Lower-lying potential curves of Br 2 are calculated by the spin-orbit configuration interaction method, and the spin-orbit branching mechanisms in the photodissociation processes are examined. The second absorption band at 227 nm is assigned unambiguously to the transition from the X 1 Σ + g to the 3 Σ + 1 u (σ g ->σ u * ) state. The calculated potential curves and the radial derivative coupling elements of Cl 2 and Br 2 are used to evaluate the nonadiabatic transition probabilities by solving the time-dependent coupled Schrodinger equations semiclassically. The different nonadiabatic behavior in Cl 2 and Br 2 is analyzed in terms of the differences in the electronic factors and the reduced mass.