We are working together with experimentalist groups based at the Daresbury Laboratory to improve the efficiency of current emitting materials and to develop new ones to be used in the next generation light sources.
Goals of the project
The aim of the project is to contribute to photocathode design providing information on the photoemission process from an atomistic point of view.
Photocathodes are a key component to modern accelerator systems and a broad international research programme is now aimed at their improvement. An empirical approach has provided materials whose efficiency is limited and a clear understanding of the electron emission mechanism is still lacking. Ab initio simulations can help to fill this gap. Density functional theory (DFT) is a general purpose and powerful method to predict, not only the electronic structure of molecules and materials, but also structure and composition in realistic conditions. Simulations can be extended, by using modern supercomputers, to systems containing thousands of atoms.
State of the art methods to compute photoemission are based on the one-step-model. However, they are computationally expensive and treat the photoemission as a single quantum mechanical process.
In this project, the simplest possible approach to compute photoemission from surfaces is being developed. Calculations are based on the well known three-step-model, widely used to interpret experimental data. This model is extended, in order to include surface effects, to a layer-by-layer decomposition of the surface electronic structure that can be computed using standard, efficient and reliable electronic structure codes (for example CASTEP). This approach is expected to reproduce satisfactorily the emission process retaining the chemical intuition needed to understand the effect of roughness, reconstruction, interaction with species and stress on the surface.