Designing photocathodes: Ab initio modelling of the photoemission process in materials for next generation light sources
File(s)
Author(s)
Mildner, Felix Constantin
Type
Thesis or dissertation
Abstract
Photocathodes are a vital component of free electron lasers (FELs) capable of producing coherent radiation pulses with lengths on the order of femtoseconds (fs). These pulse lengths are a temporal resolution synchrotron radiation light sources can not achieve, allowing measurements of protein dynamics, chemical reactions, electron dynamics and more. Challenges in developing photocathodes for FELs include extending photocathode lifetimes, improving quantum efficiency (QE), lowering emittance and mean transverse energy (MTE), and lowering work functions. While a substantial amount of experimental and theoretical work has been put towards these challenges, they still persist to this day.
Addressing these challenges includes a direct link between the electronic structure of ab initio modelled photocathode surfaces and experimentally measured photoemission (PE) spectra. With advances in ab initio methods and computing capabilities, modelling photocathode surface structures can now be performed routinely. Therefore, the link from model to experiment can be established by calculating the PE response with an appropriate emission model.
Such PE models exist, but are normally applied to modelled bulk materials. This ignores the effects of surface structures, adsorbates, and contaminants on the PE response. Within this thesis an emission model to calculate the PE spectrum of a surface is developed from previous work. Additional capabilities for angle resolved data are implemented, an approach to calculate the PE of supercell surface models is introduced, and the code implementation is optimised for the efficient modelling of large surface models. The PE spectrums for a series of metallic surfaces and heterogeneous interfaces are calculated. Experimental MTE spectra are compared to the calculated values and detailed atomic and electronic contributions are analysed. The detailed analysis of contributions allows for the linking of modelled electronic structure to experimentally observable changes in the PE spectrum.
Addressing these challenges includes a direct link between the electronic structure of ab initio modelled photocathode surfaces and experimentally measured photoemission (PE) spectra. With advances in ab initio methods and computing capabilities, modelling photocathode surface structures can now be performed routinely. Therefore, the link from model to experiment can be established by calculating the PE response with an appropriate emission model.
Such PE models exist, but are normally applied to modelled bulk materials. This ignores the effects of surface structures, adsorbates, and contaminants on the PE response. Within this thesis an emission model to calculate the PE spectrum of a surface is developed from previous work. Additional capabilities for angle resolved data are implemented, an approach to calculate the PE of supercell surface models is introduced, and the code implementation is optimised for the efficient modelling of large surface models. The PE spectrums for a series of metallic surfaces and heterogeneous interfaces are calculated. Experimental MTE spectra are compared to the calculated values and detailed atomic and electronic contributions are analysed. The detailed analysis of contributions allows for the linking of modelled electronic structure to experimentally observable changes in the PE spectrum.
Date Issued
2025-10-31
Date Awarded
2026-02-01
Copyright Statement
Attribution-NonCommercial 4.0 International Licence (CC BY-NC)
License URL
Advisor
Harrison, Nicholas
Sponsor
Science and Technology Facilities Council (Great Britain)
Engineering and Physical Sciences Research Council
UK Research and Innovation
Grant Number
EP/S023259/1
Publisher Department
Department of Materials
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)