Degradation of Alkali-Based Photocathodes from Exposure to Residual Gases: A First-Principles Study

Photocathodes are a key component in the production of electron beams in systems such as X-ray freeelectron lasers and X-ray energy-recovery linacs. Alkali-based materials display high quantum efficiency (QE), however, their QE undergoes degradation faster than metal photocathodes even in the high vacuum conditions where they operate. The high reactivity of alkali-based surfaces points to surface reactions with residual gases as one of the most important factors for the degradation of QE. To advance the understanding on the degradation of the QE, we investigated the surface reactivity of common residual gas molecules (e.g., O2, CO2, CO, H2O, N2, and H2) on one of the best-known alkali-based photocathode materials, cesium antimonide (Cs3Sb), using first-principles calculations based on density functional theory. The reaction sites, adsorption energy, and effect in the local electronic structure upon reaction of these molecules on (001), (110), and (111) surfaces of Cs3Sb were computed and analyzed. The adsorption energy of these molecules on Cs3Sb follows the trend of O2 (−4.5 eV) > CO2 (−1.9 eV) > H2O (−1.0 eV) > CO (−0.8 eV) > N2 (−0.3 eV) ≈ H2 (−0.2 eV), which agrees with experimental data on the effect of these gases on the degradation of QE. The interaction strength is determined by the charge transfer from the surfaces to the molecules. The adsorption and dissociation of O containing molecules modify the surface chemistry such as the composition, structure, charge distribution, surface dipole, and work function of Cs3Sb, resulting in the degradation of QE with exposure to O2, CO2, H2O, and CO.