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- Zhou Hagström, Nanna, 1993-, et al.
(författare)
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Megahertz-rate Ultrafast X-ray Scattering and Holographic Imaging at the European XFEL
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The advent of X-ray free-electron lasers (XFELs) has revolutionized fundamental science, from atomic to condensed matter physics, from chemistry to biology, giving researchers access to X-rays with unprecedented brightness, coherence, and pulse duration. All XFEL facilities built until recently provided X-ray pulses at a relatively low repetition rate, with limited data statistics. Here, we present the results from the first megahertz repetition rate X-ray scattering experiments at the Spectroscopy and Coherent Scattering (SCS) instrument of the European XFEL. We illustrate the experimental capabilities that the SCS instrument offers, resulting from the operation at MHz repetition rates and the availability of the novel DSSC 2D imaging detector. Time-resolved magnetic X-ray scattering and holographic imaging experiments in solid state samples were chosen as representative examples, providing an ideal test-bed for operation at megahertz rates. Nevertheless, our results are relevant and applicable to any other non-destructive XFEL experiments in the soft X-ray range.
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- Baronio, Cesare M., et al.
(författare)
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The amide I spectrum of parallel β-sheet proteins
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The amide I absorption of the polypeptide backbone has long been used to analyze the secondary structure of proteins. This approach has gained additional attention in the context of amyloid diseases where a particular focus is on the distinction between parallel and antiparallel β-sheets because these structures often discriminate between pre-fibrillar structures and fibrils. Some earlier infrared spectra with typical features of antiparallel β-sheets were interpreted as arising from the parallel β-sheets of fibrils. Therefore, the ability of infrared spectroscopy to distinguish between both types of β-sheets is debated. While it is established that regular, antiparallel β-sheets give rise to a high wavenumber band near 1690 cm-1, it is less clear whether or not this band may also occur for parallel β-sheets. Here we present and analyze the amide I spectra of two β-helix proteins, SV2 and Pent. The overall shape of the proteins is that of a cuboid which has parallel β-sheets on its four sides, which are connected by bends. The main features of their amide I spectrum are a band at 1665, and two bands between 1645 and 1628 cm-1. Both proteins exhibit also a weak component band near 1690 cm-1. Calculations of the amide I spectrum indicate that the absorption at high wavenumbers is not caused by the parallel β-sheets but by the bends between the β-strands. We therefore suggest to modify the interpretation of the amide I spectrum as follows: a high wavenumber band near 1690 cm-1 may be caused by other structures than antiparallel β-sheets. However, when the spectrum consists of only two distinct bands, one near 1690 cm-1 and one near 1630 cm-1, then an assignment to antiparallel β-sheets is consistent with the literature.
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- Degerman, David, et al.
(författare)
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Pressure Jumping as a Tool to Circumvent the Pressure Gap in High-Pressure X-ray Photoelectron Spectroscopy, Demonstrated Using CO and CO2 hydrogenation on Rh(211)
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Operando probing by x-ray photoelectron spectroscopy (XPS) of certain hydrogenation reactions are often limited by the scattering of photoelectrons in the gas phase. This work describes a method designed to partially circumvent this so called pressure gap. By performing a rapid switch from a high pressure (where acquisition is impossible) to a lower pressure we can for a short while probe a “remnant” of the high pressure surface. The time dynamics during the re-equilibration to the new pressure may also be studied. This methodology is demonstrated using the CO2 the CO hydrogenation reaction over Rh(211). In the CO2 hydrogenation reaction, the remnant surface of a 2 bar pressure shows a higher degree of CHx adsorbates and less chemisorbed CO compared to static operando spectra acquired at lower pressures. Hydrogenated CO2 shows as intermediates during the re-equilibration period. In the CO hydrogenation reaction, we observe that CHx accumulated faster during the 275 mbar low pressure regime, indicating that the termination of hydrocarbons is the most affected part of the reaction mechanism by the ~1100 mbar pressure gap.
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- Lömker, Patrick, et al.
(författare)
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Operando XPS and SXRD study of the Fischer-Tropsch reaction on flat and stepped Co at up to around 1 bar
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The Fischer-Tropsch catalytic reaction on Co has still several unknows regarding the surface chemical state of the catalyst and presence of various adsorbed intermediates appearing under operando conditions. Here, we report an operando X-ray photoelectron spectroscopy study of the flat Co(0001) and stepped Co(10-14) model catalyst surfaces during Fischer-Tropsch synthesis conditions at 150 mbar – 975 mbar pressure and 406 K – 548 K temperature in a H2/CO gas mixture using X-ray photoelectron spectroscopy as well surface X-ray diffraction. We observe that the Co surface remains metallic at all conditions with only adsorbed species. There are a number of different intermediates on the surface such as adsorbed CO in only on-top position, C/-CH intermediates, -CH2- species and various longer hydrocarbon molecules. The coverage of the hydrocarbon species decreases with increasing temperature but increase strongly with pressure. The stepped surface shows a significant increase in adsorbed hydrocarbon species which is a clear indication of higher reactivity. There is no observation of any CHO or CH2O species indicating a direct CO dissociation pathway and that to our observation only hydrocarbon species participate in the chain growth. The relative high coverage of CO that is almost constant for the various conditions points to that CO dissociation is one of the rate-limiting steps. The rapid increase in the coverage of hydrocarbon species at high pressure conditions and on the stepped surface further indicates that the final hydrogen induced termination step is also rate-limiting. By following the time dependence of the C 1s region with rapid change in the flow of reactants, we demonstrate that the surface species are highly dynamic resulting in delays in reaching steady-state conditions.
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