The photodissociation of [C4H4]+' fragment ions at the ion cyclotron resonance time-scale competes with relaxation of the internal energy by infrared emission. As a result the fraction of photodissociating ions increases with light intensity. The experiments indicate that [C4H4]+' from benzene and 1,Shexadiyne consists of a mixture of 60% vinyl acetylene ions, 10% butatriene ions and 30% cyclic ions. This confirms previous conclusions from studies of the ioh-molecule reactions of [C,H,]+' with benzene. ~~ ._ -