Crossed molecular beams and quasiclassical trajectory studies of the reaction O(1D)+H2(D2)

Spiers Memorial Lecture: New directions in molecular scattering

The field of molecular scattering is reviewed as it pertains to gas-gas as well as gas-surface chemical reaction dynamics. We emphasize the importance of collaboration of experiment and theory, from which new directions of research are being pursued on increasingly complex problems. We review both experimental and theoretical advances that provide the modern toolbox available to molecular-scattering studies. We distinguish between two classes of work. The first involves simple systems and uses experiment to validate theory so that from the validated theory, one may learn far more than could ever be measured in the laboratory. The second class involves problems of great complexity that would be difficult or impossible to understand without a partnership of experiment and theory. Key topics covered in this review include crossed-beams reactive scattering and scattering at extremely low energies, where quantum effects dominate. They also include scattering from surfaces, reactive scattering and kinetics at surfaces, and scattering work done at liquid surfaces. The review closes with thoughts on future promising directions of research.

Spin-dependent reactivity and spin-flipping dynamics in oxygen atom scattering from graphite

The formation of two-electron chemical bonds requires the alignment of spins. Hence, it is well established for gas-phase reactions that changing a molecule's electronic spin state can dramatically alter its reactivity. For reactions occurring at surfaces, which are of great interest during, among other processes, heterogeneous catalysis, there is an absence of definitive state-to-state experiments capable of observing spin conservation and therefore the role of electronic spin in surface chemistry remains controversial. Here we use an incoming/outgoing correlation ion imaging technique to perform scattering experiments for O(³P) and O(¹D) atoms colliding with a graphite surface, in which the initial spin-state distribution is controlled and the final spin states determined. We demonstrate that O(¹D) is more reactive with graphite than O(³P). We also identify electronically nonadiabatic pathways whereby incident O(¹D) is quenched to O(³P), which departs from the surface. With the help of molecular dynamics simulations carried out on high-dimensional machine-learning-assisted first-principles potential energy surfaces, we obtain a mechanistic understanding for this system: spin-forbidden transitions do occur, but with low probabilities.

Universal crossed beam imaging studies of polyatomic reaction dynamics

Crossed-beam imaging studies of polyatomic reactions show surprising dynamics not anticipated by extrapolation from smaller model systems.

Experimental and Theoretical Study of the O(1D) + HD Reaction

This work addresses the kinetics and dynamics of the gas-phase reaction between O(¹D) and HD molecules down to low temperature. Here, measurements were performed by using a supersonic flow (Laval nozzle) reactor coupled with pulsed laser photolysis for O(¹D) production and pulsed-laser-induced fluorescence for O(¹D) detection to obtain rate constants over the 50-300 K range. Additionally, temperature-dependent branching ratios (OD + H/OH + D) were obtained experimentally by comparison of the H/D atom atom yields with those of a reference reaction. In parallel, theoretical rate constants and branching ratios were calculated by using three different techniques; mean potential phase space theory (MPPST), the statistical quantum mechanical method (SQM), and ring polymer molecular dynamics (RPMD). Although the agreement between experimental and theoretical rate constants is reasonably good, with differences not exceeding 30% over the entire temperature range, the theoretical branching ratios derived by the MPPST and SQM methods are as much as 50% larger than the experimental ones. These results are presented in the context of earlier work, while the possible origins of the discrepancies between experiment and theory are discussed.

A combined theoretical and experimental investigation of the kinetics and dynamics of the O(1D) + D2 reaction at low temperature

The O(¹D) + H₂ reaction is a prototype for simple atom-diatom insertion type mechanisms considered to involve deep potential wells. While exact quantum mechanical methods can be applied to describe the dynamics, such calculations are challenging given the numerous bound quantum states involved. Consequently, efforts have been made to develop alternative theoretical strategies to portray accurately the reactive process. Here we report an experimental and theoretical investigation of the O(¹D) + D₂ reaction over the 50-296 K range. The calculations employ three conceptually different approaches - mean potential phase space theory, the statistical quantum mechanical method and ring polymer molecular dynamics. The calculated rate constants are in excellent agreement over the entire temperature range, exhibiting only weak temperature dependence. The agreement between experiment and theory is also very good, with discrepancies smaller than 26%. Taken together, the present and previous theoretical results validate the hypothesis that long-lived complex formation dominates the reaction dynamics at low temperature.

An Experimental Study of the Kinetics of OH/OD(v = 1,2,3) + SO2: The Limiting High-Pressure Rate Coefficients as a Function of Temperature

The kinetics of the reaction OH/OD(v = 1,2,3) + SO₂ were studied using a photolysis/laser-induced fluorescence technique. The rate coefficients OH/OD(v = 1,2,3) + SO₂, k₁, over the temperature range of 295-810 K were used to determine the limiting high-pressure limit k₁^∞. This method is usually applicable if the reaction samples the potential well of the adduct HOSO₂ and if intramolecular vibrational relaxation is fast. In the present case, however, the rate coefficients showed an additional fast removal contribution as evidenced by the increase in k₁ with vibrational level; this behavior together with its temperature dependence is consistent with the existence of a weakly bound complex on the potential energy surface prior to adduct formation. The data were analyzed using a composite mechanism that incoporates energy-transfer mechanisms via both the adduct and the complex, and yielded a value of k₁^∞(295 K) equal to (7.2 ± 3.3) × 10^-13 cm³ molecule^-1 s^-1 (errors at 1σ), a factor of between 2 and 3 smaller than the current recommended IUPAC and JPL values of (2.0_-1.0^+2.0) and (1.6 ± 0.4) × 10^-12 cm³ molecule^-1 s^-1 at 298 K, respectively, although the error bars do overlap. k₁^∞ was observed to only depend weakly on temperature. Further evidence for a smaller k₁^∞ is presented in the companion paper.

State-to-State Dynamics of Elementary Bimolecular Reactions

The study of state-to-state dynamics of elementary bimolecular reactions has provided remarkable insights into chemical reactivity at the most fundamental level. This review covers exciting developments in this important field in the past decade. I focus on recent studies of quantum-state-resolved molecular-beam reactive-scattering studies of elementary chemical reactions, from triatomic to polyatomic systems. Researchers have made great advances in the fundamental understanding of many elementary chemical reactions through state-to-state dynamics studies. The strong interaction between theory and experiment has significantly enhanced our understanding of the dynamics of these reactions. I hope this review provides a glimpse of this exciting research field to both experts and beginners.

Dynamics of insertion reactions of H2 molecules with excited atoms

Recent progress in the study of insertion reactions of hydrogen molecules with excited atoms is reviewed in this article. In particular, the dynamics of the reaction of O(1D), N(2D), C(1D), and S(1D) with H2 and its isotopomers, which have received a great deal of attention over the past decade, are examined in detail. All of these systems have in common the existence of several potential energy surfaces (PES) correlating with the reagents' states, and consequently, they can give rise to reaction following different adiabatic and nonadiabatic pathways. The main contribution, however, arises from their ground singlet PESs which feature the existence of deep wells with small or null barriers for insertion. Accordingly, these reactions proceed mainly via formation of relatively long-lived collision complexes and display an overall nearly statistical behavior. In spite of their similarities, the various reactions have peculiar characteristics caused by important differences of their respective PESs. The contribution of excited PES to the global reactivity, which has also become an important issue and a challenge both for theory and experiment, is also examined. The different theoretical approaches are discussed in the text, along with the experimental results obtained by a variety of techniques. The recent exact quantum treatments of these reactive systems together with the development of a rigorous statistical model have contributed to a very accurate description which in many cases matches very well the detailed measurements. The quasi-classical trajectory (QCT) method has also provided a fairly accurate description of the reaction dynamics for these systems. In particular, the analysis in terms of collision times has yielded interesting clues about the reaction mechanisms.

Experimental and quantum dynamical study on an asymmetric insertion reaction: state-to-state dynamics of O(1D) + HD(1Sigmag+, v' = 0, j' = 0)-->OH(Pi, v", N") + D(2S)

Quantum-state-resolved differential cross sections of the O((1)D) + HD --> OH + D reaction at the collision energy of 7.11 kJ/mol has been determined experimentally and theoretically. The results of the time-dependent wave-packet calculations are overall in good agreement with the crossed beam scattering data, providing a benchmark example of an asymmetric insertion reaction at the state-to-state scattering level. The good agreement between experiment and theory suggests that the underlying ground potential energy surface is generally correct and that the nonadiabatic effect involving the electronic excited pathway is apparently small in this system.

Experimental and Theoretical Differential Cross Sections for the N(2D) + H2 Reaction

In this paper, we report a combined experimental and theoretical study on the dynamics of the N(2D) + H2 insertion reaction at a collision energy of 15.9 kJ mol(-1). Product angular and velocity distributions have been obtained in crossed beam experiments and simulated by using the results of quantum mechanical (QM) scattering calculations on the accurate ab initio potential energy surface (PES) of Pederson et al. (J. Chem. Phys. 1999, 110, 9091). Since the QM calculations indicate that there is a significant coupling between the product angular and translational energy distributions, such a coupling has been explicitly included in the simulation of the experimental results. The very good agreement between experiment and QM calculations sustains the accuracy of the NH2 ab initio ground state PES. We also take the opportunity to compare the accurate QM differential cross sections with those obtained by two approximate methods, namely, the widely used quasiclassical trajectory calculations and a rigorous statistical method based on the coupled-channel theory.

Experimental and Theoretical Investigations of the Inelastic and Reactive Scattering Dynamics of O(3P) + D2

This paper presents a combined experimental and theoretical study of the dynamics of O((3)P) + D(2) collisions, with emphasis on a center-of-mass (c.m.) collision energy of 25 kcal mol(-1). The experiments were conducted with a crossed-molecular-beams apparatus, employing a laser detonation source to produce hyperthermal atomic oxygen and mass spectrometric detection to measure the product angular and time-of-flight distributions. The novel beam source, which enabled these experiments to be conducted, contributed unique challenges to the experiments and to the analysis, so the experimental methods and approach to the analysis are discussed in detail. Three different levels of theory were used: (1) quasiclassical trajectories (QCT), (2) time-independent quantum scattering calculations based on high-quality potential surfaces for the two lower-energy triplet states, and (3) trajectory-surface-hopping (TSH) studies that couple the triplet surfaces with the lowest singlet surface using a spin-orbit Hamiltonian derived from ab-initio calculations. The latter calculations explore the importance of intersystem crossing in the dynamics. Both experiment and theory show that inelastically scattered O atoms scatter almost exclusively in the forward direction, with little or no loss of translational energy. For the reaction, O((3)P) + D(2) --> OD + D, the experiment shows that, on average, approximately 50% of the available energy goes into product translation and that the OD product angular distributions are largely backward-peaked. These results may be interpreted in light of the QCT and TSH calculations, leading to the conclusion that the reaction occurs mainly on triplet potential energy surfaces with, at most, minor intersystem crossing to a singlet surface. Reaction on either of the two low-lying reactive triplet surfaces proceeds through a rebound mechanism in which the angular distributions are backward-peaked and the product OD is both vibrationally and rotationally excited. The quantum scattering results are in good agreement with QCT calculations, indicating that quantum effects are relatively small for this reaction at a collision energy of 25 kcal mol(-1).

Dynamics of the C(D1)+D2 reaction: A comparison of crossed molecular-beam experiments with quasiclassical trajectory and accurate statistical calculations

In this paper we report a combined experimental and theoretical study on the dynamics of the insertion reaction C((1)D)+D(2) at 15.5 kJ mol(-1) collision energy. Product angular and velocity distributions have been obtained in crossed beam experiments and quasiclassical trajectory (QCT) and rigorous statistical calculations have been performed on the recent and accurate ab initio potential energy surface of Bussery-Honvault, Honvault, and Launay at the energy of the experiment. The molecular-beam results have been simulated using the theoretical calculations. Good agreement between experiment and both QCT and statistical predictions is found.

Determination of the internal state distribution of the SD product from the S(1D)+D2 reaction

The S(1D)+D2-->SD+D reaction has been studied through a photolysis-probe experiment in a cell. S(1D) reagent was prepared by 193 nm photolysis of CS2, and the SD(X 2Pi) product was detected by laser fluorescence excitation. The nascent rotational/fine-structure state distribution of the SD(X 2Pi) product was determined. This reaction, previously studied theoretically and in a crossed molecular beam experiment, is known to proceed through formation and decay of a long-lived collision complex involving the deep well in the H2S ground electronic state. The determined SD rotational state distribution in the v=0 vibrational level was found to be approximately statistical, with a small preference for formation of the F1 (Omega=3/2) fine-structure manifold over F2 (Omega=1/2). The branching into the Lambda doublet levels was also investigated, and essentially equal populations of levels of A' and A" symmetry were found. The present results are compared with previous investigations of this reaction and the analogous O(1D)+D2 reaction.

Crossed beam studies of the reactions of atomic oxygen in the ground 3P and first electronically excited 1D states with hydrogen sulfide

The reactions of both ground, (3)P, and electronically excited, (1)D, oxygen atoms with hydrogen sulfide, H(2)S, have been investigated by means of the crossed molecular beams method with mass spectrometric detection at different collision energies. Amongst the possible reaction channels those leading to HSO+H for the O((3)P) reaction and to HSO/HOS+H and SO+H(2) for the O((1)D) reaction have been identified and investigated. The dynamics of the channels leading to HSO/HOS+H are elucidated for the reactions of both states and the trend with increasing the collision energy analyzed. Noteworthily, the formation of SO+H(2) products appears to be an open channel for the O((1)D) reaction, at least for the highest collision energy investigated (11.8 kcal/mol). Finally, the recent experimental and theoretical estimates of the enthalpy of formation of the HSO radical have been critically analyzed to evaluate their conformity with the present experimental data.

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.