A model of photoemission from coated surfaces is significantly modified by first providing a better account of the electron scattering relaxation time that is used throughout the theory, and second by implementing a distribution function based approach (“Moments”) to the emission probability. The latter allows for the evaluation of the emittance and brightness of the electron beam at the photocathode surface. Differences with the Fowler-Dubridge model are discussed. The impact of the scattering model and the Moments approach on the estimation of quantum efficiency from metal surfaces, either bare or partially covered with cesium, are compared to experiment. The estimation of emittance and brightness is made for typical conditions, and the derivation of their asymptotic limits is given. The adaptation of the models for beam simulation codes is briefly discussed.
Photocathodes are a critical component many linear accelerator based light sources. The development of a custom-engineered photocathode based on low work function coatings requires an experimentally validated photoemission model that accounts the complexity of the emission process. We have developed a time-dependent model accounting for the effects of laser heating and thermal propagation on photoemission. It accounts for surface conditions (coating, field enhancement, and reflectivity), laser parameters (duration, intensity, and wavelength), and material characteristics (reflectivity, laser penetration depth, and scattering rates) to predict current distribution and quantum efficiency (QE) as a function of wavelength. The model is validated by (i) experimental measurements of the QE of cesiated surfaces, (ii) the QE and performance of commercial dispenser cathodes (B, M, and scandate), and (iii) comparison to QE values reported in the literature for bare metals and B-type dispenser cathodes, all for various wavelengths. Of particular note is that the highest QE for a commercial (M-type) dispenser cathode found here was measured to be 0.22% at 266nm, and is projected to be 3.5 times larger for a 5ps pulse delivering 0.6mJ∕cm2 under a 50MV∕m field.
A model of photoemission from cesium antimonide ͑Cs 3 Sb͒ that does not rely on adjustable parameters is proposed and compared to the experimental data of Spicer ͓Phys. Rev. 112, 114 ͑1958͔͒ and Taft and Philipp ͓Phys. Rev. 115, 1583 ͑1959͔͒. It relies on the following components for the evaluation of all relevant parameters: ͑i͒ a multidimensional evaluation of the escape probability from a step-function surface barrier, ͑ii͒ scattering rates determined using a recently developed alpha-semiconductor model, and ͑iii͒ evaluation of the complex refractive index using a harmonic oscillator model for the evaluation of reflectivity and extinction coefficient.
Photocathodes are key components of electron injectors for X-ray free electron laser and X-ray energy recovery linacs, which generate brilliant, ultrafast, and coherent X-rays for the exploration of matter with ultrahigh resolutions in both space and time. Whereas alkali-based semiconducting photocathodes display a higher quantum efficiency (QE) in the visible light spectrum than their metallic counterparts, their lifetimes are much shorter due to the high reactivity of alkali-based surfaces to the residual gases in the vacuum chamber. Overcoming the tradeoff between QE and lifetimes has been a great challenge in the accelerator community. Herein, based on ab initio density functional calculations, we propose an approach to overcome this tradeoff by coating with atomically thin two-dimensional (2D) nanomaterials. On one hand, the 2D coating layers can enhance the lifetimes of photocathodes by preventing the chemical reactions with the residual gases. On the other hand, the 2D coating layers can effectively engineer the work function of photocathodes, thus controlling their QE. A monolayer of insulating BN reduces the work function, whereas a monolayer of semi-metallic graphene or semiconducting molybdenum disulfide (MoS 2) increases the work function. This phenomenon originates from the induced interfacial dipoles. The reduction of work function by BN implies that it is capable of maintaining the high QE of semiconducting photocathodes in addition to enhance their lifetimes. This study advances our understandings on the surface chemistry of coated photocathodes and opens new technological avenues to fabricate photocathodes with high QE and longer lifetimes.
A theoretical expression for the intrinsic emittance of a photocathode is developed based on a method of evaluating moments of the emission distribution function. The method is first used to reevaluate the well-known rms emittance of a thermionic source, and then, using analogous approximations but with an updated theoretical model of photoemission, an equation for the intrinsic emittance and brightness of a photocathode of comparable simplicity to the thermionic case is obtained in the limit of weak field and low temperature.
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