| 1. |
- Beye, Martin, et al.
(författare)
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Chemical Bond Activation Observed with an X-ray Laser
- 2016
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Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 7:18, s. 3647-3651
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 2. |
- Curcio, Davide, et al.
(författare)
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Ultrafast electronic linewidth broadening in the C 1s core level of graphene
- 2021
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Ingår i: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 104:16
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Tidskriftsartikel (refereegranskat)abstract
- We show that the presence of a transiently excited hot electron gas in graphene leads to a substantial broadening of the C 1s line probed by time-resolved x-ray photoemission spectroscopy. The broadening is found to be caused by an exchange of energy and momentum between the photoemitted core electron and the hot electron gas, rather than by vibrational excitations. This interpretation is supported by a quantitative line-shape analysis that accounts for the presence of the excited electrons. Fitting the spectra to this model directly yields the electronic temperature of the system, in good agreement with electronic temperature values obtained from valence band data. Furthermore, we show how the momentum change of the outgoing core electrons leads to a detectable but very small change in the time-resolved photoelectron diffraction pattern and to a nearly complete elimination of the core level binding energy variation associated with the presence of a narrow σ band in the C 1s state.
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| 3. |
- Dendzik, Maciej, et al.
(författare)
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Observation of an Excitonic Mott Transition Through Ultrafast Core-cum-Conduction Photoemission Spectroscopy
- 2020
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Ingår i: Physical Review Letters. - : American Physical Society (APS). - 0031-9007 .- 1079-7114. ; 125:9
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Tidskriftsartikel (refereegranskat)abstract
- Time-resolved soft-x-ray photoemission spectroscopy is used to simultaneously measure the ultrafast dynamics of core-level spectral functions and excited states upon excitation of excitons in WSe2. We present a many-body approximation for the Green's function, which excellently describes the transient core-hole spectral function. The relative dynamics of excited-state signal and core levels clearly show a delayed core-hole renormalization due to screening by excited quasifree carriers resulting from an excitonic Mott transition. These findings establish time-resolved core-level photoelectron spectroscopy as a sensitive probe of subtle electronic many-body interactions and ultrafast electronic phase transitions.
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| 4. |
- Durr, Hermann A., et al.
(författare)
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A Closer Look Into Magnetism : Opportunities With Synchrotron Radiation
- 2009
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Ingår i: IEEE transactions on magnetics. - 0018-9464 .- 1941-0069. ; 45:1, s. 15-57
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Forskningsöversikt (refereegranskat)abstract
- The unique properties of synchrotron radiation, such as broad energy spectrum, variable light polarization, and flexible time structure, have made it an enormously powerful tool in the study of magnetic phenomena and materials. The refinement of experimental techniques has led to many new research opportunities, keeping up with the challenges put up by modern magnetism research. In this contribution, we review some of the recent developments in the application of synchrotron radiation and particularly soft X-rays to current problems in magnetism, and we discuss future perspectives.
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| 5. |
- Gahl, Cornelius, et al.
(författare)
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A femtosecond X-ray/optical cross-correlator
- 2008
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Ingår i: Nature Photonics. - : Springer Science and Business Media LLC. - 1749-4885 .- 1749-4893. ; 2:3, s. 165-169
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Tidskriftsartikel (refereegranskat)abstract
- For a fundamental understanding of ultrafast dynamics in chemistry, biology and materials science it has been a long-standing dream to record a molecular movie in which both the atomic trajectories and the chemical states of every atom in matter are followed in real time(1). Free- electron lasers provide this perspective as they deliver brilliant femtosecond X-ray pulses spanning a wide photon energy range, which is necessary to gather element- specific and chemical- stateselective information with femtosecond time resolution. The key challenge lies in synchronizing the free- electron lasers with separate optical lasers. We exploit the peak brilliance of the free- electron laser in Hamburg(2,3) ( FLASH) and establish X- raypulseinduced transient changes of the optical reflectivity in GaAs as a powerful tool for X- ray/ optical cross- correlation. This constitutes a breakthrough in the path towards recording a molecular movie and - equally importantly - opens up the field of femtosecond X- ray- induced dynamics, only accessible with high- brilliance X- ray free- electron lasers.
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| 6. |
- Herzog, Gerd, et al.
(författare)
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In-situ GISAXS investigation of polystyrene nanoparticle spray deposition onto a silicon substrate
- 2013
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Ingår i: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 29:36, s. 11260-11266
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Tidskriftsartikel (refereegranskat)abstract
- We investigated the spray deposition and subsequent self-assembly during drying of a polystyrene nanoparticle dispersion with in situ grazing incidence small-angle X-ray scattering at high time resolution. During the fast deposition of the dispersion and the subsequent evaporation of the solvent, different transient stages of nanoparticle assembly can be identified. In the first stage, the solvent starts to evaporate without ordering of the nanoparticles. During the second stage, large-scale structures imposed by the breakup of the liquid film are observable. In this stage, the solvent evaporates further and nanoparticle ordering starts. In the late third drying stage, the nanoparticles self-assemble into the final layer structure.
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| 7. |
- Schnadt, Joachim, 1973-
(författare)
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Studies of Model Nanostructured Materials : Geometric and Electronic Structure, and sub-10 fs Charge Transfer Dynamics
- 2002
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A number of nanostructured systems has been investigated by means of Photoemission and X-ray absorption spectroscopies and by Scanning tunneling microscopy with emphasis on the geometric and electronic structure and the excited-state charge transfer dynamics. These systems comprise aromatic molecules on semiconductor surfaces (titanium dioxide), metal clusters, and an alkali-C60 compound. Electronic and geometric structure are complementary to each other, and changes in the geometric structure are accompanied by changes in the electronic structure. Therefore, a detailed investigation of the latter makes it possible to draw conclusions on the former. In particular, this close relationship has been used to characterise the adsorbate geometry of the pyridine-carboxylic acid monomers on rutile TiO2(110), which is determined by the strong substrate bond as well as interadsorbate interactions. Similarly, it has been found that bi-isonicotinic acid adsorbs on a nanostructured anatase TiO2 film by forming strong bonds between both carboxylic groups and the substrate titanium atoms. For deposited metal clusters, the core binding energies are found to mirror the cluster size.Resonant core electron spectroscopies have been employed to elucidate the excited-state charge transfer dynamics with respect to the transfer of an excited electron from the bi-isonicotinic acid and isonicotinic acid adsorbates to a titanium dioxide semiconductor surface. An important aspect has been the development of new variations of the method in order to be able to unravel the spectra of these relatively complex systems. While a strong excitonic effect localises the excited electron on the adsorbate for the lowest excited state, excitation to the higher excited states leads to an ultrarapid charge transfer on a low- to sub-femtosecond timescale.In LiC60 the character of the alkali-C60 bond has been investigated. The balance between the different energy contributions points to an ionic bond with an important element of covalency.
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