Bimolecular Ni+(2D5/2)+C3H8 reaction dynamics in real time

A beam of Ni+(D-2(5/2)) is formed at a sharp zero of time by resonant two-photon ionization with a nanosecond dye laser pulse and crossed with a beam of propane gas under single-collision conditions at collision energies of 0.01 and 0.21 eV. The ion-molecule reaction occurs in field-free space in the extraction region of a time-of-flight mass spectrometer. After a variable time delay t(ext) = 1-8 mu s, a fast high-voltage pulse extracts product ions and residual reactant ions into a field-free flight tube for mass analysis. In contrast with many earlier studies of this reaction under more energetic conditions, at 0.01 eV collision energy we find that Ni+(D-2(5/2)) reacts with C3H8 to form long-lived NiC3H8+ complexes almost exclusively (greater than or equal to 96%) on the time scale 0-25 mu s after initiation of the collision. Retarding field analysis of the decay of the long-lived NiC3H8+ complexes reveals that on a 6-24 mu s time scale 28% revert to Ni+ + C3H8 and 6% form NiC2H4+ + CR4 elimination products; the remaining complexes have not yet decayed at t = 25 mu s. At 0.21 eV collision energy, both CH4 and H-2 elimination products are formed promptly (in less than 1 mu s) and also over the entire range of time scales studied, 0.5-25 mu s. Even at this higher collision energy, about 25% of the long-lived complexes survive beyond t = 25 mu s. The apparent energetic threshold observed here for the first time provides new evidence of a potential energy barrier to elimination products comparable to the energy of ground-state reactants. In addition, direct measurement of the time scale of the reaction under carefully controlled conditions provides new dynamical information that serves as a benchmark for the theoretical treatment presented in the accompanying paper.