I*(2P1/2) and Cl*(2P1/2) production from chloroiodobenzenes in the ultraviolet

The relative quantum yields of I*(P-2(1/2)) and Cl*(P-2(1/2)) production, phi*(I) and phi*(Cl), respectively, have been measured at four different ultraviolet excitation wavelengths, e.g., 222, 236, 266, and 280 nm in the photodissociation of o-, m-, and p-chloroiodobenzenes. The measured I* and Cl* quantum yields are, with some exceptions, higher than those obtained, respectively, from iodobenzene and chlorobenzene, at the same wavelengths. While at most wavelengths the major fraction of the iodine atoms is produced in the excited state, the opposite is true for the chlorine atoms. Both direct and indirect dissociation pathways are involved in the production of I* atoms whereas Cl* is produced only by indirect pathways since direct excitation of the sigma*(C-Cl) <-- n(Cl) transition is not possible at these wavelengths. The halogen atom in the ortho position is found to be most effective in enhancing the yield of the other spin-orbit excited halogen atom in the photolysis. While the nature of the initial transition, the extent of intersystem crossing in the excited states and the exit channel effects need to be considered in interpreting the quantum yield results, some factors seem to be more effective in influencing the final outcome. Induced dipole-induced dipole and quadrupole-quadrupole interactions between the two halogen atoms (I and Cl in this case) seem to play an important role in the exit channel dynamics. These electrostatic interactions facilitate the intersystem transfer in the excited state and subsequent production of the spin-orbit excited halogen atoms.