Ionization and fragmentation dynamics of singly and multiply charged ions
Keywords: Electron ionization
Velocity-map imaging
Coulomb explosion
Interstellar medium
Astrochemistry
Ab initio trajectory simulation
Plasma chemistry
Ionization cross-section

This thesis investigates the ionization and fragmentation dynamics of gas-phase molecules, particularly those relevant to astrochemistry and of interest in velocity-map imaging experiments into the dynamics of Coulomb explosions. These two topics are covered in Part I and Part II, respectively.
Part I focuses on the redesign of a total electron ionization cross-section measurement apparatus. The aim was to satisfy a number of key experimental criteria, such as achieving full collection efficiency for the ions and electrons generated in the experiment, and obtaining accurate absolute pressure and temperature measurements. A combination of SIMION ion and electron trajectory simulations and experimental optimisation led to the final version of the instrument. The absolute total cross-section data obtained using the apparatus (including nine interstellar molecules measured) are considered to be highly reliable without significant sources of systematic errors, and the results show good agreement with cross-sections predicted using the binary-encounter Bethe (BEB) model, especially at low incident electron energy. In common with previous studies, the maximum in the experimentally measured cross-section correlates linearly with the molecular polarizability volume.
The instrument developed in this thesis will be used to perform further measurements on many more molecules, and the absolute total cross-section data will be combined with relative partial ionization cross-section data measured on an electron-molecule crossed-beam velocity-map imaging apparatus in the same laboratory to determine absolute partial ionization cross-sections for each fragment ion. A larger project to establish a comprehensive database which contains both total and partial ionization cross-sections of molecules relevant to astrochemistry and plasma science is proposed, and is currently under development.
Part II of the thesis is devoted to the ionization and fragmentation dynamics of multiply charged ions, for which Coulomb explosion plays an important role as a dissociation mechanism. While femtosecond-laser-pulse-induced Coulomb explosion experiments have been performed extensively, the corresponding simulation work has relied on simple Coulomb-only models. In the present work, we develop a new trajectory simulation code in which the forces on each atom are calculated via electronic structure methods at each trajectory time step. The electronic structure calculations and classical trajectory calculations are performed within Gaussian and Matlab, respectively, packaged within a single executable Linux shell script. All Gaussian and Matlab parameters may be set and adjusted to calculate outcomes such as evolution of atomic charges, bond orders, bond lengths, atomic velocities, atomic forces, and SCF energies of the molecular system. In addition to providing additional insight into the product scattering distributions observed in Coulomb-explosion imaging experiments, the simulations also allow a number of experimentally inaccessible features to be investigated, including details of the fragmentation mechanism and the extensive charge redistribution that occurs in the early stages of the Coulomb explosion process.
The new simulation program is the good starting point towards full simulation as per experimental conditions to re-produce measured velocity-map images. Future work will develop multi-trajectory simulations based on a range of initial conditions sampled statistically. With ever-increasing CPU power and proper technical considerations, this perspective is no longer out of reach.