We present a comprehensive first-principles investigation of the electronic and optical properties of CsK 2 Sb, a semiconducting material for ultra-bright electron sources for particle accelerators. Our study, based on density-fuctional theory and many-body perturbation theory, provides all the ingredients to model the emission of this material as a photocathode, including band gap, band dispersion, and optical absorption. An accurate description of these properties beyond the meanfield picture is relevant to take into account many-body effects. We discuss our results in the context of state-of-the-art electron sources for particle accelerators to set the stage towards improved modeling of quantum efficiency, intrinsic emittance, and other relevant quantities determining the macroscopic characteristics of photocathodes for ultra-bright beams.

Durch Oxydation von J2 rnit rduchender Salpetersaure in Gegenwart von Perfluorcarbonsaureanhydriden wurden J(OCOCF3)3 (1) und J(OCQC3F7)3 (2) dargestellt. Die analoge Oxydation von Jz in Gegenwart von Pentafluorbenzoesaure und Trifluoressigsaureanhydrid lieferte J(OCOCsF& (3). Pentafluorjodbenzol lieB sich in gleicher Weise in C6F5J(OCOCF3)2 (4, C6F5J(OCOC6F5)2 (5) und C~F S J ( O C O C~F~)~ (6) iiberfiihren. 4, 5 und 6 lassen sich mit waDriger KJ-Lasung wieder zu C6Fs.I reduzieren, mit gesattigter waisriger NaHC03-LOsung zu C,jFsJO hydrolysieren. -IR-Spektroskopische Befunde sprechen fur weitgehend kovalente Bindungen innerhalb der Struktur J -0 -CO-Rf. -In Gegenwart von Pyridin entsteht aus J2 und CF3C02Ag in Benzol [J(NCsH5)]0COCF3 (S), sowie aus J2 und C6F5C02Ag [J(NCSHS)I~COC~FS (9). Uber Perfluoracyloxy-Verbindungen des Jods ist bisher im Gegensatz zu den Acyloxy-Verbindungen nur wenig bekannt. Die Bildung von JI-Perfluoracyloxy-Verbindungen JOCORf (R, = perfluorierter organischer Rest, z. B. CF3, C3F7) wird beim Abbau von Silbersalzen von Perfluorcarbonsauren mit elementarem Jod bei hoherer Temperatur angenommen. So bildet sich z. B. CF3J in ausgezeichneten Ausbeuten bei der Einwirkung von J2 auf AgOCOCF3 1-4) offensichtlich uber JOCOCF3, dessen Isolierung als festes, farbloses 1 : 1-Addukt mit Pyridin uns gelungen ist. Das Auftreten von J(OCOCF3)3 war bisher nur von Beringer und Mitarbb.5) erwahnt, die Substanz selbst aber nicht isoliert und charakterisiert worden. Wir erhielten Jod-tris-trifluoracetat (1) bei der Oxydation von Jod mit rauchender Salpetersaure nach Fouquk6) in Gegenwart von Trifluoressigsaureanhydrid gemal3 (1) 7 4 J -0 -< Rr 7) E. Spinner, J. chern. SOC. [London] 1964,4217. 8) I. 0. Halford, J. chem. Physics 21 830 (1965). 9) S. Forskn, Spectrochirn. Acta [London] 18, 595 (1962).

The development of novel photocathode materials for ultra-bright electron sources demands efficient and cost-effective strategies that provide insight and understanding of the intrinsic material properties given the constraints of growth and operational conditions. To address this question, we propose a viable way to establish correlations between calculated and measured data on core electronic states of Cs-K-Sb materials. To do so, we combine first-principles calculations based on all-electron density-functional theory on the three alkali antimonides Cs3Sb, Cs2KSb, and CsK2Sb with x-ray photoemission spectroscopy (XPS) on Cs-K-Sb photocathode samples. Within the GW approximation of many-body perturbation theory, we obtain quantitative predictions of the band gaps of these materials, which range from 0.57 eV in Cs2KSb to 1.62 eV in CsK2Sb and manifest direct or indirect character depending on the relative potassium content. Our theoretical electronic-structure analysis also reveals that the core states of these systems have binding energies that depend only on the atomic species and their crystallographic sites, with largest shifts of the order of 2 eV and 0.5 eV associated to K 2p and Sb 3d states, respectively. This information can be correlated to the maxima in the XPS survey spectra, where such peaks are clearly visible. In this way, core-level shifts can be used as fingerprints to identify specific compositions of Cs-K-Sb materials and their relation with the measured values of quantum efficiency. Our results represent the first step towards establishing a robust connection between the experimental preparation and characterization of photocathodes, the ab initio prediction of their electronic structure, and the modeling of emission and beam formation processes.

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.

Most known methods for the determination of the structure of macromolecular complexes are limited or at least restricted at some point by their computational demands. Recent developments in information technology such as multicore, parallel and GPU processing can be used to overcome these limitations. In particular, graphics processing units (GPUs), which were originally developed for rendering real-time effects in computer games, are now ubiquitous and provide unprecedented computational power for scientific applications. Each parallel-processing paradigm alone can improve overall performance; the increased computational performance obtained by combining all paradigms, unleashing the full power of today's technology, makes certain applications feasible that were previously virtually impossible. In this article, state-of-the-art paradigms are introduced, the tools and infrastructure needed to apply these paradigms are presented and a state-of-the-art infrastructure and solution strategy for moving scientific applications to the next generation of computer hardware is outlined.