Peter M. Hierl

A chemical accelerator for the study of the dynamics of ion—molecule reactions over the energy range 0.1–100 eV

Abstract A beam instrument for the study of ion—molecule reactions is described. Both angular and velocity distributions of reactively scattered ions can be measured, as well as reaction cross sections as a function of ion energy. Such data are sufficient to characterize completely the dynamics of the reaction studied. Because of its wide energy range, high mass resolution, and high overall sensitivity, this instrument appears to have more extensive capabilities than previously described beam instruments. In addition, excitation functions and kinematic data are provided for the reations N 2 + + H 2 → N 2 H + + H and N 2 + + D 2 → N 2 D + + D over the energy range 0.078–14 eV (c.m.).

Orientation isotope effect in ion–molecule reactions

A simple kinematic model is proposed to account for the kinetic isotope effect (KIE) upon the reactions of simple ions X+ with HD. This model is based upon (1) the fact that the displacement of the center of polarizability from the center of mass in the HD molecule will affect the alignment of the reactants, (2) the fact that, for many ions X+, reaction must occur by a surface crossing mechanism, and (3) the assumption that the ratio XH+/(XH++XD+) equals the fraction of intimate collisions in which the H end of HD is oriented towards the ion at the moment the reactants pass over the centrifugal barrier in the effective radial potential (or cross over to the X–HD+ surface if the crossing occurs before the centrifugal barrier is reached). Use of the ion–induced dipole potential for the reactants permits the derivation of an analytic expression for the KIE. With no adjustable parameters, this model accounts quantitatively for the very different KIE’s observed in the reactions of Ar+ and Kr+ with HD at low co...

Determination of the Temperature-Dependent OH− (H2O) + CH3I Rate Constant by Experiment and Simulation

Abstract Experimental and simulation studies of the OH–(H2O) + CH3I reaction give temperature dependent rate constants which are in excellent agreement. Though there are statistical uncertainties, there is an apparent small decrease in the rate constant as the temperature is increased from − 60 to 125 ℃, and for this temperature range the rate constant is ∼ 1.6 times smaller than that for the unsolvated reactants OH– + CH3I. Previous work [J. Phys. Chem. A 117 (2013) 14019] for the unsolvated reaction found that the SN2 and proton transfer pathways, forming CH3OH + I– and CH2I– + H2O, have nearly equal probabilities. However, for the microsolvated OH–(H2O) + CH3I reaction the SN2 pathways dominate. An important contributor to this effect is the stronger binding of H2O to the OH– reactant than to the proton transfer product CH2I–, increasing the barrier for the proton transfer pathway. The effect of microsolvation on the rate constant for the OH–(H2O)0,1 + CH3I reactions agrees with previous experimental studies for X–(H2O)0,1 + CH3Y reactions. The simulations show that there are important non-statistical attributes to the entrance- and exit-channel dynamics for the OH–(H2O) + CH3I reaction.

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

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 reaction dynamics, 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 a...

Chemical accelerator studies of isotope effects on collision dynamics of ion–molecule reactions: Kr++HD

The reaction of Kr+ with HD has been studied as a function of relative collision energy over the range 0.08–3.1 eV (c.m.) by measuring integral reaction cross sections and the velocity vector distributions of product ions formed when a collimated, energy selected beam of Kr+ impinges on HD under single collision conditions. The ratio σ (KrH+)/σ (KrD+) passes through a sharp maximum (?2.5) at about 0.7 eV relative collision energy and decreases by a factor of 10 at higher energies. The isotopic product velocity vector distributions show a high but not perfect degree of symmetry about the center of mass at low energies but are extremely anisotropic at high energies, with the KrH+ being strongly forward scattered and the KrD+ being back scattered.

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

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.

Transmission characteristics of collimated hole threshold photoelectron detectors

Abstract The threshold photoelectron-coincidence photoion mass spectrometer is a powerful tool for the study of state-selected ions. Threshold photoelectron detectors used in this apparatus transmit small but significant numbers of electrons at energies up to two orders of magnitude higher than the energy resolution of this detector. It is frequently important to know the fraction of the total coincidence count which results from non-threshold electrons, and to correct for them. This requires measurement of the transmission characteristics of the threshold photoelectron detector over a wide range of energies. We have developed a simple experimental method for checking the transmission performance of such a detector, as well as a mathematical formalism for calculating the transmission function in closed form.

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

Chemical 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 ...

Translational energy dependence of reaction mechanism: Xe++CH4→XeH++CH3

The dynamics of the exoergic ion–molecule reaction Xe+(CH4,CH3)XeH+ were studied by chemical accelerator techniques over the relative translational energy range 0.2 to 8 eV. Results of the kinematic measurements are reported as scattering intensity contour maps in Cartesian velocity space. Center‐of‐mass angular and energy distributions, derived from these maps, provide information on the reaction mechanism and on the partitioning of available energy between internal and translational modes in the products. The results suggest that reaction proceeds via the formation of a long‐lived complex at low collision energies (below 0.5 eV) and by a direct mechanism approaching spectator stripping at higher energies.

Role of impact parameter in branching reactions: Chemical accelerator studies of the reaction Xe++CH4→XeCH3++H

Integral reaction cross sections and product velocity distributions have been measured for the ion–molecule reaction Xe+(CH4,H)XeCH3+ over the relative reactant translational energy range of 0.7–5.5 eV by chemical accelerator techniques. The kinematic results indicate that reaction proceeds in a direct manner by a rebound mechanism over the energy range studied, suggesting that this substitution reaction occurs predominantly in small impact parameter collisions. This finding contrasts with the results obtained for the competing reaction, Xe+(CH4,CH3)XeH+, where the strong forward scattering of the XeH+ product indicates that H‐atom abstraction occurs primarily in large impact parameter collisions.