Showing 1–20 of 20 results for author: Usenko, S

X-ray Driven Trihydrogen Formation on Silica Nanosurfaces

Authors: Samuel Sahel-Schackis, Adam Summers, Ritika Dagar, Alexandra Feinberg, Martin Grassl, Simon Dold, Rebecca Boll, Yevheniy Ovcharenko, Chris Aikens, Cesar Costa Vera, Alberto De Fanis, Avijit Duley, Felix Gerke, Daniel Jost, Regina Leiner, Michael Meyer, Ilana J. P. Molesky, Razib Obaid, Jeffrey Powell, Nils Rennhack, Björn Senfftleben, Hendrik Tackenberg, Paul Tuemmler, Sergey Usenko, Christian Peltz , et al. (7 additional authors not shown)

Abstract: The trihydrogen cation ($\mathrm{H_3^+}$) initiates the ion-molecule reactions that build molecular complexity in interstellar space. Whether its canonical formation reaction, $\mathrm{H_2^+ + H_2 \rightarrow H_3^+ + H}$, proceeds on inorganic surfaces under radiation-driven ionization has remained untested. Here we drive $\mathrm{H_3^+}$ formation on hydrated silica nanoparticles using intense 1.… ▽ More The trihydrogen cation ($\mathrm{H_3^+}$) initiates the ion-molecule reactions that build molecular complexity in interstellar space. Whether its canonical formation reaction, $\mathrm{H_2^+ + H_2 \rightarrow H_3^+ + H}$, proceeds on inorganic surfaces under radiation-driven ionization has remained untested. Here we drive $\mathrm{H_3^+}$ formation on hydrated silica nanoparticles using intense 1.88 keV X-ray pulses, combining ion velocity map imaging, electron time-of-flight spectroscopy, and single-particle coherent diffractive imaging to resolve this chemistry on individual particles. The self-induced surface electric field on the V/nm scale drives interfacial charge transfer and water fragmentation. This field is the dominant parameter governing the relative yields of $\mathrm{H^+}$, $\mathrm{H_2^+}$, and $\mathrm{H_3^+}$ across particle size, composition, and aggregation. Density functional theory and nonadiabatic quantum molecular dynamics simulations trace this field-driven charge transfer, directly analogous to band bending at semiconductor photoelectrodes. These results establish surface-field-driven charge transfer as a unifying mechanism between radiation dominated astrophysical environments and field-driven surface catalysis. △ Less

Submitted 6 August, 2026; originally announced August 2026.

Photoinduced enhancement of chemical shift sensitivity to local vibrations

Authors: Ana Martínez Gutiérrez, Oliver Alexander, Pablo Estévez Alonso, Lorenzo Paoloni, Terry Mullins, André Al-Haddad, Thomas M. Baumann, Rebecca Boll, Christoph Bostedt, Simon Dold, Alberto De Fanis, Gianluca Geloni, Markus Ilchen, Iyas Ismail, Björn Lautenschlager, Tommaso Mazza, Dooshaye Moonshiram, Solène Oberli, Dawei Peng, Ralph Püttner, Svitozar Serkez, Marc Simon, Florian Trinter, Sergey Usenko, Michael Meyer , et al. (4 additional authors not shown)

Abstract: The advent of novel free-electron laser sources enabling time-resolved x-ray photoelectron spectroscopy (tr-XPS) provides a unique opportunity to monitor local chemical environments in real time by measuring sub-eV shifts in core-electron binding energies. These shifts reflect the interplay between electronic excitation and nuclear motion, an interplay that remains largely unexplored. In our combi… ▽ More The advent of novel free-electron laser sources enabling time-resolved x-ray photoelectron spectroscopy (tr-XPS) provides a unique opportunity to monitor local chemical environments in real time by measuring sub-eV shifts in core-electron binding energies. These shifts reflect the interplay between electronic excitation and nuclear motion, an interplay that remains largely unexplored. In our combined theoretical and experimental study of fluoropyridine (C$_5$H$_4$FN), we investigate this link by monitoring the evolving chemical environment at the N and F atomic sites as the photoexcited $S_1$ state relaxes to the ground state via a conical intersection. We find that the F site responds primarily to vibrational relaxation, showing minimal sensitivity to the electronic excited state. In contrast, excitation to $S_1$ induces a measurable energy shift at the N site and significantly enhances its sensitivity to local vibrations within the ring. This behavior arises from a photoinduced redistribution of charge, which also increases the Coulomb interaction between the 1s electron at the N atom and the atomic partial charge at an adjacent C atom. This insight opens new avenues for exploring ultrafast dynamics and conical intersection pathways in more complex systems, from photostable DNA bases to light-harvesting materials. △ Less

Submitted 16 June, 2026; originally announced June 2026.

Probing the structure of cyclic hydrocarbon molecules with X-ray-induced Coulomb explosion imaging

Authors: Kurtis D. Borne, Rebecca Boll, Thomas M. Baumann, Surjendu Bhattacharyya, Martin Centurion, Keyu Chen, Benjamin Erk, Alberto De Fanis, Ruaridh Forbes, Markus Ilchen, Edwin Kukk, Huynh V. S. Lam, Xiang Li, Lingyu Ma, Tommaso Mazza, Michael Meyer, Terence Mullins, J. Pedro F. Nunes, Asami Odate, Shashank Pathak, Daniel Rivas, Philipp Schmidt, Florian Trinter, Sergey Usenko, Anbu S. Venkatachalam , et al. (5 additional authors not shown)

Abstract: Coulomb explosion imaging (CEI) is a powerful experimental technique that maps a molecule's geometric structure onto the momenta of ionic molecular fragments produced by rapid multiple ionization. Here, we apply CEI induced by pulses from an X-ray free-electron laser in order to image and distinguish complex hydrocarbon isomers with the chemical formula C7H8: toluene, cycloheptatriene, and 1,6-hep… ▽ More Coulomb explosion imaging (CEI) is a powerful experimental technique that maps a molecule's geometric structure onto the momenta of ionic molecular fragments produced by rapid multiple ionization. Here, we apply CEI induced by pulses from an X-ray free-electron laser in order to image and distinguish complex hydrocarbon isomers with the chemical formula C7H8: toluene, cycloheptatriene, and 1,6-heptadiyne. The measured fragment-ion momentum distributions show discernible differences between the three isomers and provide signatures of specific carbon and hydrogen sites in the molecule. In contrast to previous work, we demonstrate that distinct 'marker atoms' are not strictly required for constructing a meaningful molecular frame of reference for the interpretation of the momentum-space data. Our work paves the way for tracking the ultrafast motion of nuclei during isomerization reactions in pure hydrocarbons. △ Less

Submitted 3 February, 2026; originally announced February 2026.

Unraveling real-time chemical shifts in the ultrafast regime

Authors: Daniel E. Rivas, Lorenzo Paoloni, Rebecca Boll, Alberto De Fanis, Ana Martínez Gutiérrez, Tommaso Mazza, Solène Oberli, Oliver Alexander, André Al-Haddad, Thomas M. Baumann, Christoph Bostedt, Simon Dold, Gianluca Geloni, Markus Ilchen, Dooshaye Moonshiram, Daniel Rolles, Artem Rudenko, Philipp Schmidt, Svitozar Serkez, Sergey Usenko, Ángel Martín Pendás, Michael Meyer, Jesús González-Vázquez, Antonio Picón

Abstract: Traditional x-ray photoelectron spectroscopy (XPS) relies upon a direct mapping between the photoelectron binding energies and the local chemical environment, which is well-characterized by an electrostatic partial charges model for systems in equilibrium. However, the extension of this technique to out-of-equilibrium systems has been hampered by the lack of x-ray sources capable of accessing mult… ▽ More Traditional x-ray photoelectron spectroscopy (XPS) relies upon a direct mapping between the photoelectron binding energies and the local chemical environment, which is well-characterized by an electrostatic partial charges model for systems in equilibrium. However, the extension of this technique to out-of-equilibrium systems has been hampered by the lack of x-ray sources capable of accessing multiple atomic sites with high spectral and temporal resolution, as well as the lack of simple theoretical procedures to interpret the observed signals. In this work we employ multi-site XPS with a narrowband femtosecond x-ray probe to unravel different ultrafast dissociation processes of a polyatomic molecule, fluoromethane (CH$_{3}$F). We show that XPS can follow the cleavage of both the C-F and C-H bonds in real time, despite these channels lying close in binding energy. Additionally, we apply the partial charges model to describe these dynamics, and verify this extension with both advanced ab-initio calculations and experimental data. These results enable the application of this technique to out-of-equilibrium systems of higher complexity, by correlating real-time information from multiple atomic sites and interpreting the measurements through a viable theoretical modelling. △ Less

Submitted 15 December, 2025; originally announced December 2025.

Enhanced One-Color-Two-Photon Resonant Ionization in Highly Charged Ions by Fine-Structure Effects

Authors: Moto Togawa, Chunhai Lyu, Chintan Shah, Marc Botz, Joschka Goes, Jonas Danisch, Marleen Maxton, Kai Köbnick, Filipe Grilo, Pedro Amaro, Katharina Kubicek, Mohammed Sekkal, Awad Mohamed, Rebecca Boll, Alberto De Fanis, Simon Dold, Tommaso Mazza, Jacobo Montano, Nils Rennhack, Björn Senfftleben, Sergey Usenko, Zoltan Harman, Christoph H. Keitel, Maurice Leutenegger, Michael Meyer , et al. (3 additional authors not shown)

Abstract: Ultraintense pulses from X-ray free-electron lasers can drive, within femtoseconds, multiple processes in the inner shells of atoms and molecules in all phases of matter. The ensuing complex ionization pathways of outer-shell electrons from the neutral to the final highly charged states make a comparison with theory enormously difficult. We resolve these pathways by preparing highly charged ions i… ▽ More Ultraintense pulses from X-ray free-electron lasers can drive, within femtoseconds, multiple processes in the inner shells of atoms and molecules in all phases of matter. The ensuing complex ionization pathways of outer-shell electrons from the neutral to the final highly charged states make a comparison with theory enormously difficult. We resolve these pathways by preparing highly charged ions in an electron beam ion trap before exposing them to the pulsed radiation. This reveals how relativistic fine-structure effects shift electronic energies, largely compensate the core-screening potential, and enable the consecutive, resonant absorption of two quasi-monochromatic X-ray photons that would generally be unfeasible. This doubly-resonant channel enhances the efficiency of two-photon ionization by more than two orders of magnitude, dominating in this regime the nonlinear interaction of light and matter with possible application for future precision X-ray metrology. △ Less

Submitted 7 November, 2025; originally announced November 2025.

Dichography: Two-frame Ultrafast Imaging from a Single Diffraction Pattern

Authors: Linos Hecht, Andre Al Haddad, Björn Bastian, Thomas M. Baumann, Johan Bielecki, Christoph Bostedt, Subhendu De, Alberto De Fanis, Simon Dold, Thomas Fennel, Fanny Goy, Christina Graf, Robert Hartmann, Georg Jakobs, Maximilian Joschko, Gregor Knopp, Katharina Kolatzki, Sivarama Krishnan, Björn Kruse, Asbjørn Ø. Lægdsmand, Bruno Langbehn, Suddhasattwa Mandal, Tommaso Mazza, Michael Meyer, Christian Peltz , et al. (25 additional authors not shown)

Abstract: We experimentally demonstrate that pairs of time-delayed ultrabright and ultrashort X-ray pulses of two different colors, delivered by modern X-ray Free Electron Lasers, can provide two time-delayed snapshots of a sample. We introduce Dichography, a method that algorithmically separates the diffraction signals overlapping on the detector and independently retrieves the two images of the specimen.… ▽ More We experimentally demonstrate that pairs of time-delayed ultrabright and ultrashort X-ray pulses of two different colors, delivered by modern X-ray Free Electron Lasers, can provide two time-delayed snapshots of a sample. We introduce Dichography, a method that algorithmically separates the diffraction signals overlapping on the detector and independently retrieves the two images of the specimen. We employ Dichography to reconstruct two views of individual xenon-doped helium nanodroplets with 20 nm spatial resolution. The consistency of structures observed in both images at delays up to 750 fs provides evidence that, under these illumination conditions, significant structural damage only occurs at longer timescales. We further validate the method by imaging pairs of silver nanoparticles intercepted by the same light pulse. Dichography enables a new class of experiments across physics, chemistry, and materials science, making a significant step toward the original promise of X-ray free-electron lasers to capture ultrafast movies of nanomatter. △ Less

Submitted 27 February, 2026; v1 submitted 27 August, 2025; originally announced August 2025.

Comments: 21 pages, 7 figures. Supplemental Material in ancillary files. Improved theoretical framework. Improved experimental results

Model-free pattern separation of two-color ultrafast X-ray diffraction

Authors: Linos Hecht, Yevheniy Ovcharenko, Asbjørn Ø. Lægdsmand, Björn Bastian, Thomas M. Baumann, Alessandro Colombo, Subhendu De, Alberto De Fanis, Simon Dold, Thomas Fennel, Robert Hartmann, Katharina Kolatzki, Sivarama Krishnan, Björn Kruse, Aaron C. Laforge, Bruno Langbehn, Suddhasattwa Mandal, Tommaso Mazza, Cristian Medina, Christian Peltz, Thomas Pfeifer, Björn Senfftleben, Keshav Sishodia, Frank Stienkemeier, Rico Mayro P. Tanyag , et al. (6 additional authors not shown)

Abstract: Two-color X-ray imaging with Free Electron Laser pulses offers a powerful approach for probing ultrafast structural dynamics in nanoscale systems, combining (near-)atomic spatial resolution with femtosecond temporal precision. The first X-ray pulse captures the object's initial state, while the second, time-delayed pulse records its subsequent evolution. A key challenge lies in disentangling the t… ▽ More Two-color X-ray imaging with Free Electron Laser pulses offers a powerful approach for probing ultrafast structural dynamics in nanoscale systems, combining (near-)atomic spatial resolution with femtosecond temporal precision. The first X-ray pulse captures the object's initial state, while the second, time-delayed pulse records its subsequent evolution. A key challenge lies in disentangling the two patterns simultaneously recorded by the same detector. We demonstrate the realization of this approach on structurally varying nanoscale particles using two X-ray pulses of different photon energies, 1 and 1.2 keV. Sub-micrometer helium nanodroplets generated in vacuum are irradiated by the two X-ray pulses separated in time by up to 750 femtoseconds. Taking advantage of the high photon-energy resolution of the imaging detector, we separate the overlapping diffraction signals by analyzing individual pixel counts and applying pattern recognition. The helium nanodroplets' spherical shape allows us to cross-validate this approach by fitting the radial scattering profiles with Mie solutions for abichromatic field. The excellent agreement between the two methods, particularly in the sparsely illuminated outer regions of the diffraction patterns where high-resolution structural information is encoded, highlights the quality of this approach and its potential for future advanced X-ray movie techniques. △ Less

Submitted 18 August, 2026; v1 submitted 27 August, 2025; originally announced August 2025.

Unraveling the relaxation dynamics of Uracil: insights from time-resolved X-ray photoelectron spectroscopy

Authors: Davide Faccialà, Matteo Bonanomi, Bruno Nunes Cabral Tenorio, Lorenzo Avaldi, Paola Bolognesi, Carlo Callegari, Marcello Coreno, Sonia Coriani, Piero Decleva, Michele Devetta, Nađa Došlić, Alberto De Fanis, Michele Di Fraia, Fabiano Lever, Tommaso Mazza, Michael Meyer, Terry Mullins, Yevheniy Ovcharenko, Nitish Pal, Maria Novella Piancastelli, Robert Richter, Daniel E. Rivas, Marin Sapunar, Björn Senfftleben, Sergey Usenko , et al. (4 additional authors not shown)

Abstract: We report a study of the electronic and nuclear relaxation dynamics of the photoexcited RNA base uracil in the gas phase, using time-resolved core level photoelectron spectroscopy together with high level calculations. The dynamics was investigated by trajectory surface-hopping calculations, and the core ionization energies were calculated for geometries sampled from these. The molecule was excite… ▽ More We report a study of the electronic and nuclear relaxation dynamics of the photoexcited RNA base uracil in the gas phase, using time-resolved core level photoelectron spectroscopy together with high level calculations. The dynamics was investigated by trajectory surface-hopping calculations, and the core ionization energies were calculated for geometries sampled from these. The molecule was excited by a UV laser and dynamics was probed on the oxygen, nitrogen and carbon site by core electron spectroscopy. Assuming a particular model, we find that the initially excited $S_2(ππ^*)$ state of uracil decays with a time constant of 17 $\pm$ 4 fs to the ground state directly, or to the $S_1(nπ^*)$ state via internal conversion. We find no evidence that the $S_1(nπ^*)$ state decays to the ground state by internal conversion; instead it decays to triplet states with a time constant of 1.6 $\pm$ 0.4 ps. Oscillations of the $S_1(nπ^*)$ state O 1s intensity as a function of time correlate with those of calculated C4=O8 and C5=C6 bond lengths, which undergo a sudden expansion following the initial $π\to π^*$ excitation. We also observe oscillations in the mean energy of the main line (core ionized ionic state), which we tentatively assign to dynamics of the hot ground state. Our calculations support our interpretation of the data, and provide detailed insight into the relaxation processes of uracil. △ Less

Submitted 24 March, 2025; originally announced March 2025.

SPRING: an effective and reliable framework for image reconstruction in single-particle Coherent Diffraction Imaging

Authors: Alessandro Colombo, Mario Sauppe, Andre Al Haddad, Kartik Ayyer, Morsal Babayan, Rebecca Boll, Ritika Dagar, Simon Dold, Thomas Fennel, Linos Hecht, Gregor Knopp, Katharina Kolatzki, Bruno Langbehn, Filipe R. N. C. Maia, Abhishek Mall, Parichita Mazumder, Tommaso Mazza, Yevheniy Ovcharenko, Ihsan Caner Polat, Dirk Raiser, Julian C. Schäfer-Zimmermann, Kirsten Schnorr, Marie Louise Schubert, Arezu Sehati, Jonas A. Sellberg , et al. (18 additional authors not shown)

Abstract: Coherent Diffraction Imaging (CDI) is an experimental technique to gain images of isolated structures by recording the light scattered off the sample. In principle, the sample density can be recovered from the scattered light field through a straightforward Fourier Transform operation. However, only the amplitude of the field is recorded, while the phase is lost during the measurement process and… ▽ More Coherent Diffraction Imaging (CDI) is an experimental technique to gain images of isolated structures by recording the light scattered off the sample. In principle, the sample density can be recovered from the scattered light field through a straightforward Fourier Transform operation. However, only the amplitude of the field is recorded, while the phase is lost during the measurement process and has to be retrieved by means of suitable, well-established phase retrieval algorithms. In this work, we present SPRING, an analysis framework tailored to X-ray Free Electron Laser (XFEL) single-shot single-particle diffraction data that implements the Memetic Phase Retrieval method to mitigate the shortcomings of conventional algorithms. We benchmark the approach on experimental data acquired in two experimental campaigns at SwissFEL and European XFEL. Imaging results on isolated nanostructures reveal unprecedented stability and resilience of the algorithm's behavior on the input parameters, as well as the capability of identifying the solution in conditions hardly treatable so far with conventional methods. A user-friendly implementation of SPRING is released as open-source software, aiming at being a reference tool for the coherent diffraction imaging community at XFEL and synchrotron facilities. △ Less

Submitted 5 March, 2025; v1 submitted 11 September, 2024; originally announced September 2024.

Comments: 30 pages, 13 figures. Authors list updated and text revised

Journal ref: npj Comput Mater 11, 265 (2025)

Attosecond soft X-ray pulses generated by chirp-dispersed manipulation in an XFEL reveal nonlinear core-electron dynamics in neon

Authors: Lars Funke, Markus Ilchen, Svitozar Serkez, Kristina Dingel, Tommaso Mazza, Terence Mullins, Thorsten Otto, Daniel E. Rivas, Sara Savio, Peter Walter, Niclas Wieland, Lasse Wülfing, Sadia Bari, Rebecca Boll, Markus Braune, Francesca Calegari, Alberto De Fanis, Winfried Decking, Andreas Duensing, Stefan Düsterer, Felix Egun, Arno Ehresmann, Benjamin Erk, Danilo Enoque Ferreira de Lima, Andreas Galler , et al. (37 additional authors not shown)

Abstract: Free-electron lasers have demonstrated their capability of generating intense attosecond X-ray pulses, which are the key to studying electron dynamics at their natural time scale and in specifically targeted electronic states, but come at the expanse of complicated generation schemes and stochastic pulse shapes. Here, we demonstrate a novel and simple operation concept based on the manipulation of… ▽ More Free-electron lasers have demonstrated their capability of generating intense attosecond X-ray pulses, which are the key to studying electron dynamics at their natural time scale and in specifically targeted electronic states, but come at the expanse of complicated generation schemes and stochastic pulse shapes. Here, we demonstrate a novel and simple operation concept based on the manipulation of the electron-bunch chirp-dispersion and working with the full 4.5 MHz repetition rate at the European XFEL in Germany. With a high-fidelity single-shot temporal characterisation, we detect X-ray pulses with durations of down to 200 attoseconds and peak powers reaching into the terawatt regime at ~1 keV photon energy. As a direct application, we present simultaneous measurements of nonlinear X-ray-matter interaction via time-resolved electron spectroscopy. Using the derived temporal pulse information and restricting the durations to a regime where individual X-ray pulses are shorter than the single-core-hole life time in neon atoms, we reveal an otherwise hidden peak-intensity dependence in the nonlinear dynamics of double-core-hole formation. Our results open the field of attosecond science to the investigation of electronic processes not only in the ground state but also in systems driven far off their equilibrium. They shed light on highly transient intermediate steps in complex electronic dynamics and thus promise to help build the conceptual bridge between fundamental physical processes and chemical photo-reactions. △ Less

Submitted 26 May, 2026; v1 submitted 7 August, 2024; originally announced August 2024.

Transmission spectroscopy of CF$_4$ molecules in intense x-ray fields

Authors: Rui Jin, Adam Fouda, Alexander Magunia, Yeonsig Nam, Marc Rebholz, Alberto De Fanis, Kai Li, Gilles Doumy, Thomas M. Baumann, Michael Straub, Sergey Usenko, Yevheniy Ovcharenko, Tommaso Mazza, Jacobo Montaño, Marcus Agåker, Maria Novella Piancastelli, Marc Simon, Jan-Erik Rubensson, Michael Meyer, Linda Young, Christian Ott, Thomas Pfeifer

Abstract: The nonlinear interaction of x-rays with matter is at the heart of understanding and controlling ultrafast molecular dynamics from an atom-specific viewpoint, providing new scientific and analytical opportunities to explore the structure and dynamics of small quantum systems. At increasingly high x-ray intensity, the sensitivity of ultrashort x-ray pulses to specific electronic states and emerging… ▽ More The nonlinear interaction of x-rays with matter is at the heart of understanding and controlling ultrafast molecular dynamics from an atom-specific viewpoint, providing new scientific and analytical opportunities to explore the structure and dynamics of small quantum systems. At increasingly high x-ray intensity, the sensitivity of ultrashort x-ray pulses to specific electronic states and emerging short-lived transient intermediates is of particular relevance for our understanding of fundamental multi-photon absorption processes. In this work, intense x-ray free-electron laser (XFEL) pulses at the European XFEL (EuXFEL) are combined with a gas cell and grating spectrometer for a high-intensity transmission spectroscopy study of multiphoton-induced ultrafast molecular fragmentation dynamics in CF$_4$. This approach unlocks the direct intra-pulse observation of transient fragments, including neutral atoms, by their characteristic absorption lines in the transmitted broad-band x-ray spectrum. The dynamics with and without initially producing fluorine K-shell holes are studied by tuning the central photon energy. The absorption spectra are measured at different FEL intensities to observe nonlinear effects. Transient isolated fluorine atoms and ions are spectroscopically recorded within the ultrashort pulse duration of few tens of femtoseconds. An isosbestic point that signifies the correlated transition between intact neutral CF$_4$ molecules and charged atomic fragments is observed near the fluorine K-edge. The dissociation dynamics and the multiphoton absorption-induced dynamics encoded in the spectra are theoretically interpreted. Overall, this study demonstrates the potential of high-intensity x-ray transmission spectroscopy to study ultrafast molecular dynamics with sensitivity to specific intermediate species and their electronic structure. △ Less

Submitted 5 July, 2024; originally announced July 2024.

Comments: 30 pages, with 7 figures, submitted to Phys. Rev. X

X-ray Coulomb explosion imaging reveals role of molecular structure in internal conversion

Authors: Till Jahnke, Sebastian Mai, Surjendu Bhattacharyya, Keyu Chen, Rebecca Boll, Maria Elena Castellani, Simon Dold, Avijit Duley, Ulrike Frühling, Alice E. Green, Markus Ilchen, Rebecca Ingle, Gregor Kastirke, Huynh Van Sa Lam, Fabiano Lever, Dennis Mayer, Tommaso Mazza, Terence Mullins, Yevheniy Ovcharenko, Björn Senfftleben, Florian Trinter, Atia Tul Noor, Sergey Usenko, Anbu Selvam Venkatachalam, Artem Rudenko , et al. (4 additional authors not shown)

Abstract: Molecular photoabsorption results in an electronic excitation/ionization which couples to the rearrangement of the nuclei. The resulting intertwined change of nuclear and electronic degrees of freedom determines the conversion of photoenergy into other molecular energy forms. Nucleobases are excellent candidates for studying such dynamics, and great effort has been taken in the past to observe the… ▽ More Molecular photoabsorption results in an electronic excitation/ionization which couples to the rearrangement of the nuclei. The resulting intertwined change of nuclear and electronic degrees of freedom determines the conversion of photoenergy into other molecular energy forms. Nucleobases are excellent candidates for studying such dynamics, and great effort has been taken in the past to observe the electronic changes induced by the initial excitation in a time-resolved manner using ultrafast electron spectroscopy. The linked geometrical changes during nucleobase photorelaxation have so far not been observed directly in time-resolved experiments. Here, we present a study on a thionucleobase, where we extract comprehensive information on the molecular rearrangement using Coulomb explosion imaging. Our measurement links the extracted deplanarization of the molecular geometry to the previously studied temporal evolution of the electronic properties of the system. In particular, the protons of the exploded molecule are well-suited messengers carrying rich information on the molecule's geometry at distinct times after the initial electronic excitation. The combination of ultrashort laser pulses to trigger molecular dynamics, intense X-ray free-electron laser pulses for the explosion of the molecule, and multi-particle coincidence detection opens new avenues for time-resolved studies of complex molecules in the gas phase. △ Less

Submitted 24 May, 2024; originally announced May 2024.

Comments: 19 pages, 8 figures

Controlled molecule injector for cold, dense, and pure molecular beams at the European x-ray free-electron laser

Authors: Lanhai He, Melby Johny, Thomas Kierspel, Karol Długołęcki, Sadia Bari, Rebecca Boll, Hubertus Bromberger, Marcello Coreno, Alberto De Fanis, Michele Di Fraia, Benjamin Erk, Mathieu Gisselbrecht, Patrik Grychtol, Per Eng-Johnsson, Tommaso Mazza, Jolijn Onvlee, Yevheniy Ovcharenko, Jovana Petrovic, Nils Rennhack, Daniel E. Rivas, Artem Rudenko, Eckart Rühl, Lucas Schwob, Marc Simon, Florian Trinter , et al. (5 additional authors not shown)

Abstract: A permanently available molecular-beam injection setup for controlled molecules (COMO) was installed and commissioned at the small quantum systems (SQS) instrument at the European x-ray free-electron laser (EuXFEL). A $b$-type electrostatic deflector allows for pure state-, size-, and isomer-selected samples of polar molecules and clusters. The source provides a rotationally cold ($T\approx1$~K) a… ▽ More A permanently available molecular-beam injection setup for controlled molecules (COMO) was installed and commissioned at the small quantum systems (SQS) instrument at the European x-ray free-electron laser (EuXFEL). A $b$-type electrostatic deflector allows for pure state-, size-, and isomer-selected samples of polar molecules and clusters. The source provides a rotationally cold ($T\approx1$~K) and dense ($ρ\approx10^8$~cm$^{-3}$) molecular beam with pulse durations up to 100~\us generated by a new version of the Even-Lavie valve. Here, a performance overview of the COMO setup is presented along with characterization experiments performed both, with an optical laser at the Center for Free-Electron-Laser Science and with x-rays at EuXFEL under burst-mode operation. COMO was designed to be attached to different instruments at the EuXFEL, in particular at the small quantum systems (SQS) and single particles, clusters, and biomolecules (SPB) instruments. This advanced controlled-molecules injection setup enables XFEL studies using highly defined samples with soft and hard x-ray FEL radiation for applications ranging from atomic, molecular, and cluster physics to elementary processes in chemistry and biology. △ Less

Submitted 10 May, 2024; originally announced May 2024.

Multiple-core-hole resonance spectroscopy with ultraintense X-ray pulses

Authors: Aljoscha Rörig, Sang-Kil Son, Tommaso Mazza, Philipp Schmidt, Thomas M. Baumann, Benjamin Erk, Markus Ilchen, Joakim Laksman, Valerija Music, Shashank Pathak, Daniel E. Rivas, Daniel Rolles, Svitozar Serkez, Sergey Usenko, Robin Santra, Michael Meyer, Rebecca Boll

Abstract: Understanding the interaction of intense, femtosecond X-ray pulses with heavy atoms is crucial for gaining insights into the structure and dynamics of matter. One key aspect of nonlinear light-matter interaction was, so far, not studied systematically at free-electron lasers -- its dependence on the photon energy. Using resonant ion spectroscopy, we map out the transient electronic structures occu… ▽ More Understanding the interaction of intense, femtosecond X-ray pulses with heavy atoms is crucial for gaining insights into the structure and dynamics of matter. One key aspect of nonlinear light-matter interaction was, so far, not studied systematically at free-electron lasers -- its dependence on the photon energy. Using resonant ion spectroscopy, we map out the transient electronic structures occurring during the complex charge-up pathways. Massively hollow atoms featuring up to six simultaneous core holes determine the spectra at specific photon energies and charge states. We also illustrate how the influence of different X-ray pulse parameters that are usually intertwined can be partially disentangled. The extraction of resonance spectra is facilitated by the fact that the ion yields become independent of the peak fluence beyond a saturation point. Our study lays the groundwork for novel spectroscopies of transient atomic species in exotic, multiple-core-hole states that have not been explored previously. △ Less

Submitted 14 March, 2023; originally announced March 2023.

Comments: Supplementary information is included

Resonance-enhanced multiphoton ionization in the x-ray regime

Authors: Aaron C. LaForge, Sang-Kil Son, Debadarshini Mishra, Markus Ilchen, Stephen Duncanson, Eemeli Eronen, Edwin Kukk, Stanislaw Wirok-Stoletow, Daria Kolbasova, Peter Walter, Rebecca Boll, Alberto De Fanis, Michael Meyer, Yevheniy Ovcharenko, Daniel E. Rivas, Philipp Schmidt, Sergey Usenko, Robin Santra, Nora Berrah

Abstract: Here, we report on the nonlinear ionization of argon atoms in the short wavelength regime using ultraintense x rays from the European XFEL. After sequential multiphoton ionization, high charge states are obtained. For photon energies that are insufficient to directly ionize a $1s$ electron, a different mechanism is required to obtain ionization to Ar$^{17+}$. We propose this occurs through a two-c… ▽ More Here, we report on the nonlinear ionization of argon atoms in the short wavelength regime using ultraintense x rays from the European XFEL. After sequential multiphoton ionization, high charge states are obtained. For photon energies that are insufficient to directly ionize a $1s$ electron, a different mechanism is required to obtain ionization to Ar$^{17+}$. We propose this occurs through a two-color process where the second harmonic of the FEL pulse resonantly excites the system via a $1s \rightarrow 2p$ transition followed by ionization by the fundamental FEL pulse, which is a type of x-ray resonance-enhanced multiphoton ionization (REMPI). This resonant phenomenon occurs not only for Ar$^{16+}$, but through multiple lower charge states, where multiple ionization competes with decay lifetimes, making x-ray REMPI distinctive from conventional REMPI. With the aid of state-of-the-art theoretical calculations, we explain the effects of x-ray REMPI on the relevant ion yields and spectral profile. △ Less

Submitted 5 November, 2021; v1 submitted 15 October, 2021; originally announced October 2021.

Comments: 8 pages, 5 figures, one supplementary material (not included)

Electronic Quantum Coherence in Glycine Molecules Probed with Ultrashort X-ray Pulses in Real Time

Authors: David Schwickert, Marco Ruberti, Přemys Kolorenč, Sergey Usenko, Andreas Przystawik, Karolin Baev, Ivan Baev, Markus Braune, Lars Bocklage, Marie Kristin Czwalinna, Sascha Deinert, Stefan Düsterer, Andreas Hans, Gregor Hartmann, Christian Haunhorst, Marion Kuhlmann, Steffen Palutke, Ralf Röhlsberger, Juliane Rönsch-Schulenburg, Philipp Schmidt, Sven Toleikis, Jens Viefhaus, Michael Martins, André Knie, Detlef Kip , et al. (3 additional authors not shown)

Abstract: Quantum coherence between electronic states of a photoionized molecule and the resulting process of ultrafast electron-hole migration have been put forward as a possible quantum mechanism of charge-directed reactivity governing the photoionization-induced molecular decomposition. Attosecond experiments based on the indirect (fragment ion-based) characterization of the proposed electronic phenomena… ▽ More Quantum coherence between electronic states of a photoionized molecule and the resulting process of ultrafast electron-hole migration have been put forward as a possible quantum mechanism of charge-directed reactivity governing the photoionization-induced molecular decomposition. Attosecond experiments based on the indirect (fragment ion-based) characterization of the proposed electronic phenomena suggest that the photoionization-induced electronic coherence can survive for tens of femtoseconds, while some theoretical studies predict much faster decay of the coherence due to the quantum uncertainty in the nuclear positions and the nuclear-motion effects. The open questions are: do long-lived electronic quantum coherences exist in complex molecules and can they be probed directly, i.e. via electronic observables? Here, we use x-rays both to create and to directly probe quantum coherence in the photoionized amino acid glycine. The outgoing photoelectron wave leaves behind a positively charged ion that is in a coherent superposition of quantum mechanical eigenstates lying within the ionizing pulse spectral bandwidth. Delayed x-ray pulses track the induced coherence through resonant x-ray absorption that induces Auger decay and by the photoelectron emission from sequential double photoionization. Sinusoidal temporal modulation of the detected signal at early times (0 - 25 fs) is observed in both measurements. Advanced ab initio many-electron simulations, taking into account the quantum uncertainty in the nuclear positions, allow us to explain the first 25 fs of the detected coherent quantum evolution in terms of the electronic coherence. △ Less

Submitted 12 April, 2022; v1 submitted 8 December, 2020; originally announced December 2020.

Comments: 61 pages, 12 figures

Journal ref: Sci. Adv. 8, eabn6848 (2022)

Attosecond interferometry with self-amplified spontaneous emission of a free-electron laser

Authors: Sergey Usenko, Andreas Przystawik, Markus Jakob, Leslie Lamberto Lazzarino, Günter Brenner, Sven Toleikis, Christian Haunhorst, Detlef Kip, Tim Laarmann

Abstract: Light-phase-sensitive techniques, such as coherent multidimensional spectroscopy, are well-established in a broad spectral range, already spanning from radio-frequencies in nuclear magnetic resonance spectroscopy to visible and ultraviolet wavelengths in nonlinear optics with table-top lasers. Here, the ability to tailor the phases of electromagnetic waves with high precision is essential. In the… ▽ More Light-phase-sensitive techniques, such as coherent multidimensional spectroscopy, are well-established in a broad spectral range, already spanning from radio-frequencies in nuclear magnetic resonance spectroscopy to visible and ultraviolet wavelengths in nonlinear optics with table-top lasers. Here, the ability to tailor the phases of electromagnetic waves with high precision is essential. In the present contribution we achieve phase control of extreme-ultraviolet pulses from a free-electron laser (FEL) on the attosecond timescale in a Michelson-type all-reflective interferometric autocorrelator. By varying the relative phase of the generated pulse replicas with sub-cycle precision we observe the field interference, i.e. the light-wave oscillation with a period of 129 as. The successful transfer of a powerful optical method towards short-wavelength FEL science and technology paves the way towards utilization of advanced nonlinear methodologies even at partially coherent soft X-ray FEL sources that rely on self-amplified spontaneous emission. △ Less

Submitted 4 May, 2017; v1 submitted 14 February, 2017; originally announced February 2017.

Comments: 24 pages, including 4 pages supplementary material, in total 6 figures

Journal ref: Nat. Commun. 8, 15626 (2017)

Femtosecond dynamics of correlated many-body states in C$_{60}$ fullerenes

Authors: Sergey Usenko, Michael Schüler, Armin Azima, Markus Jakob, Leslie L. Lazzarino, Yaroslav Pavlyukh, Andreas Przystawik, Markus Drescher, Tim Laarmann, Jamal Berakdar

Abstract: Fullerene complexes may play a key role in the design of future molecular electronics and nanostructured devices with potential applications in light harvesting using organic solar cells. Charge and energy flow in these systems is mediated by many-body effects. We studied the structure and dynamics of laser-induced multi-electron excitations in isolated C$_{60}$ by two-photon photoionization as a… ▽ More Fullerene complexes may play a key role in the design of future molecular electronics and nanostructured devices with potential applications in light harvesting using organic solar cells. Charge and energy flow in these systems is mediated by many-body effects. We studied the structure and dynamics of laser-induced multi-electron excitations in isolated C$_{60}$ by two-photon photoionization as a function of excitation wavelength using a tunable fs UV laser and developed a corresponding theoretical framework on the basis of \emph{ab initio} calculations. The measured resonance line width gives direct information on the excited state lifetime. From the spectral deconvolution we derive a lower limit for purely electronic relaxation on the order of $τ_\mathrm{el}=10^{+5}_{-3}$ fs. Energy dissipation towards nuclear degrees of freedom is studied in time-resolved techniques. The evaluation of the non-linear autocorrelation trace gives a characteristic time constant of $τ_\mathrm{vib}=400\pm100$ fs for the exponential decay. In line with the experiment, the observed transient dynamics is explained theoretically by nonadiabatic (vibronic) couplings involving the correlated electronic, the nuclear degrees of freedom (accounting for the Herzberg-Teller coupling), and their interplay. △ Less

Submitted 5 October, 2016; v1 submitted 25 May, 2016; originally announced May 2016.

Comments: 20 pages, 7 figures

Journal ref: New J. Phys. 18, 113055 (2016)

Retarded Many-Sphere Hydrodynamic Interactions in a Viscous Fluid

Authors: P. P. J. M. Schram, A. S. Usenko, I. P. Yakimenko

Abstract: An alternative method is suggested for the description of the velocity and pressure fields in an unbounded incompressible viscous fluid induced by an arbitrary number of spheres moving and rotating in it. Within the framework of this approach, we obtain the general relations for forces and torques exerted by the fluid on the spheres. The behavior of the translational, rotational, and coupled fri… ▽ More An alternative method is suggested for the description of the velocity and pressure fields in an unbounded incompressible viscous fluid induced by an arbitrary number of spheres moving and rotating in it. Within the framework of this approach, we obtain the general relations for forces and torques exerted by the fluid on the spheres. The behavior of the translational, rotational, and coupled friction and mobility tensors in various frequency domains are analyzed up to the terms of the third order in the dimensionless parameter equal to the ratio of a typical radius of a sphere to the penetration depth of transverse waves and a certain power of the dimensionless parameter equal to the ratio of a typical radius of a sphere to the distance between the centers of two spheres. We establish that the retardation effects can essentially affect the character of the hydrodynamic interactions between the spheres. △ Less

Submitted 20 August, 2004; originally announced August 2004.

Comments: 60 pages, RevTeX

Velocity and Pressure Fields Induced by Spheres in an Unbounded Fluid

Authors: A. S. Usenko

Abstract: We propose a procedure for the determination of the time-dependent velocity and pressure fields of an unbounded incompressible viscous fluid in an external force field induced by an arbitrary number of spheres moving and rotating in it as well as the forces and torques exerted by the fluid on the particles. Within the completely linearized scheme, we express the velocity and pressure fields of t… ▽ More We propose a procedure for the determination of the time-dependent velocity and pressure fields of an unbounded incompressible viscous fluid in an external force field induced by an arbitrary number of spheres moving and rotating in it as well as the forces and torques exerted by the fluid on the particles. Within the completely linearized scheme, we express the velocity and pressure fields of the fluid in terms of induced surface force densities and derive the explicit form for all quantities contained in these relations not imposing any additional restrictions on the size of particles, distances between them, and the frequency range. We show the incorrectness of similar results obtained earlier by several authors because these results are expressed in terms of nonexistent inverse tensors. We explain the reasons leading to this and propose a procedure for the elimination of divergent quantities. In the stationary case, using the proposed procedure, we obtained the translational, rotational, and coupled friction and mobility tensors for a system containing an arbitrary number of spheres up to, respectively, the second, forth, and third orders in the dimensionless parameter equal to the ratio of a typical radius of a sphere to a typical distance between two spheres. In various particular cases, the results obtained in the present paper agree with the well-known results derived by other methods. △ Less

Submitted 17 April, 2002; originally announced April 2002.

Comments: 71 pages, revtex