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Title: Designing new materials for photocathodes
Author: Camino, Bruno
ISNI:       0000 0004 6348 628X
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2017
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Free electron lasers (FELs) are state of the art in terms of generating light pulses. By using free electrons as lasing medium, FELs provide tunable radiation in the infrared to X-rays wavelength range and on the femtosecond timescale. Their use in biology, chemistry and materials science allows for the probing of dynamic processes at levels never reached before. In order to design the new generation of FELs, a bright and low emittance electron beam must be generated from the photocathode. For this reason widespread experimental and computational efforts are taking place worldwide in order to generate more efficient materials for photocathode applications. However, a clear understanding of the emission process and, in particular, how the process is influenced by the surface structure needs further investigation. In this work, a computational tool based on the well know three step model (3SM) for photoemission is presented. In its original formulation, the 3SM is based on the bulk electronic structure of materials. In this study it is extended to explicit models of the surfaces. This approach retains the simple chemical intuition and it allows the disentanglement of the effect of the atomic, electronic and chemical structure of the surface on the observed photoemission. This is achieved by using a layer-by-layer decomposition of the surface electronic structure that is calculated through reliable density functional theory (DFT) calculations. Test calculations on clean copper, silver and magnesium surfaces are reported in this thesis and compared to the measured quantum efficiency. The ability of the model to simulate the influence of surface modifications on the observed quantum efficiency are also reported: the adsorption of oxygen, hydrogen and cesium on the Mg (0001) surface and the presence of steps on the Ag (111) surface are discussed. Hydrogen and oxygen were selected because they are well known contaminants of photocathode surfaces and cesium was simulated because it is commonly used to enhance the quantum efficiency of photoemitting materials. The calculations allow the known effects of surface states and work function changes to be rationalised. Furthermore, the simulations of the adsorbate covered surfaces generated some counter intuitive results that can be explained by the model developed in this work. The computational tool presented in this thesis has already provided some insights on how surface chemistry and reconstruction influence the quantum efficiency of materials currently used in photocathodes and will be used in future studies to generate guidelines for the design of more efficient photocathodes.
Supervisor: Harrison, Nicholas M. Sponsor: Science and Technology Facilities Council (Great Britain)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral