Alkali-antimonide photocathodes were grown on Si(100) and studied by means of XPS and UHV-AFM to validate the growth procedure and morphology of this material. The elements were evaporated sequentially at elevated substrate temperatures (first Sb, second K, third Cs). The generated intermediate K-Sb compound itself is a photocathode and the composition of K2.4Sb is close to the favored K3Sb stoichiometry. After cesium deposition, the surface layer is cesium enriched. The determined rms roughness of 25 nm results in a roughness domination of the emittance in the photoinjector already above 3 MV/m

Alkali antimonides have a long history as visible-light-sensitive photocathodes. This work focuses on the process of fabrication of the bi-alkali photocathodes, K2CsSb. In-situ synchrotron x-ray diffraction and photoresponse measurements were used to monitor phase evolution during sequential photocathode growth mode on Si(100) substrates. The amorphous-to-crystalline transition for the initial antimony layer was observed at a film thickness of 40 Å . The antimony crystalline structure dissolved upon potassium deposition, eventually recrystallizing upon further deposition into K-Sb crystalline modifications. This transition, as well as the conversion of potassium antimonide to K2CsSb upon cesium deposition, is correlated with changes in the quantum efficiency.

Nano-roughness limits the emittance of electron beams that can be generated by high efficiency photocathodes, such as the thermally reacted alkali antimonide thin films. However there is an urgent need for photocathodes that can produce an order of magnitude or more lower emittance than present day systems in order to increase the transverse coherence width of the electron beam. In this paper we demonstrate a method for producing alkali antimonide cathodes with near atomic smoothness with high reproducibility.Photoemission based electron sources for the next generation x-ray high repetition rate, high brightness light sources such as Energy Recovery Linacs 1 and Free Electron Lasers 2 need to satisfy several criteria, namely: high (>1%) quantum efficiency (QE) in the visible range, smallest possible intrinsic emittance, fast (sub-ps) response time and a long operational lifetime. During the past decade, alkali-antimonides (eg. K 2 CsSb) have emerged as the only class of materials that satisfies all these requirements with a high QE >5% and a low intrinsic emittance in the range of 0.36-0.5 µm per mm rms laser spot size in green (520-545 nm) light 3-5 . Additionally, alkali-antimonides also show promise as sources of ultra-cold electrons for ultrafast electron diffraction 6 applications and Inverse Compton Scattering based Gamma ray sources 7 . Although alkali antimonides have many excellent characteristics, the synthesis process leads to relatively high levels of roughness 8 . K 2 CsSb photocathodes are typically grown as thin films over conducting substrates by thermal evaporation of ∼10-30 nm of Sb followed by sequential thermal evaporation and reaction of K and Cs respectively 3,9 . The films created by this process are not ordered and can have a root mean square (rms) surface roughness as high as 25 nm with a period of roughly 100 nm 8 . Such a surface roughness can distort the electric field near the cathode surface causing the intrinsic emittance to drastically increase. Ignoring the contribution of the slope effect 10,11 , to first order, the intrinsic emittance after accounting for this electric field effect can be given by in = 2 in0 + 2 f , where in0 is the intrinsic emittance of the cathode at near zero electric field and f is the enhancement to the intrinsic emittance at an electric field of f MV/m (typically in the range of 1-20 MV/m) at the cathode surface. In RF/SRF based electron guns, used for high bunch charge applications, the electric field at the cathode surface can be greater than 20 MV/m. In this case the electric field enhancement of the intrinsic emittance can be as high as 2 µm per mm rms laser spot size making these cathodes unusable.12 . The smallest possible intrinsic emittance is limited by the lattice temperature of the cathode to in0 = 0.22 a) Electronic mail: fjun@lbl.gov µm at room temperature and can be obtained by exciting electrons with near threshold photons 13 . However, in alkali-antimonides the smallest possible emittance is limited to a higher value even at photoemission...

High quantum efficiency photocathodes are mandatory for the operation of photoinjector driven electron accelerators with high average current and high brightness beams. Photocathodes based on bi-alkali antimonides, e.g., CsK 2 Sb, exhibit high quantum efficiencies for visible light and can be operated close to the photoemission threshold, thus they are suitable candidates to provide high current and low emittance electron beams. In this paper, a codeposition procedure of K and Cs on Sb resulting in high quantum efficiency photocathodes is presented and compared to the sequential growth procedure that was established for photomultiplier and accelerator applications. In-situ x-ray photoelectron spectroscopy is applied to gain insights into the reaction pathway of antimony with alkali metals, and to optimize the growth process of CsK 2 Sb on Mo. It has been found that the average stoichiometry of the samples is similar after both procedures. The study also presents the behavior of the photocurrent at cryogenic temperatures, the influence of cooling and warmup cycles on the photocathode lifetime and our experience with storage and transport. This work demonstrates that our codeposition growth procedure reproducibly delivers high quantum efficiency photocathodes, and that their quantum efficiency, when excited with green photons, is not influenced by cryogenic temperatures.

Bi-alkali antimonide photocathodes are the best known sources of electrons for high current and/or high bunch charge applications like Energy Recovery Linacs or Free Electron Lasers.Despite their high quantum efficiency in visible light and low intrinsic emittance, the surface roughness of these photocathodes prohibits their use as low emittance cathodes in high accelerating gradient superconducting and normal conducting radio frequency photoguns and limits the minimum possible intrinsic emittance near the threshold. Also, the growth process for these materials is largely based on recipes obtained by trial and error and is very unreliable. In this paper, using X-ray diffraction, we investigate the different structural and chemical changes that take place during the growth process of the bi-alkali antimonide material K 2 CsSb.Our measurements give us a deeper understanding of the growth process of alkaliantimonide photocathodes allowing us to optimize it with the goal of minimizing the surface roughness to preserve the intrinsic emittance at high electric fields and increasing its reproducibility.

K2CsSb is a promising photocathode candidate to serve as an electron source in next-generation light sources such as Free Electron Lasers (FEL) and Energy Recovery Linacs (ERL). As the traditional recipe for creation of K2CsSb photocathodes typically results in a rough surface that deteriorates electron beam quality, significant effort has been made to explore novel growth methods for K2CsSb photocathodes. In this paper, a method of ternary co-evaporation of K, Cs, and Sb is described. By using in-situ synchrotron X-ray techniques, the quality of the photocathode is characterized during and after the growth. K2CsSb photocathodes grown by this method on Si (100) and MgO (001) substrates show strong (222) texture, and the two photocathodes exhibit 1.7% and 3.4% quantum efficiencies at a wavelength of 530 nm, with a rms surface roughness of about 2–4 nm. This represents an order of magnitude reduction in roughness compared to typical sequential deposition and should result in a significant improvement in the brightness of the generated electron beam.

Theoretical and experimental results for the charge state and energy loss of low-energy He scattered off a Ni(llO) surface are compared. A first-principles theory is used to analyze both the charge state and the energy loss. Charge-exchange processes are of primordial importance to explain the energy-loss spectra. Energy straggling plays an important role in the system under study.PACS numbers: 79.20.Rf

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