Dynamical responses of azobenzene initiated in its lowest excited state S 1 are shown in Fig. . Panel (a) displays the S 1 (orange) and S 0 (blue) population evolution. Since the total, S 1 + S 0 , population is conserved, we concentrate on the S 1 population dynamics, which can be subdivided in several stages. The population remains constant within the first 25 fs. This establishes a characteristic time which the photoinduced wavepacket needs to travel from the initial Franck-Condon region to the IC region. From 25 fs to 90 fs, the population exhibits a fast decay: S 1 looses half of its initial value at around 50 fs. Then the slower decay is observed. .