Electron diffraction using a cold atom source
AuthorSpeirs, Rory William
AffiliationSchool of Physics
Document TypePhD thesis
Access StatusOpen Access
© 2017 Dr. Rory William Speirs
Observing matter on atomic length and time scales simultaneously is now routinely achieved in ultrafast electron and X-ray imaging techniques, but continued advances in both approaches promise to deliver huge leaps in our understanding of all kinds of atomic structures and processes. Both technologies rely on the generation of ultrabright, ultrashort duration electron bunches, with these bunches being used directly to probe the sample in electron diffraction, or to generate ultrabright X-ray pulses in X-ray free electron lasers. The Cold Atom Electron and Ion Source (CAEIS) was conceived as a possible way to generate ultrafast electron bunches that are brighter than can currently be produced, with the aim of enabling next-generation structural determination techniques, particularly those based on electron diffraction. The CAEIS generates electrons by near-threshold photoionisation of an atomic gas, which has been shown to produce electron bunches with temperature as low as 10K. Such cold electron bunches have the potential to be much brighter than those generated from solid photocathode sources, which typically have temperatures in the thousands of Kelvin range. Extremely cold ions are also generated in the CAEIS, which show great potential for use in ion microscopy and milling. This thesis presents work on a number of different aspects of the continued development of the cold atom electron and ion source, with a particular emphasis on progress towards ultrafast single-shot electron diffraction based experiments. The brightness degrading effects of space-charge repulsion are investigated using nanosecond duration ion bunches as analogues of ultrafast, picosecond duration electron bunches. Ion bunch shaping was achieved through tailoring of the spatial profile of lasers used in ionisation of the atomic gas. It was found that atomic fluorescence could substantially reduce the fidelity with which the ion bunch profile could be controlled, but methods were developed to circumvent the fluorescence problem. The improved shaping procedures allowed generation of uniformly filled ellipsoidal bunches, which theoretically will not suffer emittance degradation under space-charge expansion. Emittance measurements following space-charge driven expansion showed that these uniformly filled ellipsoids did indeed have reduced emittance growth compared to other profiles. Photoexcitation and field-ionisation processes involved in generation of cold electrons on ultrafast timescales were investigated, with the aim of determining the mechanisms that affect the ultimate electron bunch duration. Bunch duration was measured for a range of excitation conditions, with the finding that previously assumed ultrafast excitation pathways in fact generated fairly slow nanosecond long bunches. Ionisation time could also be a million times slower than assumed if atoms were excited below the classical ionisation threshold. Identification of the conditions required for ultrafast excitation and ionisation ultimately allowed generation of ultrafast cold electron bunches with duration of tens of picoseconds. Electron diffraction using nanosecond long electron bunches was achieved in both transmission and reflection modes for a variety of large samples of inorganic crystals. Bunches were of sufficiently high charge to allow identification of features of a crystal structure using only a single shot. Bragg peaks could also be identified by averaging together many images formed using ultrafast, but low-charge bunches. Simulations were performed to determine the feasibility of using electrons generated in the CAEIS for electron coherent diffractive imaging of nanoscale apertures. It was found that it should be possible to successfully reconstruct the object plane wavefield, even taking into account realistic experimental parameters for partial coherence and noise.
Keywordsatomic physics; ultrafast electron diffraction; single-shot electron diffraction; cold atom electron source; diffraction imaging; optical physics; electron beam physics; ion beam physics; Rydberg spectroscopy; ultrafast ionisation
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