Electron-induced dissociation dynamics studied using covariance-map imaging

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.

Imaging the Dynamics of the Electron Ionization of C2F6

The dissociation of C₂F₆ following electron ionization at 100 eV has been studied using multimass velocity-map ion imaging and covariance-map imaging analysis. Single ionization events form parent C₂F₆⁺ cations in an ensemble of electronic states, which follow a multiplex of relaxation pathways to eventually dissociate into ionic and neutral fragment products. We observe CF₃⁺, CF₂⁺, CF⁺, C⁺, F⁺, C₂F₅⁺, C₂F₄⁺, C₂F₂⁺, and C₂F⁺ ions, all of which can reasonably be formed from singly charged parent ions. Dissociation along the C–C bond typically forms slow-moving, internally excited products, whereas C–F bond cleavage is rapid and impulsive. Dissociation from the à state of the cation preferentially forms C₂F₅⁺ and neutral F along a purely repulsive surface. No other electronic state of the ion will form this product pair at the electron energies studied in this work, nor do we observe any crossing onto this surface from higher-lying states of the parent ion. Multiply charged dissociative pathways are also explored, and we note characteristic high kinetic energy release channels due to Coulombic repulsion between charged fragments. The most abundant ion pair we observe is (CF₂⁺, CF⁺), and we also observe ion pair signals in the covariance maps associated with almost all possible C–C bond cleavage products as well as between F⁺ and each of CF₃⁺, CF₂⁺, CF⁺, and C⁺. No covariance between F⁺ and C₂F₅⁺ is observed, implying that any C₂F₅⁺ formed with F⁺ is unstable and undergoes secondary fragmentation. Dissociation of multiply charged parent ions occurs via a number of mechanisms, details of which are revealed by recoil-frame covariance-map imaging.