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

Abstract

Photocathodes are a key component in the production of electron beams in systems such as X-ray free-electron 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.

Towards the operation of Cs-K-Sb photocathodes in superconducting rf photoinjectors

“…In the first 19 h after preparation, the QE continues to increase due to the ongoing reactions in the photocathode layer. After this period, the QE decreases, and an increase of the photoemission threshold is observed; this is likely related to the adsorption of residual gas molecules and their reaction with the highly reactive photocathode layer [38]. By the end of the measurements (125 h) the QE reached 0.7% and the threshold calculated was found to be 2.39 eV.…”

Overcoming the quantum efficiency-lifetime tradeoff of photocathodes by coating with atomically thin two-dimensional nanomaterials

“…Various mechanisms lead to the short operational lifetimes of semiconducting photocathodes, for examples, ion back bombardment or thermally induced Cs loss and irreversible chemical reactions with oxygen containing residual gases. 3,5,[8][9][10][11] All these processes result in the deviation from optimum Cs composition at the surface, which increases the work function and deteriorates the QE. 11 Therefore, suppressing or eliminating damage to the surfaces of semiconducting photocathodes is essential to extending their lifetimes.…”

Photoemission and degradation of semiconductor photocathode

“…It has been confirmed that oxygen would result in a significant decrease of QE for GaAs in an experiment [44]. In addition, the first-principle calculation indicates that oxygen is the dominant poisonous gas to degrade alkali-based photocathodes [20]. Therefore, semiconductors degradation by oxygen can be representative for the influence of residual gases.…”

“…Previous works have reported the decay of cathodes' QE with the presence of "poisonous" gases, like O 2 , H 2 O, CO 2 . Some studies have been dedicated on the chemical analysis of surface reactivity of common residual gas molecules [20]. The conclusion was that the oxygen has the most detrimental effect on QE.…”