Understanding the ultrafast electron photoemission process, from simulation to experiment*

The ongoing efforts to develop a reliable ultrafast electron diffraction and imaging system require a stable source of photoemitted electrons and an understanding of how the properties of the generated bunch depend on the photocathode. In order to gain more understanding of this process, we combine the three-step photoemission model with N-particle electron simulations. By using the Fast Multipole Method to treat space charge effects, we are able to follow the time evolution of pulses containing over $10^6$ electrons and investigate the role of laser fluence and extraction field on the total number of electrons that escape the surface. The results of these simulations are compared to experimental images of the photoemission process collected using the shadow imaging technique. We are able to show good quantitative agreement both for the number of electrons generated and the pulse parameters. We also see evidence of a virtual cathode limit, which gives an upper limit to the number of electrons that is is possible to extract. The extension of these results to various extraction fields, laser pulse shapes and photocathode material parameters, represents a very interesting future development, allowing to better optimize the materials used in electron pulse generation.