Photo induced three body decay

Abstract

The photo induced three body decay : ABC hnu A B C, where a molecule ABC decays into three fragments A, B and C upon irradiation, is reviewed. Various experimental and theoretical techniques for the investigation of this reaction and their application to a wide range of molecular species are discussed. Emphasis is laid on the distinction between concerted and stepwise processes, consisting of one single or two consecutive kinetic events, respectively. The concerted fragmentation scheme is further classified as being of either synchronous or asynchronous character, depending on whether or not the bond breaking processes take place in unison. The three body decays of acetone, azomethane and s-tetrazine are discussed in detail as prototypes for these mechanisms. A novel kinematic analysis approach, based on the evaluation of fragment kinetic energy distributions, is presented and applied to the ultraviolet photodissociation of phosgene. Competing pathways are found to be operative, dominated by the asynchronous concerted mechanism with preferential forward scattering of the carbon monoxide fragment. The synchronous concerted decay plays a minor role under significant excitation of the in-plane and out-of-plane bending modes of the parent molecule. Finally the power of the newly developed method for the analysis of the three body decay of a small polyatomic molecule is highlighted.