Multiple steps and multiple excitations in photoisomerization of azobenzene

Detailed understanding of femtosecond-scale photochemical processes requires dynamical simulations that are complementary to interpretations based on transitions between energy surfaces. For cis to trans photoisomerization of azobenzene following a 100 fs laser pulse, we find that the mechanism is rotation about the central NN bond, and the process is complete in less than 1 ps. The initial excitation and subsequent de-excitation are each achieved via multiple steps, with the molecule always in a superposition of electronic states, as the 3N nuclear degrees of freedom are excited by the laser pulse.