Dynamics of the ion–molecule reaction Kr+(H2,H)KrH+

This is the publisher's version, also available electronically from http://scitation.aip.org/content/aip/journal/jcp/65/4/10.1063/1.433219

Role of impact parameter in branching reactions

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Energy Dependence of Energy Partition in Products of Direct Reactions: Crossed‐Beam Studies and a New Model

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Direct Mechanism of Reaction CH3 ++CH4→C2H5 ++H2

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Chemical Accelerator Studies of Isotope Effects on Collision Dynamics of Ion–Molecule Reactions: Elaboration of a Model for Direct Reactions

This is the publisher's version, also available electronically from http://scitation.aip.org/content/aip/journal/jcp/53/2/10.1063/1.1674042.Crossed‐beam studies on isotopic variants of the reaction Ar+ + H2→ArH+ are reported. Both velocity and angular distributions of the ionic product as a function of initial translational energy, down to 0.1 eV (center of mass), have been measured. At lowest energies there is a gain in the translational energy of the products over that of the reactants, but at higher energies there is increasing conversion of kinetic into internal energy. While this represents the most probable course of the reaction there is a fairly wide distribution about the median values. Results confirm that this reaction is predominantly direct at all energies and provide no evidence for intermediate persistent complex formation. They are also consistent with a model for direct reactions previously proposed. The data on reaction with HD permit further development of this mechanism. The reactants are mutually accelerated by their long‐range attractive potential until hydrogen atom transfer occurs. The liberated H (or D) atom is reflected from the ArD+(ArH+ and the products separate, being decelerated in the process by the attractive potential acting between them. This “polarization–reflection” model yields a reasonable value for the radius at which transfer occurs, and it accounts quantitatively for the magnitudes of, and isotopic effects on, the median product velocities. It also predicts the significant back scattering observed at very low as well as very high energies. With appropriate modification for the attractive potentials involved the model can provide a simple representation of direct reactions in general

Excitation functions for the reactions of Ar^+ with CH4, CD4, and CH2D2

This is the publisher's version, also available electronically from http://scitation.aip.org/content/aip/journal/jcp/63/11/10.1063/1.431267.Integral reaction cross sections as a function of initial translational energy (0.4–30 eV c.m.) are reported for isotopic variants of the exoergic ion‐molecule reaction Ar++CH4 → ArH++CH3. The excitation functions, which maximize at about 5 eV and decrease at lower collision energies, appear to possess translational energy thresholds at about 0.1 eV. At the higher energies there is a large isotope effect favoring abstraction of H over D. The observed threshold behavior, unusual for exoergic reactions of positive ions, is discussed in terms of the formation of an ArCH4 + intermediate complex at low collision energies

Observation of a stripping threshold for the reaction N2 ^++CH4→N2H^++CH3

This is the publisher's version, also available electronically from http://scitation.aip.org/content/aip/journal/jcp/64/9/10.1063/1.432690Chemical accelerator studies on isotopic variants of the reaction N2 ++CH4→N2H++CH3 are reported. Reaction cross sections, as well as velocity and angular distributions of the ionic products have been measured as a function of initial translational energy over the energy range 0.65–35 eV (center of mass). The results are similar to those recently reported for the reaction of Ar+ with CH4. The excitation function maximizes at about 5 eV (c.m.) and decreases at lower collision energies, appearing to possess a threshold at 0.1 eV. At the higher energies there is a large isotope effect favoring abstraction of H over D. The product velocity vector distribution is strongly peaked forward of the center of mass, indicating that the reaction is predominantly direct over the energy range studied. The spectator stripping model, although providing a reasonable first approximation to the reaction dynamics, overestimates the product translational energy by approximately 0.1 eV. This behavior is presumed to be caused by a basin in the potential energy hypersurface for this reaction. If, however, an N2CH4 + complex is formed at low collision energies, it appears to decompose via reaction channels other than that resulting in N2H+ formation

Observation of a translational energy threshold for a highly exoergic ion‐molecule reaction

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Chemical accelerator studies of reaction dynamics: Ar^+ + CH4 → ArH^+ + CH3

This is the publisher's version, also available electronically from http://scitation.aip.org/content/aip/journal/jcp/62/7/10.1063/1.430836.Chemical accelerator studies on isotopic variants of the reaction Ar+ + CH4 → ArH+ + CH3 are reported. Velocity and angular distributions of the ionic product as a function of initial translational energy have been measured over the energy range 0.39–25 eV center-of-mass (c.m.). The asymmetry of the product distribution with respect to the center of mass indicates that the reaction is predominantly direct over the energy range studied. The dynamics of the reaction are approximated by the spectator stripping model: The reaction exothermicity appears as product internal energy and product excitation increases with collision energy at the rate predicted by this model. The internal degrees of freedom of the neutral product have little effect on reactiondynamics, and product excitation appears to reside principally in the ionic product. Deviations from the spectator stripping model suggest the existence of a basin in the potential energy hypersurface for this reaction; the ArCH4 + complex which may be formed at low collision energies, however, preferentially decomposes via reaction channels other than that resulting in ArH+ formation

Translational energy dependence of cross sections for reactions of OH− (H2O) n with CO2 and SO2

This is the publisher's version, also available electronically from http://scitation.aip.org/content/aip/journal/jcp/80/10/10.1063/1.446510.A tandem mass spectrometer has been used to measure cross sections for reactions of the solvated negative ions OH−(H2O) n , where 0≤n≤3, with the neutral molecules CO2 and SO2 over the range of reactant translational energy 0.15–25.0 eV (LAB). The reactions observed include solvent switching, collisional dissociation, and charge transfer. The exoergic solvent switching reactions are very rapid, having cross sections which exceed a hundred square Angstroms at low energies. These cross sections decrease approximately as (energy)−0.5 up to 1 eV and then decrease much more rapidly at higher collision energies. Estimates of bond dissociation energies for the cluster ions are derived from the measured translational energy thresholds for the endothermic collisional dissociationreactions