Data are presented on the metastable H2 loss from H5+, D5+, H7 , and Hg+ ions formed in a high-pressure, temperature-variable ion source with coaxial electron beam, ion exit geometry. The metastable reactions occur in the second field free region of a reverse geometry mass spectrometer and require parent ion lifetimes of ~10-6 s. Substantial structure is observed in the kinetic energy distributions, derived from the metastaWe peak shapes, for Hs+, D5+, and H7+. Statistical theory modeling predicts a smooth P(Et) with no structure and a lifetime in the 10~9 s range for H5+. A theoretical model is developed for H5+ that combines ab initio potential surfaces with a high-order WKB tunneling formalism. This model nicely reproduces the observed structure and predicts metastable reactions with lifetimes of ~10"6 s. The structure is due to a superposition of tunneling from various bound variational levels of the H3+-H2 stretch for different angular momentum states of the H5+ molecule. A Boltzmann distribution of J0 and v states can only account for the lowest energy portion of P(Et). The experimental distribution is used to extract a J0 distribution required by the model to fit the data. The extracted distribution has substantial intensity at the highest values of J0 that support bound variations (J0 = 29 for H5+). Several possible mechanisms are suggested that could generate these high values of J0 under our experimental conditions. Statistical phase space theory accurately fits the P(Et) for metastable H2 loss from H9+ indicating tunneling is a minor process, while H2 loss from H7+ appears to occur both by tunneling and statistical processes.

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Articles you may be interested inLow energy ion-molecule reaction dynamics: Complex and direct collisions of O− with NH3Calculations on the rate of the ion-molecule reaction between NH+ 3 and H2The influence of vibrational excitation and collision energy on the ion-molecule reaction NHj (V2) + ND3 has been investigated using a recently constructed quadrupole-octopole-quadrupole mass spectrometer. The NHj reagent ions are prepared state selectively with 0-7 quanta il! the !!2 umbrella bending mode by (2+ 1) resonance enhanced multiphoton ionization through the B or C' Rydberg states of ammonia. Reactive collisions between the mass-filtered ion beam and a thermal distribution of neutral reagent molecules occur with controlled collision energies (0.5-10.0 eV center of mass) within the octopoie ion guide, enabling product ions to be collected independent of scattering dynamics. The reaction of NHj with ND3 has three major product channels: (1) deuterium abstraction, (2) charge transfer, and (3) proton transfer. Each of these channels exhibits a strong dependence on ion vibrational excitation and collision energy. Product branching ratios and relative cross sections are reported and compared with previous results. Briefly, both deuterium abstraction and charge transfer are enhanced by vibrational excitation, whereas proton transfer is suppressed. As the collision energy increases, the branching fraction for charge transfer increases sharply, that for proton transfer decreases, and that for deuterium abstraction remains nearly unchanged. These results point to a short-lived collision complex in which vibration and translation play inequivalent roles.one-photon VUV ion preparation with guided ion techniques 3772