Monte Carlo Simulations of Photoelectron Emission from CsBr Coated Cs3Sb Photocathodes

摘要: Protective films can improve the robustness of alkali antimonide photocathodes, but they also inevitably alter the photoemission properties. Previous studies have shown that coating films reduce the quantum efficiency (QE) and modify the kinetic-energy distribution of emitted electrons. However, a comprehensive understanding of the microscopic mechanisms responsible for these changes is still lacking, which hinders further optimization of coated photocathodes. In this work, we developed a five-step Monte Carlo framework to describe the complete photoemission process of a CsBr-coated Cs$_3$Sb photocathode. The framework includes photoelectron generation, electron transport in Cs$_3$Sb, electron transmission at the Cs$_3$Sb-CsBr interface, electron transport and scattering in the CsBr coating film, and electron emission from the coating film into vacuum. Simulation results indicate that QE attenuation is determined by the combined effects of the interface barrier and scattering in the coating film. The interface barrier suppresses the transmission of low-energy electrons and accounts for the rapid QE decrease in thin coating films, whereas the cumulative contribution of scattering increases with coating-film thickness. Moreover, the simulations show that the mean transverse energy (MTE) can be substantially reduced through energy filtering of photoelectrons at the Cs$_3$Sb-CsBr interface, suggesting a promising route for manipulating the intrinsic emittance of photocathodes.