Sputtering yields for different ions and materials at low ion energies have a similar energy dependence. Due to this similarity, yield data can be characterized by a normalized energy function and two parameters for each ion target combination. One of these parameters is the threshold energy. An energy scaling can be based on this parameter. The other parameter is a multiplication factor. Both parameters depend mainly on the ion and target mass M1 and M2 and on the surface binding energy EB. An analytic expression for the normalized functions and both the parameters is given. This empirical relation also allows an estimate of unknown sputtering data, if M1, M2, and EB are noted. A physical interpretation of the empirical relation is given for the case M1≪M2, as in this case special collision processes which dominate the sputtering can be identified.

An Analytical Formula and Important Parameters for Low-Energy Ion Sputtering: Erratum of the Publication in Journal of Applied Physics Vol. 51, S. 2861 ff, (1980)

The design and performance of a laser‐excited cesium antimonide (Cs3Sb) photoemissive electron source operating in an electron‐gun configuration is described. Such an electron source has advantages of higher brightness, beam shapeability, low‐temperature operation, external modulability, and ease of service compared with conventional thermal and field emission sources. With an uncooled photoemissive source operating at 2.9 A/cm2, cathode life exceeded 50 h without degradation. Photoemissive electron sources should be especially useful in applications requiring high‐resolution imaging or high‐frequency pulsed operation.

Practical laser‐activated photoemissive electron source

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 ad...

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

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 (MoS2) 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. Coating alkali-based photocathodes with atomically thin hBN enhance their lifetime whilst improving their quantum efficiency. A team led by Enrique Batista at Los Alamos National Laboratory performed ab initio density functional theory calculations on photocathodes coated with a variety of 2D materials. While monolayers of semi-metallic graphene and semiconducting MoS2 were found to increase the photocathode work function, a monolayer of insulating hBN led to a work function reduction whilst offering protection of the highly reactive photocathode surface from irreversible chemical reactions. Such work function reduction, key to achieving high quantum efficiency, was ascribed to the formation of induced dipole moments at the interface between hBN and the Cs3Sb cathode surface, pointing out of Cs3Sb. hBN monolayers are thus promising coating materials for alkali-based semiconducting photocathodes.

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

A high current, high current density cesium antimonide photoemissive electron source was constructed and tested over an extended period of time with small degradation. A photocurrent density of more than 3 A/cm2 was obtained with a performance drop of about 15% over a 70‐h period. The slow decay was caused by cesium desorption from the cathode. Cesium evaporated onto the surface at the end of the test rejuvenated the photocathode to its original response. Such a photoemissive source has particular applications to electron beam lithography machines and high resolution, nondestructive electron microscopes.

L.R. Danielson

Papers

Laser illuminated high current photocathodes