Photodissociation of molecular beams of aryl halides

A molecular beam photodissociation study has been made of a number of simple aryl iodides and bromides. The angular distribution of photofragments is characterized by an anisotropy parameter β which was measured for each molecule. The values of β depend on the lifetime of the excited state compared to the rotational correlation times of the molecule as well as on the orientation of the transition dipole with respect to the C–X bond. Knowing these orientations we can extract excited state liftimes as follows: methyl iodide, 0.07 picoseconds (psec); iodobenzene, 0.5 psec; α-iodonaphthalene, 0.9 psec; and 4-iodobiphenyl, 0.6 psec. It is concluded that methyl iodide directly dissociates but that the aryl compounds predissociate. The analogous aryl bromides have small anisotropy parameters and it is estimated that they live in the excited state perhaps two orders of magnitude longer than the aryl iodides. The data suggest a mechanism for predissociation in which S2 undergoes intersystem crossing to a triplet state localized on the C–I bond either directly or via S1.