M. Brouard

Product rotational polarization in photon‐initiated bimolecular reactions

This paper concerns the semiclassical description, calculation and measurement of angular momentum polarization in the products of elementary gas‐phase bimolecular reactions. A unified, semiclassical treatment of the centre‐of‐mass correlated (k,k′,j′) angular distribution involving the reagent and product relative velocity and the product angular momentum vectors is described, and is related to other methodologies already existing in the literature. Explicit expressions are provided enabling experimentalists to extract rotational polarization information from crossed‐molecular beam and photon‐initiated reaction studies, under a variety of experimental conditions. Furthermore, the strategy developed is well suited to the theoretical calculation of reaction product polarization, in particular, using classical trajectory methods. An illustrative example of such a calculation is presented, and the centre‐of‐mass polarization data provided is used to simulate the laboratory frame rotational moments that can b...

The stereochemistry of the O(1D)+N2O→NO+NO reaction via velocity‐aligned photofragment dynamics

Velocity‐aligned, superthermal O(1D) atoms generated via the photodissociation of N2O have been employed to investigate the stereodynamics of the title reaction. The power of this experimental technique, when coupled with Doppler‐resolved, polarized laser‐induced fluorescence probing of the reaction products, is demonstrated by reference to the specific reaction channel leading to NO(υ’=0)+NO(υ’=16,17), which is shown to proceed via direct stripping dynamics. Furthermore, the observed product‐state selective linear and angular momenta disposals imply that the reaction is stereodynamically constrained to occur via collinear collision geometries.

Interference structures in the differential cross-sections for inelastic scattering of NO by AR

Inelastic scattering is a fundamental collisional process that plays an important role in many areas of chemistry, and its detailed study can provide valuable insight into more complex chemical systems. Here, we report the measurement of differential cross-sections for the rotationally inelastic scattering of NO(X2Π1/2, v=0, j=0.5, f) by Ar at a collision energy of 530 cm(-1) in unprecedented detail, with full Λ-doublet (hence total NO parity) resolution in both the initial and final rotational quantum states. The observed differential cross-sections depend sensitively on the change in total NO parity on collision. Differential cross-sections for total parity-conserving and changing collisions have distinct, novel quantum-mechanical interference structures, reflecting different sensitivities to specific homonuclear and heteronuclear terms in the interaction potential. The experimental data agree remarkably well with rigorous quantum-mechanical scattering calculations, and reveal the role played by total parity in acting as a potential energy landscape filter.

Fourier moment analysis of velocity-map ion images

An alternative to inverse Abel transform and forward convolution methods is presented for extracting dynamical information from velocity-map ion images. Unlike most competing methods, that presented here does not require the probed three-dimensional distribution to possess cylindrical symmetry. The new method involves analysis of the Fourier moments of images measured in different experimental geometries, and allows speed distributions, angular differential cross sections, and angular momentum alignment and orientation to be determined from raw images of the products of photodissociation and photon-initiated bimolecular reactions. The methodology is developed within the semiclassical framework of Dixon’s bipolar moment formalism [R. N. Dixon, J. Chem. Phys. 85, 1866 (1986)], although it is equally applicable to other common formulations of the product scattering distribution. To allow a comparison of the method with the Abel inversion, which requires that the velocity distribution of the probed product ha...

Product state-resolved stereodynamics: quasiclassical study of the reaction O(1D) + HD → OH(OD) (ν′, j′) + D(H)

Abstract State-resolved integral and differential cross sections and product energy partitioning have been calculated for the reaction O ( 1 D )+ HD (ν = 0, j = 0–1) → OH(OD) + D(H) by means of quasi-classical trajectory (QCT) calculations at a collision energy of 0.197 eV (19 kJ mol −1 ) using the potential energy surface of Schinke and Lester. The results indicate the dominance of an insertion mechanism producing HOD intermediates and reveal a strong dependence of the differential scattering cross section on the product quantum state. This dependence has been related to the product state-resolved opacity functions and the estimated lifetimes of the reactive collisional trajectories.

Velocity-map imaging study of the O(3P)+N2 product channel following 193 nm photolysis of N2O

Velocity-map imaging has been used to characterize the velocity and angular momentum alignment distributions of the O(3PJ) products of N2O photolysis at 193 nm. The measured velocity and spatial anisotropy distributions indicate that around 60% of the available energy appears in product translation, with the remainder being released into internal excitation of the N2 cofragment. The measured O(3PJ) alignment parameters have been interpreted in terms of an instantaneous dissociation model, which suggests preferential population of ML=0 states and an electron density distribution peaking perpendicular to the direction of the breaking bond. This is consistent with dissociation on a surface of Σ− symmetry, the most likely candidate correlating with ground state products being the asymptotic 3Σ−(3A″) surface. There are several possible routes to this surface, and these are considered in light of the measured velocity distributions and velocity-dependent spatial anisotropy parameters of the O(3P) products relat...

The dynamics of the Cl+C2H6→HCl(v', j')+C2H5 reaction at 0.24 eV: Is ethyl a spectator?

The hydrogen atom abstraction reaction between Cl(2P3/2) and ethane has been studied at a mean collision energy of 0.24 eV. The experiments were performed in a coexpansion of molecular chlorine and ethane, with the atomic Cl reactants generated by laser photodissociation of Cl2 at 355 nm. HCl(v′,j′) products were detected quantum state selectively using (2+1) resonantly enhanced multiphoton ionization, coupled with velocity-map ion imaging. The ion images were used to determine center-of-mass angular and kinetic energy release distributions. Several analysis methods were employed and have been carefully assessed. It is shown that the single beam experiments can be used with confidence to determine both center-of-mass angular and energy release distributions. For the title reaction the angular distribution is found to be forward peaking, with on average 22% of the available energy channeled into internal excitation of the ethyl coproducts. Possible sources of this internal excitation are discussed.

An experimental and quasiclassical study of the product state resolved stereodynamics of the reaction O(1D2) + H2(υ = 0) → OH (X2Π32; υ = 0, N, f) + H

The product state resolved dynamics of the photon initiated reaction N2O+hν(193 nm) → O(1D2)+N2, O(1D2)+H2(υ = 0) → OH(X2Π32; υ = 0, N = 5, f) + H, conducted at 300 K and at a mean collision energy of 10 kJ mol−1, have been studied experimentally and computationally. Product state-resolved differential cross-sections of the scattered OH fragments have been probed through analysis of their polarised, Doppler resolved laser induced fluoresence spectra and compared with the predictions of quasiclassical trajectory (QCT) computations conducted on the Schinke-Lester ab initio ground state surface (SL1). The experiments confirm the QCT prediction of strongly peaked backward scattering in the channels generating OH(υ = 0, N = 5). The two individually probed Λ-doublet components display identical angular distributions. Although most collision complexes are estimated to have lifetimes less than their mean rotational periods, the total differential cross-section, summed over all product channels, is predicted to display forward and backward peaks with a moderate bias in favour of backward scattering.

Product rotational polarization. The stereodynamics of the F + H2 reaction

Abstract The angular momentum polarization of the products of the reaction F + H2 (ν = 0, j = 0) → HF(ν′) + H is calculated via the QCT methodology at a collision energy of 0.119 eV. The HF rotational angular momentum distribution is found to display both alignment and orientation, the latter along the y-axis, perpendicular to the k-k′ scattering plane, which depend sensitively on the product vibrational level. The origin of polarization behaviour is traced back to different dynamical mechanisms leading to production of HF(ν′ = 0), and to a lesser extent HF (ν′ = 1), compared with higher product vibrational states, with the former originating primarily from repulsive insertion type trajectories, and the latter primarily from repulsive abstraction type trajectories.

Peripheral chemical reactions

Abstract Forward scattering of the products of direct exoergic atom transfer reactions is proposed as an indication for a peripheral attraction: a reaction in which an atom at the periphery of the reactants is abstracted. Model computations for the O( 1 D) + N 2 O reaction are used to illustrate the proposed mechanism. The opacity function has a peak at higher impact parameters which is correlated with the forward scattering. Potential energy surfaces which do not manifest peripheral attraction lead to backwards scattering of the products.