Velocity map imaging of chemical reaction dynamics

Lead Research Organisation: University of Bristol
Department Name: Chemistry

Abstract

The dynamics of molecular photodissociation and bimolecular reactions will be studied
using a molecular beam and velocity map imaging apparatus. Experimental determinations
of product quantum state population distributions and differential cross sections will be
compared with computational simulations using accurate potential energy surfaces. The
research will contribute to the activities supported by EPSRC Programme Grant
EP/L005913/1 Chemical Applications of Velocity and Spatial Imaging.

Publications

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publication icon
Wang XD (2019) Observation of Rainbows in the Rotationally Inelastic Scattering of NO with CH. in The journal of physical chemistry. A

Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509619/1 01/10/2016 30/09/2021
1792909 Studentship EP/N509619/1 01/10/2016 31/03/2020 Frederick Cascarini
 
Description The reaction of chlorine and propene has been studied computationally using a previously published, fully dimensional empirical valence bond (EVB) potential energy surface. This reaction has both fundamental significance, as a model system to study competition between direct and indirect H-atom abstraction pathways, and wider importance for our understanding of Cl-atom initiated oxidation chemistry of alkenes in the Earth's atmosphere. The work has revealed new details about the variation of the reaction mechanism at three different collision energies ranging from those typical of ambient conditions through to highly excited conditions.

The results of these calculations demonstrated significant variations in the pathways from reactants to products. For example, at the lower collision energies studied, a higher proportion of reactions formed intermediates surviving a few picoseconds, prior to dissociation and progression towards hydrogen abstraction. The 1-chloropropyl radical is the dominant transient intermediate. Despite these mechanistic variations, the distribution of product population over internal states of the HCl is unaffected by collision energy changes Notably, we observe very few signs of the 'roaming' style large-amplitude dynamics previously argued to control the H-atom abstraction, for which the chlorine explores longer-range regions of the shallow potential well around the propene C=C double bond.

Experimentally, a new crossed-molecular beam and velocity map imaging apparatus designed for the study of bimolecular reactive and inelastic scattering has completed assembly, testing and refinement. We have progressed from unimolecular photodissociation studies (in collaboration with Dr Jochen Mikosch (Max-Born Institute, Berlin) on 2,4-dibromofluorobenzene photodissociation, to inelastic collisions of nitric oxide with methane. The latter study has generated NO quantum-state resolved scattering images for collisions which change rotational and spin-orbit states. Analysis of the results is ongoing, but the images reveal clear differences in the scattering dynamics leading to NO in different rotational angular momentum states. Work is now underway to study the reactive scattering of CN radicals with deuterated methane.
Exploitation Route Our work will primarily act to enable further research in the fields covered. The work on 2,4-dibromofluorobenzene is planned to be taken forward with far-UV transient absorption studies in Berlin. The work on chlorine and propene contributes both to an improved understanding of the dynamics of a significant atmospheric process, and therefore is of relevance to further studies in the field of atmospheric chemistry, and towards understanding both the significance and limitations of the 'roaming' style dynamics, which is of relevance to a wide range of similar reactions. The NO scattering work with methane is currently the largest system to have quantum-state resolved inelastic scattering experiments performed on it, to the best of our knowledge, and the significant trends in dynamics will be of interest to future experiments on a similar scale in this field.
Sectors Chemicals