Photofragment velocity-map imaging of organic molecules

Photofragment velocity-map imaging (VMI) has generally been employed to investigate the photodissociation dynamics of relatively small molecular systems (< 5 atoms). The work reported in this thesis focuses on the application of this technique for the investigation of the unimolecular photodissociation of larger chemical systems, which are of interest to a broad cross section of the chemical community. Typically, VMI studies involve state-selective detection of one particular fragmentation product, and so are often limited to the investigation of a single dissociation channel. By employing vacuum ultra-violet (VUV) photoionization, we are able to detect most, if not all of the fragments resulting from the dissociation of a neutral species, with ‘universal’ ionization being achieved in the ideal case when the fragment ionization energies are all lower than the VUV photon energy. This capability becomes particularly important when investigating larger systems, since these often display complex dynamics with multiple competing fragmentation pathways. Our approach allows us to investigate the different photofragmentation processes occurring for a particular system, to evaluate the relative importance of the active dissociation channels, and to gain insight into the energy partitioning amongst the fragments.
A study of the UV photodissociation of two neutral alkyl iodide molecules demonstrates the first use in our laboratory of ‘universal’ ionization in combination with VMI. Studies into the photofragmentation processes resulting from 193 nm photoexcitation of neutral N,N-dimethylformamide, a small-molecule model for a peptide bond, and a number of neutral cyclic alkenes, which undergo the retro-Diels-Alder reaction, are also presented. The remaining studies presented in this thesis have investigated the photofragmentation processes of ionic species, generated by means of VUV photoionization. In the case of ion dissociation each fragmentation channel necessarily produces one charged species, which may be detected using the VMI technique. Therefore, such studies provide an insight into all of the active channels. An in-depth VMI study of the UV photodissociation of two ethyl halide cations is presented, which demonstrates the successful investigation of the multiple photofragmentation pathways of these ionic species. The remainder of the cation photodissociation studies are of relevance to a number of common processes known to occur in mass spectrometry, including the McLafferty rearrangement, the retro-Diels-Alder reaction, and ‘peptide’ bond fragmentation. By velocity-map imaging the products of these reactions, further information is obtained concerning these dissociation processes, which are no doubt of interest to the wider chemical community.
This work forms part of the velocity-map imaging mass spectrometry (VMImMS) project. VMImMS involves imaging each of the fragmentation products that result from dissociation of a parent molecule of interest, with the aim of increasing the amount of information that can be obtained from a mass-spectrometry-type experiment. The work presented in this thesis demonstrates that VMImMS allows us to unravel details of the dissociation dynamics of both neutral and ionic species, and is potentially a powerful technique for investigating the fragmentation processes of increasingly complex systems.

Keywords: VMI
dynamics
photofragmentation
VMImMS
velocity-map imaging
photodissociation
alkyl halide
N-dimethylformamide
fragmentation
N
universal ionization
DMF
photofragment
dissociation
velocity-map imaging mass spectrometry
Advisor: Vallance, C