Rotational State Resolved Differential Cross Sections for the Reaction F + D2 → DF + D at Collision Energies 140−240 meV

INVESTIGATION OF THE CONTRIBUTION FOR THE NONCOLLINEAR CHANNEL OF THE F(2P3/2,2P1/2) + H2/D2 REACTIONS ON FOUR DIABATIC POTENTIAL ENERGY SURFACES

We performed the nonadiabatic time-dependent wave packet calculation on the four diabatic potential energy surfaces, which have the different barrier height, to investigate the contribution of the noncollinear channel for the F (2P) + H2/D2 (v = j = 0) reactions. The reaction probabilities, integral cross-sections, and rate constants are presented. The results indicate that the probabilities as the function of the collision energy have an obvious translation. The reactive activity of the reactions comes from the noncollinear reactive channel. The bent barrier height would decrease the reactive activity. The integral cross-sections are in the order of AWS < LWA-5 < LWA-78 ≈ MASW, which is opposite to that of the bent barrier height. At the lower temperature, the difference of the rate constants is unambiguous. As the temperature increases, the difference reduces. At the higher temperature, the rate constants computed on the four potential energy surfaces are close.

Comparison of experimental time-of-flight spectra of the HF products from the F+H2 reaction with exact quantum mechanical calculations

High resolution HF product time-of-flight spectra measured for the reactive scattering of F atoms from n-H2(p-H2) molecules at collision energies between 69 and 81 meV are compared with exact coupled-channel quantum mechanical calculations based on the Stark-Werner ab initio ground state potential energy surface. Excellent agreement between the experimental and computed rotational distributions is found for the HF product vibrational states v'=1 and v'=2. For the v'=3 vibrational state the agreement, however, is less satisfactory, especially for the reaction with p-H2. The results for v'=1 and v'=2 confirm that the reaction dynamics for these product states is accurately described by the ground electronic state 1 (2)A' potential energy surface. The deviations for HF(v'=3, j' > or =2) are attributed to an enhancement of the reaction resulting from the 25% fraction of excited ((2)P(12)) fluorine atoms in the reactant beam.

Simulation of the reactive scattering of F+D2 on a model family of potential energy surfaces with various topographies: the correlation approach

The connection between the salient features of the potential energy surface (PES) and the dynamical characteristics of the elementary collision process is studied using a correlation approach based on quasiclassical trajectory simulations. The method is demonstrated for the reaction F + D2 --> D + DF for which the scattering characteristics were calculated on a model family of PES's based on a London-Eyring-Polanyi-Sato-type five-parameter equation. The correlations between the reactive cross section and the vibrational and rotational quantum numbers and the scattering angle of the DF product, and the various parameters of the collinear and noncollinear PES's, such as the location and height of the minimal barrier and the Sato coefficients, are reported. Although usually correlations between two variables suffice, in some cases coefficients of correlation among three variables are required. The role of nonlinear parameter dependencies in computing the correlation coefficients is also considered. The correlation approach makes it possible to examine a large set of potential surfaces without intermediate human control and obtain quantitative information.

Rapid solid-state metathesis routes to aluminum nitride

Metathesis (exchange) reactions offer the possibility of controlling temperature through a judicious choice of precursors. Here, a reaction between AlCl(3) and Ca(3)N(2) is found to produce phase-pure aluminum nitride (AlN) in seconds. The CaCl(2) byproduct salt, whose formation drives this highly exothermic reaction, is simply washed away after reaction completion. SEM images demonstrate that the AlN product is a micron-sized powder, while TEM shows well-formed crystallites. Thermodynamic calculations indicate that a reaction temperature of 2208 K could be reached under adiabatic conditions. Using an in situ thermocouple and a stainless steel reactor vessel to hold the precursors, a reaction temperature of 1673 K is measured 0.8 s after initiation. Switching to a thermally insulating ceramic vessel produces a maximum reaction temperature of 2010 K because of the more nearly adiabatic conditions. The high reaction temperature appears to be critical to forming phase-pure AlN. Experiments with Li(3)N, instead of Ca(3)N(2), produce lower temperatures (1513 K), resulting in both Al and Al(2)O(3) impurities.

Effect of a single quantum rotational excitation on state-to-state dynamics of the O(1D)+H2-->OH+H reaction

Crossed molecular beams scattering experiments on the O(1D)+H2 reaction have been carried out in order to study the effect of the reagent (H2) rotational excitation on the detailed dynamics of this benchmark insertion reation. Experimental results indicate that a single quantum rotational excitation of H2 has a significant impact on the product state distributions at the forward and backward scattering directions, while very little effect has been found in the sideway scattering direction. No clear patterns of this effect are found in the OH-product state distributions, indicating that the single quantum excitation on the dynamics is rather complicated.

Crossed-beam studies of neutral reactions: state-specific differential cross sections

Crossed-molecular-beam and laser techniques have enabled experimentalists to measure the state-resolved differential cross sections of elementary chemical reactions. This article reviews recent progress in this area. Particular emphasis is placed on some intriguing physical phenomena associated with a few benchmark reactions and how these measurements help in answering fundamental questions about reaction dynamics. We examine specifically the geometric phase effects in the reaction H + D2, the dynamical resonance phenomenon in F + HD, the unusually large spin-orbit reactivity in Cl((2)P) + H2, the insertion reaction O((1)D) + H2, and the mode-specific reactivity in Cl + CH4(nu). The give-and-take between experiment and theory in unraveling the physical picture of the dynamics is illustrated throughout this review.

A Quantum State-Resolved Insertion Reaction: O((1)D) + H(2)(J = 0) --> OH((2) product operator product operator product operator, v, N) + H((2)S)

The O((1)D) + H(2) --> OH + H reaction, which proceeds mainly as an insertion reaction at a collisional energy of 1.3 kilocalories per mole, has been investigated with the high-resolution H atom Rydberg "tagging" time-of-flight technique and the quasiclassical trajectory (QCT) method. Quantum state-resolved differential cross sections were measured for this prototype reaction. Different rotationally-vibrationally excited OH products have markedly different angular distributions, whereas the total reaction products are roughly forward and backward symmetric. Theoretical results obtained from QCT calculations indicate that this reaction is dominated by the insertion mechanism, with a small contribution from the collinear abstraction mechanism through quantum tunneling.

CROSSED-BEAM STUDIES OF REACTION DYNAMICS

▪ Abstract  This article reviews recent progress in our understanding of gas-phase neutral reaction dynamics as made possible by improvements in the crossed molecular beam scattering technique for measuring reactive differential cross sections. A selection of crossed-beam studies on systems that play a fundamental role in our basic understanding of reaction phenomena are discussed to illustrate the capabilities of the experimental method. The examples include benchmark atom-diatom abstraction and insertion reactions, and four-atom radical reactions for which state-to-state, state-resolved, or state-averaged differential cross sections have recently been measured. The results are discussed in the light of the latest related theoretical developments regarding the treatment of potential energy surfaces and the dynamics of the systems. Recent results on crossed-beam studies of chemically relevant reactions of carbon, nitrogen, and oxygen atoms are also reviewed, and the latest developments in the technique are noted.