| 1. |
- Schönhense, G., et al.
(author)
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Suppression of the vacuum space-charge effect in fs-photoemission by a retarding electrostatic front lens
- 2021
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In: Review of Scientific Instruments. - : American Institute of Physics (AIP). - 0034-6748 .- 1089-7623. ; 92:5
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Journal article (peer-reviewed)abstract
- The performance of time-resolved photoemission experiments at fs-pulsed photon sources is ultimately limited by the e-e Coulomb interaction, downgrading energy and momentum resolution. Here, we present an approach to effectively suppress space-charge artifacts in momentum microscopes and photoemission microscopes. A retarding electrostatic field generated by a special objective lens repels slow electrons, retaining the k-image of the fast photoelectrons. The suppression of space-charge effects scales with the ratio of the photoelectron velocities of fast and slow electrons. Fields in the range from -20 to -1100 V/mm for E-kin = 100 eV to 4 keV direct secondaries and pump-induced slow electrons back to the sample surface. Ray tracing simulations reveal that this happens within the first 40 to 3 mu m above the sample surface for E-kin = 100 eV to 4 keV. An optimized front-lens design allows switching between the conventional accelerating and the new retarding mode. Time-resolved experiments at E-kin = 107 eV using fs extreme ultraviolet probe pulses from the free-electron laser FLASH reveal that the width of the Fermi edge increases by just 30 meV at an incident pump fluence of 22 mJ/cm(2) (retarding field -21 V/mm). For an accelerating field of +2 kV/mm and a pump fluence of only 5 mJ/cm(2), it increases by 0.5 eV (pump wavelength 1030 nm). At the given conditions, the suppression mode permits increasing the slow-electron yield by three to four orders of magnitude. The feasibility of the method at high energies is demonstrated without a pump beam at E-kin = 3830 eV using hard x rays from the storage ring PETRA III. The approach opens up a previously inaccessible regime of pump fluences for photoemission experiments.
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| 2. |
- Müller, L., et al.
(author)
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Note : Soft X-ray transmission polarizer based on ferromagnetic thin films
- 2018
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In: Review of Scientific Instruments. - : American Institute of Physics Inc.. - 0034-6748 .- 1089-7623. ; 89:3
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Journal article (peer-reviewed)abstract
- A transmission polarizer for producing elliptically polarized soft X-ray radiation from linearly polarized light is presented. The setup is intended for use at synchrotron and free-electron laser beamlines that do not directly offer circularly polarized light for, e.g., X-ray magnetic circular dichroism (XMCD) measurements or holographic imaging. Here, we investigate the degree of ellipticity upon transmission of linearly polarized radiation through a cobalt thin film. The experiment was performed at a photon energy resonant to the Co L3-edge, i.e., 778 eV, and the polarization of the transmitted radiation was determined using a polarization analyzer that measures the directional dependence of photo electrons emitted from a gas target. Elliptically polarized radiation can be created at any absorption edge showing the XMCD effect by using the respective magnetic element.
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| 3. |
- Schlueter, C., et al.
(author)
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New HAXPES Applications at PETRA III
- 2018
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In: Synchrotron Radiation News. - : Informa UK Limited. - 0894-0886 .- 1931-7344. ; 31:4, s. 29-35
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Journal article (peer-reviewed)
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| 4. |
- Schlueter, C., et al.
(author)
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The new dedicated HAXPES beamline P22 at PETRAIII
- 2019
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In: 13th International Conference on Synchrotron Radiation Instrumentation (SRI2018). - : American Institute of Physics (AIP). - 9780735417823
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Conference paper (peer-reviewed)abstract
- A new undulator beamline (P22) for hard X-ray photoelectron spectroscopy (HAXPES) was built at PETRA III (DESY, Hamburg) to meet the increasing demand for HAXPES-based techniques. It provides four special instruments for high-resolution studies of the electronic and chemical structure of functional nano-materials and catalytic interfaces, with a focus on measurements under operando and/or ambient conditions: (i) a versatile solid-state spectroscopy setup with optional wide-angle lens and in-situ electrical characterization, (ii) a HAXPEEM instrument for sub-µm spectro-microscopy applications, (iii) an ambient pressure system (> 1 bar) for operando studies of catalytic reactions and (iv) a time-of-flight spectrometer as a full-field k-microscope for measurements of the 4D spectral function ρ(EB,k). The X-ray optics were designed to deliver high brightness photon flux within the HAXPES energy range 2.4 – 15 keV. An LN2-cooled double-crystal monochromator with interchangeable pairs of Si(111) and (311) crystals is optionally combined with a double channel-cut post-monochromator to generate X-rays with variable energy bandpass adapted to the needs of the experiment. Additionally, the beam polarization can be varied using a diamond phase plate integrated into the beamline. Adaptive beam focusing is realized by Be compound refractive lenses and/or horizontally deflecting mirrors down to a spot size of ∼20x17 µm2 with a flux of up to 1.1x1013 ph/s (for Si(111) at 6 keV).
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| 5. |
- Schönhense, G., et al.
(author)
-
Time-of-flight photoelectron momentum microscopy with 80-500 MHz photon sources : Electron-optical pulse picker or bandpass pre-filter
- 2021
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In: Journal of Synchrotron Radiation. - 0909-0495. ; 28, s. 1891-1908
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Journal article (peer-reviewed)abstract
- The small time gaps of synchrotron radiation in conventional multi-bunch mode (100-500 MHz) or laser-based sources with high pulse rate (∼80 MHz) are prohibitive for time-of-flight (ToF) based photoelectron spectroscopy. Detectors with time resolution in the 100 ps range yield only 20-100 resolved time slices within the small time gap. Here we present two techniques of implementing efficient ToF recording at sources with high repetition rate. A fast electron-optical beam blanking unit with GHz bandwidth, integrated in a photoelectron momentum microscope, allows electron-optical 'pulse-picking' with any desired repetition period. Aberration-free momentum distributions have been recorded at reduced pulse periods of 5 MHz (at MAX II) and 1.25 MHz (at BESSY II). The approach is compared with two alternative solutions: A bandpass pre-filter (here a hemispherical analyzer) or a parasitic four-bunch island-orbit pulse train, coexisting with the multi-bunch pattern on the main orbit. Chopping in the time domain or bandpass pre-selection in the energy domain can both enable efficient ToF spectroscopy and photoelectron momentum microscopy at 100-500 MHz synchrotrons, highly repetitive lasers or cavity-enhanced high-harmonic sources. The high photon flux of a UV-laser (80 MHz, <1 meV bandwidth) facilitates momentum microscopy with an energy resolution of 4.2 meV and an analyzed region-of-interest (ROI) down to <800 nm. In this novel approach to 'sub-μm-ARPES' the ROI is defined by a small field aperture in an intermediate Gaussian image, regardless of the size of the photon spot.
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| 6. |
- Shypylenko, A., et al.
(author)
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Effect of ion implantation on the physical and mechanical properties of Ti-Si-N multifunctional coatings for biomedical applications
- 2016
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In: Materials and Design. - : Elsevier BV. - 0264-1275. ; 110, s. 821-829
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Journal article (peer-reviewed)abstract
- In the present work, multifunctional Ti-Si-N coatings have been deposited using CAVD method with the aim of studying their chemical, physical, structural and mechanical properties. Coatings of Ti-Si-N were modified by high-intensity ion implantation using copper ions with dose D = 2 × 1017 ions/cm2 and energy E = 60 keV. The results demonstrated that ion implantation has an effect on the grain size, hardness, and Young modulus of the Ti-Si-N coating. Additionally, the effect of Cu implantation on the bioactive properties of coatings was investigated by contact antimicrobial essay. The results show a high release of Cu ions in the cultivation liquid and the low efficiency of the < 20% Cu doping towards E. coli bacteria. Our results bring understanding to the low dosage ion implantation of multifunctional surfaces towards applications and general drawbacks of ion implantation as bioactive tailoring method.
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| 7. |
- Shypylenko, A., et al.
(author)
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Influence of ion implantation on the physical and mechanical properties of multifunctional coatings based on Ti-Si-N
- 2016
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In: Proceedings of the 6th International Conference Nanomaterials: Applications and Properties, NAP 2016. - 9781509025138
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Conference paper (peer-reviewed)abstract
- Current paper presents the results of investigating of multifunctional Ti-Si-N coatings. Ti-Si-N coatings have been deposited using CAVD method with the aim of studying their chemical, physical, structural and mechanical properties. Coatings of Ti-Si-N were modified by high-intensity ion implantation using copper ions with dose D = 2 × 1017 ions/cm2 and energy E = 60 keV. The results demonstrated that ion implantation has effect on grain size, hardness, and Young modulus of the Ti-Si-N coating.
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| 8. |
- Chernenkaya, A., et al.
(author)
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Microscopic origin of the charge transfer in single crystals based on thiophene derivatives : A combined NEXAFS and density functional theory approach
- 2016
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In: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 145:3
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Journal article (peer-reviewed)abstract
- We have investigated the charge transfer mechanism in single crystals of DTBDT-TCNQ and DTBDT-F4TCNQ (where DTBDT is dithieno[2,3-d;2′,3′-d′] benzo[1,2-b;4,5-b′]dithiophene) using a combination of near-edge X-ray absorption spectroscopy (NEXAFS) and density functional theory calculations (DFT) including final state effects beyond the sudden state approximation. In particular, we find that a description that considers the partial screening of the electron-hole Coulomb correlation on a static level as well as the rearrangement of electronic density shows excellent agreement with experiment and allows to uncover the details of the charge transfer mechanism in DTBDT-TCNQ and DTBDT-F4 TCNQ, as well as a reinterpretation of previous NEXAFS data on pure TCNQ. Finally, we further show that almost the same quality of agreement between theoretical results and experiment is obtained by the much faster Z+1/2 approximation, where the core hole effects are simulated by replacing N or F with atomic number Z with the neighboring atom with atomic number Z+1/2.
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| 9. |
- Kalha, Curran, et al.
(author)
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Hard x-ray photoelectron spectroscopy : a snapshot of the state-of-the-art in 2020
- 2021
-
In: Journal of Physics. - : Institute of Physics Publishing (IOPP). - 0953-8984 .- 1361-648X. ; 33:23
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Research review (peer-reviewed)abstract
- Hard x-ray photoelectron spectroscopy (HAXPES) is establishing itself as an essential technique for the characterisation of materials. The number of specialised photoelectron spectroscopy techniques making use of hard x-rays is steadily increasing and ever more complex experimental designs enable truly transformative insights into the chemical, electronic, magnetic, and structural nature of materials. This paper begins with a short historic perspective of HAXPES and spans from developments in the early days of photoelectron spectroscopy to provide an understanding of the origin and initial development of the technique to state-of-the-art instrumentation and experimental capabilities. The main motivation for and focus of this paper is to provide a picture of the technique in 2020, including a detailed overview of available experimental systems worldwide and insights into a range of specific measurement modi and approaches. We also aim to provide a glimpse into the future of the technique including possible developments and opportunities.
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| 10. |
- Medjanik, K., et al.
(author)
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Near-Edge x‑ray absorption fine structure investigation of the quasi-One-Dimensional organic conductor (TMTSF)2PF6
- 2016
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In: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 120:43, s. 8574-8583
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Journal article (peer-reviewed)abstract
- We present high-resolution near-edge X-ray absorption fine structure (NEXAFS) measurements at the P L2/3 edges, F K edge, C K edge, and Se M2/3 edges of the quasi-one-dimensional (1D) conductor and superconductor (TMTSF)2PF6. NEXAFS allows probing the donor and acceptor moieties separately; spectra were recorded between room temperature (RT) and 30 K at normal incidence. Spectra taken around RT were also studied as a function of the angle (θ) between the electric field of the X-ray beam and the 1D conducting direction. In contrast with a previous study of the S L2/3-edges spectra in (TMTTF)2AsF6, the Se M2/3 edges of (TMTSF)2PF6 do not exhibit a well-resolved spectrum. Surprisingly, the C K-edge spectra contain three well-defined peaks exhibiting strong and nontrivial θ and temperature dependence. The nature of these peaks as well as those of the F K-edge spectra could be rationalized on the basis of first-principles DFT calculations. Despite the structural similarity, the NEXAFS spectra of (TMTSF)2PF6 and (TMTTF)2AsF6 exhibit important differences. In contrast with the case of (TMTTF)2AsF6, the F K-edge spectra of (TMTSF)2PF6 do not change with temperature despite stronger donor−anion interactions. All these features reveal subtle differences in the electronic structure of the TMTSF and TMTTF families of salts.
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