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 are used. It is shown that the singlet-triplet (S-T) transition to the lowest triplet state a³A'' ← X¹A' 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 visible light near the peak of the solar actinic flux through the S-T transition indicates the importance of SOC effects in atmospheric photochemistry of heavy 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 S-T transition b³A' ← X¹A' enters into the region of the first singlet-singlet (S-S) A¹A'' ← X¹A'' absorption and produces an appreciable contribution to the total cross section at wavelengths λ ≃ 340-360 nm in addition to the S-S absorption. The S-T absorption decreases the concentration and the lifetime of tropospheric hypobromous acid and influences the ozone depletion problem to a great extent.