Picosecond photofragment spectroscopy. IV. Dynamics of consecutive bond breakage in the reaction C_2F_4I_2→C_2F_4+ 2I

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.