Photodissociation of aryl and aryl–alkyl halides at 193 nm: Fragment translational energy distributions

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Abstract
Molecular beams of a number of aryl and aryl–alkyl halide molecules were photodissociated using an excimer laser at 193 nm and fragment translational energy distributions measured. Previous work had shown that dissociation of aryl halide molecules takes place by a spin–orbit dependent crossing from an intermediate (π, π*) delocalized state to a triplet (σ, σ*) state localized on the C–X bond (X=Cl,Br,I). The vibrational energy of the intermediate state remains in the aromatic framework; thus the fragment translational energy distribution reflects only the electronic but not the vibrational energy of the intermediate state. This fact is exploited in identifying the electronic nature of this state. Present work indicates that both phenyl and naphthyl halide molecules dissociate at 193 nm to produce vibrationally hot aryl radicals. The actual dissociation pathway depends on the competition between intersystem crossing and internal conversion which are functions of both ring size and halogen substituents. Iodobenzene dissociates from S3, S2, and S1 whereas chlorobenzene internally converts to S1 entirely before dissociating. Bromobenzene also dissociates from all three singlet states whereas 2‐bromonaphthalene dissociates from S1 even though it is excited to S4 at 193 nm. Fluorination (C6F5I and C6F5Br) drastically reduced the kinetic energy of the fragments which seems to indicate dissociation from a highly distorted aryl ring configuration which may be related to the well‐known benzene structural isomers. At lower photon energies it had been found that the flux of fragments of aryl iodides was anisotropic with respect to the E vector of the dissociating light but the flux from aryl bromides was isotropic. The same results are found at 193 nm with a somewhat different interpretation. In all cases, C6H5CH2X and C6H5C2H4X produced translational energy distributions peaking at zero energy which implies that intersystem crossing is so slow that the molecule internally converts from S3 to S0 with subsequent statistical unimolecular decomposition.

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