Optimization of some parameters for high efficiency caesium-antimony semitransparent photocathodes

Bhide, G.K.; Rangarajan, L M; Bhat, Bashir Mohi Ud Din

doi:10.1088/0022-3727/4/4/315

Cited by 5 publications

(2 citation statements)

References 11 publications

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“…Previously, the contaminating effect due to residual gases was investigated by exposing the photocathodes to individual gas species under controlled conditions. ,− As early as 1969, Decker studied the decay characteristics of alkali-Sb photocathodes exposed to residual environment gases . Later, the poisoning effect on Cs 3 Sb photocathodes was investigated by a few other groups. , The discovery of negative electron affinity (NEA) GaAs-Cs photocathode has stimulated more interest in the degradation of alkali-based photocathodes in the community. ,,,, It has been demonstrated that oxygen containing molecules, such as O 2 , CO 2 , CO, and H 2 O, produce significant degradation to the QE, while N 2 and H 2 have no effect to the QE. ,,,,,, Based on these experiments, it has been proposed that O 2 , CO 2 , and H 2 O experience dissociative adsorption on the surface of alkali-based photocathodes, while CO adsorbs molecularly on the surface without dissociating into C and O atoms. , The O atoms dissociated from the O 2 , CO 2 , and H 2 O molecules change the surface chemistry by deviating from the optimum Cs/O composition ratio and results in strong reduction of the QE. ,, Recently, a theoretical kinetic model was presented that describes the experimental data of QE degradation on the basis of three thermodynamic states: a desorbed gas molecule, a physisorbed state, and a reacted chemisorbed state . Whereas these experiments provide data on the kinetics of gases interacting with photocathodes, which is confirmed with the kinetic model mentioned above, no comprehensively theoretical efforts have been undertaken to elucidate the underlying physics and chemistry involved in the degradation process.…”

Section: Introductionmentioning

confidence: 99%

“…5,8,9,11,14 It has been demonstrated that oxygen containing molecules, such as O 2 , CO 2 , CO, and H 2 O, produce significant degradation to the QE, while N 2 and H 2 have no effect to the QE. 2,5,8,9,11,14,15 Based on these experiments, it has been proposed that O 2 , CO 2 , and H 2 O experience dissociative adsorption on the surface of alkali-based photocathodes, while CO adsorbs molecularly on the surface without dissociating into C and O atoms. 5,8 The O atoms dissociated from the O 2 , CO 2 , and H 2 O molecules change the surface chemistry by deviating from the optimum Cs/O composition ratio and results in strong reduction of the QE.…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2017

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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.

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