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Funding Information: The authors acknowledge SOLEIL for the provision of synchrotron radiation (proposal numbers: 20161265, 20171063, 20180483, 20181721, and 20191506) facilities at the GALAXIES beamline. Some measurements were carried out at the CoESCA endstation at the BESSY-II electron storage ring operated by the Helmholtz-Zentrum Berlin für Materialien und Energie. The research leading to this result has been supported by the project CALIPSOplus under the Grant Agreement 730872 from the EU Framework Programme for Research and Innovation HORIZON 2020. The authors acknowledge research funding from the Swedish Research Council (Grant Nos. VR 2016-04590, VR 2018-04125, VR 2018-04330, VR 2018-06465), Swedish Energy Agency (P50626-1, P43549-1), the Göran Gustafsson foundation, the Swedish Foundation for Strategic Research (project nr. RMA15-0130), and the Carl Tryggers foundation (Grant No. CTS 18:59). | openaire: EC/H2020/730872/EU//CALIPSOplus A heterojunction is the key junction for charge extraction in many thin film solar cell technologies. However, the structure and band alignment of the heterojunction in the operating device are often difficult to predict from calculations and, due to the complexity and narrow thickness of the interface, are difficult to measure directly. In this study, we demonstrate a technique for direct measurement of the band alignment and interfacial electric field variations of a fully functional lead halide perovskite solar cell structure under operating conditions using hard X-ray photoelectron spectroscopy (HAXPES). We describe the design considerations required in both the solar cell devices and the measurement setup and show results for the perovskite, hole transport, and gold layers at the back contact of the solar cell. For the investigated design, the HAXPES measurements suggest that 70% of the photovoltage was generated at this back contact, distributed rather equally between the hole transport material/gold interface and the perovskite/hole transport material interface. In addition, we were also able to reconstruct the band alignment at the back contact at equilibrium in the dark and at open circuit under illumination.

Commercial use of lead halide perovskites requires improved thermal stability and therefore a better understanding of their degradation mechanisms. The thermal degradation of three clean perovskite single crystal surfaces (MAPbI(3), MAPbBr(3), FAPbBr(3)) was investigated using synchrotron-based photoelectron spectroscopy. Central findings are that the halide has a large impact on thermal stability and that the degradation of formamidnium results in the formation of a new organic species at the FAPbBr(3) crystal surface.

Auger photoelectron coincidence spectroscopy APECS has been used to examine the electron correlation and itinerance effects in transition metals Cu, Ni and Co. It is shown that the LVV Auger, in coincidence with 2p photoelectrons, spectra can be represented using atomic multiplet positions if the 3d shell is localized atomic like and with a self convoluted valence band for band like itinerant materials as explained using the Cini Sawatzky model. For transition metals, the 3d band changes from band like to localized with increasing atomic number, with the possibility of a mixed behavior. Our result shows that the LVV spectra of Cu can be represented by atomic multiplet calculations, those of Co resemble the self convolution of the valence band and those of Ni are a mixture of both, consistent with the Cini Sawatzky model

The chemical shift of core level binding energies makes electron spectroscopy for chemical analysis (ESCA) a workhorse analytical tool for science and industry. For some elements, close lying and overlapping spectral features within the natural life time broadening restrict applications. We establish how the core level binding energy chemical shift can be picked up experimentally by the additional selectivity through Auger electron photoelectron coincidence spectroscopy (APECS). Coincident measurement of Ni 3p photoemission with different MVV Auger regions from specific decay channels, narrows the 3p core-levels to a width of 1.2 eV, resolves the spin–orbit splitting of 1.6 eV and determines the chemical shift of Ni 3p levels of a Ni(111) single crystal and its oxidized surface layer to 0.6 eV.

Development of functional and stable solid polymer electrolytes SPEs for battery applications is an important step towards both safer batteries and for the realization of lithium based or anode less batteries. The interface between the lithium and the solid polymer electrolyte is one of the bottlenecks, where severe degradation is expected. Here, the stability of three different SPEs poly ethylene oxide PEO , poly amp; 949; caprolactone PCL and poly trimethylene carbonate PTMC together with lithium bis trifluoromethanesulfonyl imide LiTFSI salt, is investigated after they have been exposed to lithium metal under UHV conditions. Degradation compounds, e.g. Li O R, LiF and LixSyOz, are identified for all SPEs using soft X ray photoelectron spectroscopy. A competing degradation between polymer and salt is identified in the outermost surface region lt;7 nm , and is dependent on the polymer host. PTMC LiTFSI shows the most severe decomposition of both polymer and salt followed by PCL LiTFSI and PEO LiTFSI. In addition, the movement of lithium species through the decomposed interface shows large variation depending on the polymer electrolyte system

The layered dichalcogenide MoS$$_{2}$$ 2 is relevant for electrochemical Li adsorption/intercalation, in the course of which the material undergoes a concomitant structural phase transition from semiconducting 2H-MoS$$_{2}$$ 2 to metallic 1T-Li$$_{\mathrm{x}}$$ x MoS$$_{2}$$ 2 . With the core hole clock approach at the S L$$_{1}$$ 1 X-ray absorption edge we quantify the ultrafast directional charge transfer of excited S3p electrons in-plane ($$\parallel$$ ‖ ) and out-of-plane ($$\perp$$ ⊥ ) for 2H-MoS$$_{2}$$ 2 as $$\tau _{2H,\parallel } = 0.38 \pm 0.08$$ τ 2 H , ‖ = 0.38 ± 0.08 fs and $$\tau _{2H,\perp } = 0.33 \pm 0.06$$ τ 2 H , ⊥ = 0.33 ± 0.06 fs and for 1T-Li$$_{\mathrm{x}}$$ x MoS$$_{2}$$ 2 as $$\tau _{1T,\parallel } = 0.32 \pm 0.12$$ τ 1 T , ‖ = 0.32 ± 0.12 fs and $$\tau _{1T,\perp } = 0.09 \pm 0.07$$ τ 1 T , ⊥ = 0.09 ± 0.07 fs. The isotropic charge delocalization of S3p electrons in the semiconducting 2H phase within the S-Mo-S sheets is assigned to the specific symmetry of the Mo-S bonding arrangement. Formation of 1T-Li$$_{\mathrm{x}}$$ x MoS$$_{2}$$ 2 by lithiation accelerates the in-plane charge transfer by a factor of $$\sim 1.2$$ ∼ 1.2 due to electron injection to the Mo-S covalent bonds and concomitant structural repositioning of S atoms within the S-Mo-S sheets. For excitation into out-of-plane orbitals, an accelerated charge transfer by a factor of $$\sim 3.7$$ ∼ 3.7 upon lithiation occurs due to S-Li coupling.

Visible light is shown to create a transient metallic S Mo S surface layer on bulk semiconducting p doped indirect bandgap 2H MoS2. Optically created electron hole pairs separate in the surface band bending region of the p doped semiconducting crystal causing a transient accumulation of electrons in the surface region. This triggers a reversible 2H semiconductor to 1T metal phasetransition of the surface layer. Electron phonon coupling of the indirect bandgap p doped 2H MoS2 enables this efficient pathway even at a low density of excited electrons with a distinct optical excitation threshold and saturation behavior. This mechanism needs to be taken into consideration when describing the surface properties of illuminated p doped 2H MoS2. In particular, light induced increased charge mobility and surface activation can cause and enhance the photocatalytic and photoassisted electrochemical hydrogen evolution reaction of water on 2H MoS2. Generally, it opens up for a way to control not only the surface of p doped 2H MoS2 but also related dichalcogenides and layered systems. The findings are based on the sensitivity of time resolved electron spectroscopy for chemical analysis with photon energy tuneable synchrotron radiation

For the layered transition metal dichalcogenide 1T-TaS2, we establish through a unique experimental approach and density functional theory, how ultrafast charge transfer in 1T-TaS2 takes on isotropic three-dimensional character or anisotropic two-dimensional character, depending on the commensurability of the charge density wave phases of 1T-TaS2. The X-ray spectroscopic core-hole-clock method prepares selectively in- and out-of-plane polarized sulfur 3p orbital occupation with respect to the 1T-TaS2 planes and monitors sub-femtosecond wave packet delocalization. Despite being a prototypical two-dimensional material, isotropic three-dimensional charge transfer is found in the commensurate charge density wave phase (CCDW), indicating strong coupling between layers. In contrast, anisotropic two-dimensional charge transfer occurs for the nearly commensurate phase (NCDW). In direct comparison, theory shows that interlayer interaction in the CCDW phase - not layer stacking variations - causes isotropic three-dimensional charge transfer. This is presumably a general mechanism for phase transitions and tailored properties of dichalcogenides with charge density waves., AF acknowledges funding by the Helmholtz Virtual Institute VI 419 and the ERC Advanced Investigator Grant No. 669531 EDAX. NM acknowledges funding by the ERC Advanced Investigator Grant No. 321319 ESUX and support from the Carl Tryggers foundation for Scientifc Research CTS. DS-P and MM acknowledge funding from EU-FP7 project THINFACE (grant No. 607232), Spanish MINECO (grant No. MAT2016-78293-C6-4-R), and Basque Dep.de Educación and UPV/EHU (grant No. IT-756-13).

A comprehensive investigation of the emission characteristics for electrons induced by X-rays of a few hundred eV at grazing-incidence angles on an atomically clean Cu(111) sample during laser excitation is presented. Electron energy spectra due to intense infrared laser irradiation are investigated at the BESSY II slicing facility. Furthermore, the influence of the corresponding high degree of target excitation (high peak current of photoemission) on the properties of Auger and photoelectrons liberated by a probe X-ray beam is investigated in time-resolved pump and probe measurements. Strong electron energy shifts have been found and assigned to space-charge acceleration. The variation of the shift with laser power and electron energy is investigated and discussed on the basis of experimental as well as new theoretical results.

For the layered transition metal dichalcogenide 1T-TaS2, we establish through a unique experimental approach and density functional theory, how ultrafast charge transfer in 1T-TaS2 takes on isotropic three-dimensional character or anisotropic two-dimensional character, depending on the commensurability of the charge density wave phases of 1T-TaS2. The X-ray spectroscopic core-hole-clock method prepares selectively in- and out-of-plane polarized sulfur 3p orbital occupation with respect to the 1T-TaS2 planes and monitors sub-femtosecond wave packet delocalization. Despite being a prototypical two-dimensional material, isotropic three-dimensional charge transfer is found in the commensurate charge density wave phase (CCDW), indicating strong coupling between layers. In contrast, anisotropic two-dimensional charge transfer occurs for the nearly commensurate phase (NCDW). In direct comparison, theory shows that interlayer interaction in the CCDW phase – not layer stacking variations – causes isotropic three-dimensional charge transfer. This is presumably a general mechanism for phase transitions and tailored properties of dichalcogenides with charge density waves.

Catching CO oxidation Details of the transition state that forms as carbon monoxide (CO) adsorbed on a ruthenium surface is oxidized to CO 2 have been revealed by ultrafast excitation and probe methods. Öström et al. initiated the reaction between CO and adsorbed oxygen atoms with laser pulses that rapidly heated the surface and then probed the changes in electronic structure with oxygen x-ray absorption spectroscopy. They observed transition-state configurations that are consistent with density functional theory and a quantum oscillator model. Science , this issue p. 978

The so-called vacuum space-charge acceleration of emitted electrons triggered by absorption of soft X-rays (a few 100 eV) under intense near-infrared laser excitation is studied experimentally and theoretically. The influence of high excitation densities on the properties of Auger and photo electrons liberated by a probe X-ray beam is investigated for grazing-incidence photons interacting with an atomically clean Cu (111) surface as a model system. Transient electron spectra have been taken in a pump–probe setup at the BESSY II storage ring using a newly developed compact electrostatic retarding Bessel-box spectrometer. Strong electron-energy shifts have been found and assigned to space-charge acceleration. Model calculations based on experimental input parameters are in good agreement with experimentally obtained energy-gain values indicating ultimate limits of ultrafast X-ray experiments with photo or Auger electrons as a probe.

The journal of physical chemistry letters 7(18), 3647 - 3651(2016). doi:10.1021/acs.jpclett.6b01543, The concept of bonding and antibonding orbitals is fundamental in chemistry. The population of those orbitals and the energetic difference between the two reflect the strength of the bonding interaction. Weakening the bond is expected to reduce this energetic splitting, but the transient character of bond-activation has so far prohibited direct experimental access. Here we apply time-resolved soft X-ray spectroscopy at a free-electron laser to directly observe the decreased bonding–antibonding splitting following bond-activation using an ultrashort optical laser pulse., Published by ACS, Washington, DC

Synchrotron radiation facilities routinely operate in a multi-bunch regime, but applications relying on time-of-flight schemes require single bunch operation. Here we show that pulse picking by resonant excitation in a storage ring creates in addition to the multi-bunch operation a distinct and separable single bunch soft X-ray source. It has variable polarization, a photon flux of up to 107–109 ph s−1/0.1%BW at purity values of 104–102 and a repetition rate of 1.25 MHz. The quasi-resonant excitation of incoherent betatron oscillations of electrons allows horizontal pulse separation at variable (also circular) polarization accessible for both, regular 30 ps pulses and ultrashort pulses of 2–3 ps duration. Combined with a new generation of angularly resolving electron spectrometers this creates unique opportunities for time-resolved photoemission studies as confirmed by time-of-flight spectra. Our pulse picking scheme is particularly suited for surface physics at diffraction-limited light sources promising ultimate spectral resolution. Although synchrotron facilities routinely operate in a multi-bunch regime for maximum average brilliance, studies relying on time-of-flight schemes require single-bunch operation. Here, Holldack et al.isolate and apply single bunch X-ray pulses from multibunch radiation using pulse picking by resonant excitation.

The simultaneous detection of energy, momentum and temporal information in electron spectroscopy is the key aspect to enhance the detection efficiency in order to broaden the range of scientific applications. Employing a novel 60 degrees wide angle acceptance lens system, based on an additional accelerating electron optical element, leads to a significant enhancement in transmission over the previously employed 30 degrees electron lenses. Due to the performance gain, optimized capabilities for time resolved electron spectroscopy and other high transmission applications with pulsed ionizing radiation have been obtained. The energy resolution and transmission have been determined experimentally utilizing BESSY II as a photon source. Four different and complementary lens modes have been characterized. (C) 2017 The Authors. Published by Elsevier B.V., Postprints der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe, 782