Three-dimensional potential energy surfaces for the electronic ground state as well as the lowest three (1)A " states have been computed using highly correlated CASSCF-MRCI wave functions and a large basis set. An approximate diabatization scheme has been employed to generate quasidiabatic potential energy surfaces for the lowest two (1)A " states. The diabatization is based on the condition that both the orbitals as well as the configuration coefficients of the diabatic wave functions change as little as possible as function of geometry. The diabatic potential energy surfaces are used in time-dependent simulations of the absorption spectrum as well as the vibrational and rotational product distributions. Excellent agreement between the computed and experimental absorption spectra and product distributions is obtained, indicating that the ab initio potentials as well as the diabatization scheme are accurate.