Picosecond photofragment spectroscopy. IV. Dynamics of consecutive bond breakage in the reaction C2F4I2→C2F4+ 2I

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Abstract
Picosecond photofragment spectroscopy of the ultraviolet (UV) photodissociation of 1,2-diiodotetrafluoroethane reveals consecutive breaking of the two C–I bonds. Spin–orbit excited (I*) atoms show a prompt rise, in agreement with a direct mode dissociation of the first bond. Ground-state (I) atoms show a biexponential buildup, one component being fast (≤1 ps) while the other component is slow (30–150 ps depending on total energy), characteristic of the second bond breaking. The transient behavior of I atoms changes with the available energy. These results are interpreted in terms of a two step model involving a weakly bound radical. Simulations of transient behavior of I atoms, based on estimated internal energy distributions from the primary step and a model for dissociation rates as a function of energy, suggest that surface crossings are relevant to the dynamics and that the quantum yield of I atoms varies with excitation energy.