Development of an Apparatus for the Study of Electron Impact Fragmentation of Molecular Clusters

The aim of the experiment described in this thesis was to generate a beam of molecular clusters and to look at fragmentation processes induced by low-energy electron impact. These processes are studies by time-of-flight detection of ionised and neutral metastable fragments. A beam of molecular clusters is generated using a pulsed supersonic expansion, and this beam is crossed with a pulsed beam of electrons. The electron pulse is 1 μs to provide the necessary time-of-flight resolution. Ionised fragments are detected using a reflectron time-of-flight mass spectrometer with a microchannel plate detector. Neutral metastable fragments are detected using a channeltron with appropriately biased meshes in front to avoid the detection of charged particles. For both types of fragment data acquisition takes place using a multichannel scaler. Detection of the ions is mass resolved, whereas the flight time of the neutral metastable fragments provides information about the kinetic energy these fragments have acquired in the fragmentation process. The ultimate goal of this experiment is to study lowenergy electron impact on biomolecules solvated in water clusters. This is relevant in the context of radiation damage studies. Recent research on radiation damage in biological organisms has demonstrated the relevance of low-energy secondary electrons produced by the radiation. The work described in this thesis involved the development of a working cluster source, which produces excellent methanol and argon mass spectra, via supersonic expansion from a nozzle. A neutral metastable detector has been designed, constructed, and implemented; time-of-flight spectra have been gathered for electron impact on argon clusters, and are in agreement with previous work, indicating the detector is operating well. The electron gun has been adapted to incorporate a deflection system for steering of the electron beam, and now operates well with stability down to 20 eV. Programs have been written using LabVIEW for control of the electron impact energy, and for the acquisition of data and excitation functions, as well as providing quick analysis of mass spectra. Further minor modifications and implementations have been conducted to optimise the overall operation of the system. This developmental work has moved the apparatus at Maynooth closer to its ultimate goal, the study of electron impact fragmentation of molecular clusters