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Physical review letters 130, 108001 (2023). doi:10.1103/PhysRevLett.130.108001, We present a femtosecond time-resolved optical pump-soft x-ray probe photoemission study in which we follow the dynamics of charge transfer at the interface of water and anatase TiO$_2$(101). By combining our observation of transient oxygen O 1s core level peak shifts at submonolayer water coverages with Ehrenfest molecular dynamics simulations we find that ultrafast interfacial hole transfer from TiO$_2$ to molecularly adsorbed water is completed within the 285 fs time resolution of the experiment. This is facilitated by the formation of a new hydrogen bond between an O$_2$c site at the surface and a physisorbed water molecule. The calculations fully corroborate our experimental observations and further suggest that this process is preceded by the efficient trapping of the hole at the surface of TiO$_2$ by hydroxyl species (-OH), that form following the dissociative adsorption of water. At a water coverage exceeding a monolayer, interfacial charge transfer is suppressed. Our findings are directly applicable to a wide range of photocatalytic systems in which water plays a critical role., Published by APS, College Park, Md.

Laser & photonics reviews 16, 2100268 (2022). doi:10.1002/lpor.202100268, The Free-Electron Laser (FEL) FLASH offers the world-wide still unique capability to study ultrafast processes with high-flux, high-rate XUV and soft X-ray pulses. The vast majority of experiments at FLASH are of pump-probe type and thus rely as well on optical ultrafast lasers. Here, a novel FEL facility laser is reported which combines the high average power output of Yb:YAG amplifiers with spectral broadening in a Herriott-type multi-pass cell and subsequent pulse compression to sub-100 fs durations. Compared to the previously employed laser utilizing optical parametric amplification, the new system comes with significantly improved noise figures, compactness, simplicity and power efficiency. It provides FLASH users 10 Hz repetition rate laser bursts with 800 𝜇s long trains of up to 800 pulses being optically synchronized to the FEL bursts with femtosecond precision. In the experimental chamber, pulses with up to 50 𝜇J energy, 60 fs FWHM duration and 1 MHz rate at 1.03 𝜇m wavelength are available and can be adjusted by computer-control. First cross-correlation measurements with the FEL pulses at the plane-grating monochromator photon beamline are demonstrated, proofing the suitability of the new laser for user experiments at FLASH., Published by Wiley VCH, Weinheim

Femtosecond light-induced phase transitions between different macroscopic orders provide the possibility to tune the functional properties of condensed matter on ultrafast timescales. In first-order phase transitions, transient non-equilibrium phases and inherent phase coexistence often preclude a non-ambiguous detection of the transition precursors and their temporal onset.Here, we present a study combining time-resolved photoelectron spectroscopy and ab-initio electron dynamics calculations elucidating the transient subpicosecond processes governing the photoinduced generation of ferromagnetic order in antiferromagnetic {F}e{R}h. The transient photoemission spectra are accounted for by assuming that not only the occupation of electronic states is modified during the photoexcitation process.Instead, the photo-generated non-thermal distribution of electrons modifies the electronic band structure. The ferromagnetic phase of FeRh, characterized by a minority band near the Fermi energy, is established $ 350 \pm 30$ fs after the laser excitation. Ab-initio calculations indicate that the phase transition is initiated by a photoinduced charge transfer from Rh to Fe.

Scientific data 7(1), 442 (2020). doi:10.1038/s41597-020-00769-8, Published by Nature Publ. Group, London

Physical review letters 125(9), 096401: 1-7 (2020). doi:10.1103/PhysRevLett.125.096401, 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 WSe$_2$. 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., Published by APS, College Park, Md.

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 WSe 2 . 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., Physical Review Letters, 125 (9), ISSN:0031-9007, ISSN:1079-7114

Characterization of the electronic band structure of solid state materials is routinely performed using photoemission spectroscopy. Recent advancements in short-wavelength light sources and electron detectors give rise to multidimensional photoemission spectroscopy, allowing parallel measurements of the electron spectral function simultaneously in energy, two momentum components and additional physical parameters with single-event detection capability. Efficient processing of the photoelectron event streams at a rate of up to tens of megabytes per second will enable rapid band mapping for materials characterization. We describe an open-source workflow that allows user interaction with billion-count single-electron events in photoemission band mapping experiments, compatible with beamlines at 3rd and 4rd generation light sources and table-top laser-based setups. The workflow offers an end-to-end recipe from distributed operations on single-event data to structured formats for downstream scientific tasks and storage to materials science database integration. Both the workflow and processed data can be archived for reuse, providing the infrastructure for documenting the provenance and lineage of photoemission data for future high-throughput experiments., Scientific Data, 7 (1), ISSN:2052-4463

Nature Communications 12, 5088 (1-8) (2021). doi:https://doi.org/10.1038/s41467-021-25347-3, Femtosecond light-induced phase transitions between different macroscopic orders provide the possibility to tune the functional properties of condensed matter on ultrafast timescales. In first-order phase transitions, transient non-equilibrium phases and inherent phase coexistence often preclude non-ambiguous detection of transition precursors and their temporal onset.Here, we present a study combining time-resolved photoelectron spectroscopy and ab-initio electron dynamics calculations elucidating the transient subpicosecond processes governing the photoinduced generation of ferromagnetic order in antiferromagnetic {F}e{R}h. The transient photoemission spectra are accounted for by assuming that not only the occupation of electronic states is modified during the photoexcitation process.Instead, the photo-generated non-thermal distribution of electrons modifies the electronic band structure. The ferromagnetic phase of FeRh, characterized by a minority band near the Fermi energy, is established $ 350 \pm 30$ fs after the laser excitation. Ab-initio calculations indicate that the phase transition is initiated by a photoinduced Rh-to-Fe charge transfer., Published by Nature Publishing Group UK, [London]