Triple evaporation growth of sodium–potassium–antimonide thin films as photoemission source for electron accelerators

Bi-alkali antimonide photocathodes are promising candidates for use as electron sources in high-brightness applications at advanced accelerator facilities. However, their preparation remains challenging due to the narrow compositional window required for optimal performance. We addressed these challenges by using a newly developed deposition system that employs effusion cells for precise flux control. In this work, we present a triple evaporation growth procedure for the preparation of Na2KSb photocathodes. This method significantly improves reproducibility compared to the traditional sequential growth process. The systematic growth of four photocathodes with varying stoichiometry demonstrated precise control over the chemical composition. Detailed x-ray photoelectron spectroscopy analysis, combined with measurements of spectral quantum efficiency and lifetime, revealed strong correlations between the stoichiometry and photoelectric properties. Quantum efficiencies exceeding 5% and 1/e lifetimes longer than 500 h were achieved. A Na2KSb photocathode produced using this method was used for the commissioning of the photoinjector at the SEALab accelerator, demonstrating the operation of an SRF photoinjector with a sodium-based photocathode for the very first time.