The dynamics of the Cl+n-C4H10→HCl (v′,j′)+C4H9 reaction at 0.32 eV

Primary vs. secondary H-atom abstraction in the Cl-atom reaction with n-pentane

Velocity map imaging (VMI) measurements and quasi-classical trajectory (QCT) calculations on a newly developed, global potential energy surface (PES) combine to reveal the detailed mechanisms of reaction of Cl atoms with n-pentane. Images of the HCl (v = 0, J = 1, 2 and 3) products of reaction at a mean collision energy of 33.5 kJ mol^-1 determine the centre-of-mass frame angular scattering and kinetic energy release distributions. The HCl products form with relative populations of J = 0-5 levels that fit to a rotational temperature of 138 ± 13 K. Product kinetic energy release distributions agree well with those derived from a previous VMI study of the pentyl radical co-product [Estillore et al., J. Chem. Phys. 2010, 132, 164313], but the angular distributions show more pronounced forward scattering. The QCT calculations reproduce many of the experimental observations, and allow comparison of the site-specific dynamics of abstraction of primary and secondary H-atoms. They also quantify the relative reactivity towards Cl atoms of the three different H-atom environments in n-pentane.

Dynamics of chlorine atom reactions with hydrocarbons: insights from imaging the radical product in crossed beams

We present a comprehensive overview of our ongoing studies applying dc slice imaging in crossed molecular beams to probe the dynamics of chlorine atom reactions with polyatomic hydrocarbons. Our approach consists in measuring the full velocity-flux contour maps of the radical products using vacuum ultraviolet "soft" photoionization at 157 nm. Our overall goal is to extend the range of chemical dynamics investigations from simple triatomic or tetraatomic molecules to systematic investigations of a sequence of isomers or a homologous series of reactants of intermediate size. These experimental investigations are augmented by high-level ab initio calculations which, taken together, reveal trends in product energy and angular momentum partitioning and offer deep insight into the reaction mechanisms as a function of structure, bonding patterns, and kinematics. We explore these issues in alkanes, for which only direct reactive encounters are found, and in unsaturated hydrocarbons, for which an addition-elimination mechanism competes with direct abstraction. The results for alkene addition-elimination in particular suggest a new view of these reactions: The only pathway to HCl elimination is accessed by means of roaming excursions of the Cl atom from the strongly bound adduct.

Determination of differential cross sections and kinetic energy release of co-products from central sliced images in photo-initiated dynamic processes

For photo-initiated inelastic and reactive collisions, dynamic information can be extracted from central sliced images of state-selected Newton spheres of product species. An analysis framework has been established to determine differential cross sections and the kinetic energy release of co-products from experimental images. When one of the reactants exhibits a high recoil speed in a photo-initiated dynamic process, the present theory can be employed to analyze central sliced images from ion imaging or three-dimensional sliced fluorescence imaging experiments. It is demonstrated that the differential cross section of a scattering process can be determined from the central sliced image by a double Legendre moment analysis, for either a fixed or continuously distributed recoil speeds in the center-of-mass reference frame. Simultaneous equations which lead to the determination of the kinetic energy release of co-products can be established from the second-order Legendre moment of the experimental image, as soon as the differential cross section is extracted. The intensity distribution of the central sliced image, along with its outer and inner ring sizes, provide all the clues to decipher the differential cross section and the kinetic energy release of co-products.

Velocity map imaging of the dynamics of bimolecular chemical reactions

The experimental technique of velocity map imaging (VMI) enables measurements to be made of the dynamics of chemical reactions that are providing unprecedented insights about reactive scattering. This perspective article illustrates how VMI, in combination with crossed-molecular beam, dual-beam or photo-initiated (Photoloc) methods, can reveal correlated information on the vibrational quantum states populated in the two products of a reaction, and the angular scattering of products (the differential cross section) formed in specific rotational and vibrational levels. Reactions studied by VMI techniques are being extended to those of polyatomic molecules or radicals, and of molecular ions. Subtle quantum-mechanical effects in bimolecular reactions can provide distinct signatures in the velocity map images, and are exemplified here by non-adiabatic dynamics on coupled potential energy surfaces, and by experimental evidence for scattering resonances.

Dynamics of CN+alkane reactions by crossed-beam dc slice imaging

The hydrogen atom abstraction reactions of CN (X (2)Sigma(+)) with alkanes have been studied using the crossed molecular beam technique with dc slice ion imaging at collision energies of 7.5 and 10.8 kcalmol. The product alkyl radical images were obtained via single photon ionization at 157 nm for the reactions of CN (X (2)Sigma(+)) with n-butane, n-pentane, n-hexane, and cyclohexane. From analysis of the images, we obtained the center-of-mass frame product angular distributions and translational energy distributions directly. The results indicate that the products are largely backscattered and that most of the available energy ( approximately 80%-85%) goes to the internal energy of the products. The reaction dynamics is discussed in light of recent kinetics data, theoretical calculations, and results for related halogen and oxygen atom reactions.

State-to-state reaction dynamics: A selective review

A selective review of state-to-state reaction dynamics experiments is presented. The review focuses on three classes of reactions that exemplify the rich history and illustrate the current state of the art in such work. These three reactions are (1) the hydrogen exchange reaction, H+H2-->H2+H and its isotopomers; (2) the H+RH-->H2+R reactions, where RH is an alkane, beginning with H+CH4-->H2+CH3 and extending to much larger alkanes; and (3) the Cl+RH-->HCl+R reactions, principally Cl+CH4-->HCl+CH3. We describe the experiments, discuss their results, present comparisons with theory, and introduce heuristic models.

Recent advances in crossed-beam studies of bimolecular reactions

A critical overview of the recent progress in crossed-beam reactive scattering is presented. This review is not intended to be an exhaustive nor a comprehensive one, but rather a critical assessment of what we have been learning about bimolecular reaction dynamics using crossed molecular beams since year 2000. Particular emphasis is placed on the information content encoded in the product angular distribution-the trait of a typical molecular beam scattering experiment-and how the information can help in answering fundamental questions about chemical reactivity. We will start with simple reactions by highlighting a few benchmark three-atom reactions, and then move on progressively to the more complex chemical systems and with more sophisticated types of measurements. Understanding what cause the experimental observations is more than computationally simulating the results. The give and take between experiment and theory in unraveling the physical picture of the underlying dynamics is illustrated throughout this review.

Imaging the dynamics of gas phase reactions

Ion imaging methods are making ever greater impact on studies of gas phase molecular reaction dynamics. This article traces the evolution of the technique, highlights some of the more important breakthroughs with regards to improving image resolution and in image processing and analysis methods, and then proceeds to illustrate some of the many applications to which the technique is now being applied--most notably in studies of molecular photodissociation and of bimolecular reaction dynamics.

Imaging photon-initiated reactions: A study of the Cl(P3∕22)+CH4→HCl+CH3 reaction

The hydrogen or deuterium atom abstraction reactions between Cl((2)P(3/2)) and methane, or its deuterated analogues CD(4) and CH(2)D(2), have been studied at mean collision energies around 0.34 eV. The experiments were performed in a coexpansion of molecular chlorine and methane in helium, with the atomic Cl reactants generated by polarized laser photodissociation of Cl(2) at 308 nm. The Cl-atom reactants and the methyl radical products were detected using (2+1) resonantly enhanced multiphoton ionization, coupled with velocity-map ion imaging. Analysis of the ion images reveals that in single-beam experiments of this type, careful consideration must be given to the spread of reagent velocities and collision energies. Using the reactions of Cl with CH(4), CD(4), and CH(2)D(2), as examples, it is shown that the data can be fitted well if the reagent motion is correctly described, and the angular scattering distributions can be obtained with confidence. New evidence is also provided that the CD(3) radicals from the Cl+CD(4) reaction possess significant rotational alignment under the conditions of the present study. The results are compared with previous experimental and theoretical works, where these are available.

'Molecular photography': velocity-map imaging of chemical events

Every chemical reaction bears its own unique fingerprint, embodied in the kinetic energy, angular distribution and rotational and vibrational motion of the newly formed reaction products. These quantities reflect the forces acting during the chemical reaction, and their measurement often provides unparalleled insight into the basic physics governing chemical reactivity. One experimental technique that has truly captured the imagination of the reaction-dynamics community is velocity-map ion imaging, which provides a visual 'snapshot' of the complete product scattering distribution in a single measurement. Originally developed to study gas-phase photodissociation, the technique is now routinely being applied to bimolecular processes, particularly inelastic and reactive scattering. This article will review recent developments in the field, using examples from studies of a range of chemical processes.