Diffractive imaging of dissociation and ground-state dynamics in a complex molecule

We have investigated the structural dynamics in photoexcited 1,2-diiodotetrafluoroethane molecules $\left(C_2{F}_4{I}_2\right)$ in the gas phase experimentally using ultrafast electron diffraction and theoretically using FOMO-CASCI excited-state dynamics simulations. The molecules are excited by an ultraviolet femtosecond laser pulse to a state characterized by a transition from the iodine $5p\text{⊥}$ orbital to a mixed $5p\text{∥}\text{σ}$ hole and $C{F}_2^{\text{∙}}$ antibonding orbital, which results in the cleavage of one of the carbon-iodine bonds. We have observed, with sub-Angstrom resolution, the motion of the nuclear wave packet of the dissociating iodine atom followed by coherent vibrations in the electronic ground state of the $C_2{F}_4{I}_2$ radical. The radical reaches a stable classical (nonbridged) structure in less than 200 fs.