecule by visib le light near the peak of the solar actinic flux through the S-T transition indicates the importance of SOC effects in atmospheric photochemistry of h eavy halogens. The transition is polarized along the O-Br bond in agreement with the experimental measurements of OH Doppler line profiles (Barnes et al., J. Phys. Chem. 100 (1996) 453). The second ST transition b(3)A' <-- X( 1)A' enters into the region of the first singlet-singlet (S-S) A(1)A " <-- X(1)A' absorption and produces an appreciable contribution to the total cro ss section at wavelengths lambda approximate to 340-360 nm in addition to t he S-S absorption. The S-T absorption decreases the concentration and the l ifetime of tropospheric hypobromous acid and influences the ozone depletion problem to a great extent. (C) 1999 Elsevier Science B.V. All rights reser ved.