Relative ionization cross sections for the title systems with articulation of all product ion channels have been measured in the collision energy range 1.5–4.0 kcal/mol using crossed supersonic molecular beams; the H2 results have been extended down to 0.5 kcal/mol by the use of a 10% H2/Ar seeded beam. The data are interpreted with a microscopic two-step model that assumes ionization near the turning point in the excited state, a centrifugal barrier criterion for ionic complex formation, and statistical partitioning of flux among the possible ionic products, i.e., phase-space theory. A full statistical calculation underestimates the amount of rearrangement ionization He*+H2→HeH++H+e− by a factor of 2, but one which excludes antiparallel coupling of orbital and rotational angular momenta in the H+2 channel is in better accord with the data. A substantial isotope effect favoring HeD+ over HeH+ in the HD reactions by a factor of 1.9±0.2 is well represented by the model.
The interactions of N2(A 3Σ+u) and N2(B 3Πg) with H2(X 1Σ+g) have been characterized through potential energy calculations, which were performed using a combination of MCSCF and multiple-reference SDCI methods. In C2v geometry with the N2 and H2 bond axes parallel, at the highest level of theory used, energy transfer from N2(A 3Σ+u) to H2(b 3Σ+u) and dissociation of the latter into H atoms is found to proceed through an adiabatic reaction path with a barrier of 0.513 eV. In C2v geometry with perpendicular orientation of N2 and H2, the two lowest 3B2 surfaces are shown to exhibit a strongly avoided crossing; the lower surface shows a favorable pathway for energy transfer from N2(B 3Πg) to H2(b 3Σ+u). In each case, energy transfer occurs via a two-electron exchange mechanism as a result of mixing between orbitals with the proper energy and symmetry. Consistent with the isoconfigurational electronic structure of N2(B) and CO(a 3Π), the results for N2(B) are similar to those which we found previously for quenching of CO(a) by H2. The overall results are shown to be consistent with available experimental kinetics data, which show quenching of N2(A) by H2 to be inefficient. Application of these results to electronic quenching and vibrational relaxation of N2(A) by other small molecules is discussed.
Quasiclassical trajectory calculations have been performed for motion on an ab initio potential energy surface to investigate energy transfer from N2(A 3∑+u, v=1, 3 and 6) to H2 and D2. Because of the unusual features of the surface, both vibrational relaxation and electronic quenching of N2(A) are observed, the latter process resulting in dissociation of the hydrogen molecule. It is deduced that coupling of the vibrational motions of the N2 and H2 molecules initiates the energy transfer process. The results are compared with experimental information on the quenching of N2(A) by σ-bonded molecules.
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