There havebeen many studies on the photodissociation dynamics of CN-containing compounds, 1 11but most of these were concerned with relatively simplemolecules. The more complicated• molecules that have been studied are dicyanoacetylene (C4N2), 2 acrylonitrile (C3H3N), 3 and cyanoacetylene (C3HN). 4-5 The discovery of these moleculesin outer space6, 7 suggests that fundamental laboratory studies of theirphotochemistry would be of value. Photochemical investigations using lasers to dissociate and to detect the fragments can supply information about product energy distributions and detailed photolysis mechanisms. 8, 9 Recent experiments have also increased our understanding of the UV multiphoton photodissociation process (UVMPPD). 3, 10-20 There are three mechanisms that have been proposed to explain UVMPPD: secondary, 12, 17 simultaneous, 13, 15 and sequential photolysis. 10, 11, 16, 20 Secondary photolysis involves the photodissociation of one of the primary photolysis products. Simultaneous photolysis implies that two or more photons are simultaneously absorbed by the ground-state parent molecule. Sequential photolysis involves the formation of a predissociative excited state via absorption of a single photon. This state can either decay by dissociation or beexcited further by absorption of another photon, and dissociate from the doubly excited state. The multiphoton mechanism for a particular molecule depends upon laser fluence, excited-state lifetimes, two-photon absorption cross sections, absorption cross sections of the primary photochemical products, and laser pulse widths. The pulse widths of laboratory excimer lasers are long enough that any mechanism may be described by a series of rate equations. 19 Simultaneous and sequential mechanisms are peculiar to laser photolysis, because they require the enormous intensity of a laser beam. The sec-ondary photolysis mechanism is important in cosmochemistry and cometary chemistry, 7 where the time between collisions is very long and the reaction rates are very slow. Under such circum-stances a principal loss mechanism for free radicals is photolysis. Fumaronitrile (trans-1, 2-dicyanoethylene) is a large molecule consisting of eight atoms. It has a conjugated bond system and a planar structure