Ultrafast structural dynamics in the photodissociation of iodobenzene captured with ultrafast electron diffraction and a genetic algorithm

We investigate the fragmentation dynamics of iodobenzene following UV-induced photodissociation using a kilo-electron-volt ultrafast electron diffraction setup. The dissociation of the iodine atom takes place on a sub-picosecond timescale via two distinct dissociation channels (fast and slow), which we have identified in the diffraction signal. We use a genetic algorithm to retrieve the structure of the phenyl ring after dissociation directly from the diffraction data, representing the ensemble-averaged geometry. The retrieved phenyl ring shows that the C_2v symmetry is preserved. The correlations among retrieved internal coordinates indicate an in-plane vibrational mode, which is assigned to the ν₁₀ (“squatting”) mode, as the dominant mode. Ab-initio calculations of the dissociation also indicate that the C_2v symmetry is preserved and the same active molecular vibration is present in both dissociation channels. The agreement between calculation and experiment suggests a similar relaxation pathway of the fragment despite different initial excited states in both the fast and slow dissociation channels.