Analysis of the H + D2reaction mechanism through consideration of the intrinsic reactant polarisation

Stereodynamic control of nonadiabatic processes in low-energy Be+(2P) + H2 (v = 0, j = 2) collisions

Controlling the relative arrangement of colliding molecules is crucial for determining the dynamical outcomes of chemical processes and has emerged as a hot spot of experimental research. Here, the quantum scattering calculations are conducted to investigate the stereodynamic control in collisions between Be+(2P) and H2 (v = 0, j = 2), which undergo nonadiabatic transitions to the electronic ground state. Stereodynamic preparation is achieved by controlling the initial alignment of the H2 bond axis relative to the scattering frame. For product BeH+ in the reactive process, the differential cross sections (DCSs) are significantly enhanced in the forward and sideways hemispheres when the alignment angle β is 60°. For the product H2 in the quenching channel, the β = 0° preparation can result in a more than one-fold increase in the DCS at a polar scattering angle of 0°. Furthermore, varying the alignment angle β also has noteworthy effects on the rotational-state distributions of BeH+ products. Specifically, β = 0° preparation can induce the disappearance of the bimodal distribution of rotational states at a collision energy of 0.05 eV.

Nonadiabatic Ultracold Quantum Reactive Scattering of Hydrogen with Vibrationally Excited HD(v = 5-9)

The results from electronically non-adiabatic and adiabatic quantum reactive scattering calculations are presented for the H + HD(v = 5-9) → H + HD(v', j') reaction at ultracold collision energies from 10 nK to 60 K. Several experimentally verifiable signatures of the geometric phase are reported in the total and vibrationally and rotationally resolved rate coefficients. Most notable is the predicted 2 orders of magnitude enhancement of the rotationally resolved ultracold rates of odd symmetry relative to those of even symmetry. Prominent shape resonances appear at higher collision energies (100 mK to 20 K), which could be measured experimentally. Significant geometric phase effects are also reported on the resonance energies and lifetimes. In particular, an enhancement (suppression) of the l = 1 (l = 2) shape resonances for HD(v = 5, 6) is predicted for even symmetry relative to those of odd symmetry.

Quantum state-to-state study for (H−(D−),HD) collisions on two potential energy surfaces

Revealing the reaction mechanisms of the H⁻/D⁻ + HD reaction – an exact quantum dynamics study on two potential energy surfaces.

A new perspective: imaging the stereochemistry of molecular collisions

The concept of the steric effect plays a central role in chemistry. This Perspective describes how the polarization of reactant molecules in space can be used to probe directly the steric effect, and highlights some of the new measurements that are made possible by coupling reactant orientation and alignment with ion imaging techniques.

The reactive collision mechanism evinced: stereodynamical control of the elementary Br + H2 → H + HBr reaction

From a kinetics standpoint, reactive molecular collisions are the building blocks of the mechanisms of chemical reactions. In contrast, a dynamics standpoint reveals molecular collisions to have their own internal mechanisms, which are not mere theoretical abstractions: through suitable preparation of the reactants internal and stereochemical states, features of the mechanisms of a reactive molecular collision can be made evident and used as "handles" to control the reaction outcome. Using time-independent quantum dynamical calculations, we demonstrate this for the Br + H2(v = 0-1, j = 2) → H + HBr reaction in the 1.0-1.6 eV range of total energies. Despite its pronounced effect on reactivity, which is in agreement with the predictions from Polanyi rules, reactant vibration is found to have little effect on the mechanism of this endoergic, late-barrier reaction. Analysis of the correlations between directional reaction properties shows that the collision stereochemistry strongly depends on the total energy, but not on how this energy is partitioned between reactant translation and vibration. The stereodynamical preferences implied by the collision mechanisms determine how and to what extent one can control the reaction. Regarding the overall reaction, the extent of control is found to be large near the reaction threshold but not when the total energy is high. Regarding state-to-state reactions, the effect of reactant stereochemistry on the product rotational state distribution is found to be nontrivial and energy dependent.

Reaction dynamics and mechanism of the Cl + HD(v = 1) reaction: a quantum mechanical study

Time-independent quantum mechanical calculations have been performed in order to characterize the dynamics and stereodynamics of Cl + HD reactive collisions. Calculations have been carried out at two different total energy values and for various initial states using the adiabatic potential energy surface by Bian and Werner [J. Chem. Phys. 2000, 112, 220]. Special attention has been paid to the reaction with HD(v = 1) for which integral and differential cross-sections have been calculated and the effect of vibrational vs translational energy on the reactivity has been examined. In addition, the reactant polarization parameters and polarization-dependent differential cross-sections have been determined. From these results, the spatial preferences of the reaction and the extent of the control of the cross sections achievable through a suitable preparation of the reactants have been also studied. The directional requirements are tighter for the HCl channel than for the DCl one. Formation of the products takes place preferentially when the rotational angular momentum of the HD molecule is perpendicular to the reactants approach direction. Cross-sections and polarization moments computed from the scattering calculations have been compared with experimental results by Kandel et al. [J. Chem. Phys. 2000, 112, 670] for the reaction with HD(v = 1) produced by stimulated Raman pumping. The agreement so obtained is good, and it improves the accordance found in previous calculations with other methodologies and potential energy surfaces.

Three-vector correlation in statistical reactions: the role of the triatomic parity

This article presents a methodology for the determination of the k-j-k' three-vector correlation assuming a statistical model for atom-diatom reactions; k and k' are the reagent-approach and product-recoil directions, respectively, and j is the rotational angular momentum of the reagent diatomic. Although the polarization of reagent angular momentum is in most cases negligible, conservation of the triatomic parity imposes a certain polarization for some combinations involving low reagent and product rotational states. Statistical and quantum-mechanical polarization-dependent differential cross sections were calculated for the barrierless D(+) + H(2)(v = 0,j) → HD(v' = 0,j') + H(+) reaction. The agreement between the two is in most cases excellent, confirming the statistical character of the reaction at low and moderate collision energies.

Quantum mechanical mechanisms of inelastic and reactive H + D(2)(v = 0, j = 2) collisions

This article analyses the mechanisms of inelastic and reactive H + D(2)(v = 0, j = 2) collisions that result in highly vibrationally excited products when the collision energy is 1.70 eV. The analytical method is entirely quantum mechanical and focuses on correlations between the polarization of the reactant molecule and the direction of product scattering. Two viewpoints are used. The "intrinsic" viewpoint reveals the reactant polarizations that lead to the largest cross section at each value of the scattering angle (the angle between the reactant-approach and product-recoil directions); the "extrinsic" viewpoint reveals how the dependence of the collision cross section on the scattering angle changes when the reactant polarization is fixed at each one of a set of experimentally feasible alternatives. Comparison of processes correlating with the same range of impact parameters is also used, to facilitate isolation and identification of directional effects. When products are scattered in the backward and sideways regions, the results for inelastic and reactive collisions are rather similar. When products are scattered in the forward region, the results for inelastic and reactive collisions are clearly different: a side-on collision geometry that largely increases the inelastic cross section hardly affects the reactive cross section. This feature is the quantum mechanical signature of the so-called "tug-of-war" mechanism.

Quantum mechanical limits to the control of atom-diatom chemical reactions through the polarisation of the reactants

This article considers the extent to which one can control the reactivity of atom-diatom systems through reactant polarisation. Three different limits for reactivity manipulation are defined: "absolute" limits that do not depend on the reaction dynamics but can only be obtained for particular combinations of quantum numbers, "unconstrained" limits that depend on dynamics but not on constraints imposed by any particular experimental setup, and "constrained" limits that depend on dynamics and also on the constraints imposed by a particular experimental setup. Methods for calculation of these limits are presented and applied to the benchmark F + H2 reaction. The variations of the maximum and minimum reactivity one can obtain are analysed in terms of reaction mechanisms and steric constraints. Tables listing the minimum and maximum values of angular momentum polarisation moments of rank up to 4, and integer and half-integer quantum numbers up to 5, are also presented.