Photodissociation of energy selected C4H+6 ions: The isomerization barrier between butyne and 1,3 butadiene ion isomers

Ions are energy selected by photoelectron photoion coincidence (PEPICO) and subsequently photodissociated by a pulsed excimer pumped dye laser. The laser is triggered by the delayed zero energy electron signal so that the laser pulse intercepts the single ion at a fixed point in the ion trajectory. The product ions are separated from the undissociated parent ions by their time of flight. Evidence is presented for both single and multiphoton dissociation of the butadiene ions. The derived kinetic energy release distribution (KERD) of the C3H+3 product ions is consistent with the statistically predicted KERD. Finally, the isomerization barrier between 1- and 2-butyne and butadiene ions is measured by probing the ion structure with the laser as a function of the C4H+6 internal energy. Because this rearrangement requires only H atom shifts, the measured barrier is associated with the ubiquitous H atom scrambling process commonly observed in unsaturated hydrocarbon ions.