Energy partitioning in photodissociation. Rotational energy disposal in the photodissociation of the cyanogen halides

Abstract

Rotational energy disposal in the vacuum u.v. photodissociation of the cyanogen halides has been studied by analysis of the CN(B→X) photofragment fluorescence spectrum at high resolution. The experimental conditions were chosen to exclude collisional relaxation and interference from competing photodissociation pathways. The level of rotational excitation in CN(B)_v= 0 increases in the order ICN < BrCN < ClCN, and the rotational distribution from BrCN at 158 nm closely approximates Boltzmann behaviour with a rotational temperature ∼1700 K. In contrast, photodissociation of ClCN at 147 nm produces a bimodal distribution while photodissociation of ICN at 185 nm leads to a distribution peaking at very low levels, but with a weak “tail” extending to the maximum available energy. A simplified quantum mechanical model is described, which accommodates most of the observed behaviour and which emphasises the constraints imposed by the requirements of energy and angular momentum conservation. In consequence, dissociation is restricted to near linear configurations in the photo-excited molecules.