Bimolecular Ni+(2D5/2) + C3H8 Reaction Dynamics in Real Time

Abstract

A beam of Ni+(2D5/2) 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 text = 1−8 μs, 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+(2D5/2) reacts with C3H8 to form long-lived NiC3H8+ complexes almost exclusively (≥96%) on the time scale 0−25 μs after initiation of the collision. Retarding field analysis of the decay of the long-lived NiC3H8+ complexes reveals that on a 6−24 μs time scale 28% revert to Ni+ + C3H8 and 6% form NiC2H4+ + CH4 elimination products; the remaining complexes have not yet decayed at t = 25 μs. At 0.21 eV collision energy, both CH4 and H2 elimination products are formed promptly (in less than 1 μs) and also over the entire range of time scales studied, 0.5−25 μs. Even at this higher collision energy, about 25% of the long-lived complexes survive beyond t = 25 μs. 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.