Coupled Electron-Nuclear Dynamics Induced and Monitored with Femtosecond Soft X-ray Pulses in the Amino Acid Glycine.

The coupling of electronic and nuclear motion in polyatomic molecules is at the heart of attochemistry. The molecular properties, transient structures, and reaction mechanism of these many-body quantum objects are defined on the level of electrons and ions by molecular wave functions and their coherent superposition, respectively. In the present contribution, we monitor nonadiabatic quantum wave packet dynamics during molecular charge motion by reconstructing both the oscillatory charge density distribution and the characteristic time-dependent nuclear configuration coordinate from time-resolved Auger electron spectroscopic data recorded in previous studies on glycine molecules [Schwickert et al. Sci. Adv. 2022, 8, eabn6848]. The electronic and nuclear motion on the femtosecond time scale was induced and probed in kinematically complete soft X-ray experiments at the FLASH free-electron laser facility. The detailed analysis of amplitude, instantaneous phase, and instantaneous frequency of the propagating many-body wave packet during its lifecycle provides unprecedented insight into dynamical processes beyond the Born-Oppenheimer approximation. We are confident that the refined experimental data evaluation helps to develop new theoretical tools to describe time-dependent molecular wave functions in complicated but ubiquitous non-Born-Oppenheimer photochemical conditions.

Electron population dynamics in resonant non-linear x-ray absorption in nickel at a free-electron laser.

Free-electron lasers provide bright, ultrashort, and monochromatic x-ray pulses, enabling novel spectroscopic measurements not only with femtosecond temporal resolution: The high fluence of their x-ray pulses can also easily enter the regime of the non-linear x-ray-matter interaction. Entering this regime necessitates a rigorous analysis and reliable prediction of the relevant non-linear processes for future experiment designs. Here, we show non-linear changes in the L3-edge absorption of metallic nickel thin films, measured with fluences up to 60 J/cm2. We present a simple but predictive rate model that quantitatively describes spectral changes based on the evolution of electronic populations within the pulse duration. Despite its simplicity, the model reaches good agreement with experimental results over more than three orders of magnitude in fluence, while providing a straightforward understanding of the interplay of physical processes driving the non-linear changes. Our findings provide important insights for the design and evaluation of future high-fluence free-electron laser experiments and contribute to the understanding of non-linear electron dynamics in x-ray absorption processes in solids at the femtosecond timescale.

Photon-shot-noise-limited transient absorption soft X-ray spectroscopy at the European XFEL.

Femtosecond transient soft X-ray absorption spectroscopy (XAS) is a very promising technique that can be employed at X-ray free-electron lasers (FELs) to investigate out-of-equilibrium dynamics for material and energy research. Here, a dedicated setup for soft X-rays available at the Spectroscopy and Coherent Scattering (SCS) instrument at the European X-ray Free-Electron Laser (European XFEL) is presented. It consists of a beam-splitting off-axis zone plate (BOZ) used in transmission to create three copies of the incoming beam, which are used to measure the transmitted intensity through the excited and unexcited sample, as well as to monitor the incoming intensity. Since these three intensity signals are detected shot by shot and simultaneously, this setup allows normalized shot-by-shot analysis of the transmission. For photon detection, an imaging detector capable of recording up to 800 images at 4.5 MHz frame rate during the FEL burst is employed, and allows a photon-shot-noise-limited sensitivity to be approached. The setup and its capabilities are reviewed as well as the online and offline analysis tools provided to users.

Relaxation dynamics of 3He and 4He clusters and droplets studied using near infrared and visible fluorescence excitation spectroscopy.

The relaxation dynamics of electronically excited 3He and 4He clusters and droplets is investigated using time-correlated near-infrared and visible (NIR/VIS) fluorescence excitation spectroscopy. A rich data set spanning a wide range of cluster and droplet sizes is produced. The spectral features broadly follow the vacuum ultraviolet excitation (VUV) spectra. However, when the NIR/VIS spectra are normalised to the VUV fluorescence, regions with distinctly different cluster size and isotope dependence are identified, enabling deeper insight into the relaxation mechanism. Particle density, location of atomic-like states and their principal quantum number, n, are found to play an important role in the relaxation. For states with n = 3 and higher, only energy within the surface region is transferred to excited atoms which are subsequently ejected from the surface and fluoresce in vacuum. For states with n = 2, energy from the entire region within clusters and droplets is transferred to the surface, leading to the ejection of excited atoms and excimers. Here, the energy is transferred by excitation hopping, which competes with radiative and non-radiative decay, making ejection and NIR/VIS fluorescence inefficient in increasingly larger droplets.

Charge-induced chemical dynamics in glycine probed with time-resolved Auger electron spectroscopy.

In the present contribution, we use x-rays to monitor charge-induced chemical dynamics in the photoionized amino acid glycine with femtosecond time resolution. The outgoing photoelectron leaves behind the cation in a coherent superposition of quantum mechanical eigenstates. Delayed x-ray pulses track the induced coherence through resonant x-ray absorption that induces Auger decay. Temporal modulation of the Auger electron signal correlated with specific ions is observed, which is governed by the initial electronic coherence and subsequent vibronic coupling to nuclear degrees of freedom. In the time-resolved x-ray absorption measurement, we monitor the time-frequency spectra of the resulting many-body quantum wave packets for a period of 175 fs along different reaction coordinates. Our experiment proves that by measuring specific fragments associated with the glycine dication as a function of the pump-probe delay, one can selectively probe electronic coherences at early times associated with a few distinguishable components of the broad electronic wave packet created initially by the pump pulse in the cation. The corresponding coherent superpositions formed by subsets of electronic eigenstates and evolving along parallel dynamical pathways show different phases and time periods in the range of ( - 0.3 ± 0.1 ) π ≤ ϕ ≤ ( 0.1 ± 0.2 ) π and 18.2 - 1.4 + 1.7 ≤ T ≤ 23.9 - 1.1 + 1.2 fs. Furthermore, for long delays, the data allow us to pinpoint the driving vibrational modes of chemical dynamics mediating charge-induced bond cleavage along different reaction coordinates.

Extreme Ultraviolet Wave Packet Interferometry of the Autoionizing HeNe Dimer.

Femtosecond extreme ultraviolet wave packet interferometry (XUV-WPI) was applied to study resonant interatomic Coulombic decay (ICD) in the HeNe dimer. The high demands on phase stability and sensitivity for vibronic XUV-WPI of molecular-beam targets are met using an XUV phase-cycling scheme. The detected quantum interferences exhibit vibronic dephasing and rephasing signatures along with an ultrafast decoherence assigned to the ICD process. A Fourier analysis reveals the molecular absorption spectrum with high resolution. The demonstrated experiment shows a promising route for the real-time analysis of ultrafast ICD processes with both high temporal and high spectral resolution.

Electronic quantum coherence in glycine molecules probed with ultrashort x-ray pulses in real time.

Here, we use x-rays to create and probe quantum coherence in the photoionized amino acid glycine. The outgoing photoelectron leaves behind the cation in a coherent superposition of quantum mechanical eigenstates. Delayed x-ray pulses track the induced coherence through resonant x-ray absorption that induces Auger decay and by photoelectron emission from sequential double photoionization. Sinusoidal temporal modulation of the detected signal at early times (0 to 25 fs) is observed in both measurements. Advanced ab initio many-electron simulations allow us to explain the first 25 fs of the detected coherent quantum evolution in terms of the electronic coherence. In the kinematically complete x-ray absorption measurement, we monitor its dynamics for a period of 175 fs and observe an evolving modulation that may implicate the coupling of electronic to vibronic coherence at longer time scales. Our experiment provides a direct support for the existence of long-lived electronic coherence in photoionized biomolecules.

Nanocellulose-Assisted Thermally Induced Growth of Silver Nanoparticles for Optical Applications.

Optically responsive materials are present in everyday life, from screens to sensors. However, fabricating large-area, fossil-free materials for functional biocompatible applications is still a challenge today. Nanocelluloses from various sources, such as wood, can provide biocompatibility and are emerging candidates for templating organic optoelectronics. Silver (Ag) in its nanoscale form shows excellent optical properties. Herein, we combine both materials using thin-film large-area spray-coating to study the fabrication of optical response applications. We characterize the Ag nanoparticle formation by X-ray scattering and UV-vis spectroscopy in situ during growth on the nanocellulose template. The morphology and optical properties of the nanocellulose film are compared to the rigid reference surface SiO2. Our results clearly show the potential to tailor the energy band gap of the resulting hybrid material.

High-gain harmonic generation with temporally overlapping seed pulses and application to ultrafast spectroscopy.

Collinear double-pulse seeding of the High-Gain Harmonic Generation (HGHG) process in a free-electron laser (FEL) is a promising approach to facilitate various coherent nonlinear spectroscopy schemes in the extreme ultraviolet (XUV) spectral range. However, in collinear arrangements using a single nonlinear medium, temporally overlapping seed pulses may introduce nonlinear mixing signals that compromise the experiment at short time delays. Here, we investigate these effects in detail by extending the analysis described in a recent publication (Wituschek et al., Nat. Commun., 11, 883, 2020). High-order fringe-resolved autocorrelation and wave packet interferometry experiments at photon energies > 23 eV are performed, accompanied by numerical simulations. It turns out that both the autocorrelation and the wave-packet interferometry data are very sensitive to saturation effects and can thus be used to characterize saturation in the HGHG process. Our results further imply that time-resolved spectroscopy experiments are feasible even for time delays smaller than the seed pulse duration.

Tracking attosecond electronic coherences using phase-manipulated extreme ultraviolet pulses.

The recent development of ultrafast extreme ultraviolet (XUV) coherent light sources bears great potential for a better understanding of the structure and dynamics of matter. Promising routes are advanced coherent control and nonlinear spectroscopy schemes in the XUV energy range, yielding unprecedented spatial and temporal resolution. However, their implementation has been hampered by the experimental challenge of generating XUV pulse sequences with precisely controlled timing and phase properties. In particular, direct control and manipulation of the phase of individual pulses within an XUV pulse sequence opens exciting possibilities for coherent control and multidimensional spectroscopy, but has not been accomplished. Here, we overcome these constraints in a highly time-stabilized and phase-modulated XUV-pump, XUV-probe experiment, which directly probes the evolution and dephasing of an inner subshell electronic coherence. This approach, avoiding any XUV optics for direct pulse manipulation, opens up extensive applications of advanced nonlinear optics and spectroscopy at XUV wavelengths.

Generation and characterization of tailored MIR waveforms for steering molecular dynamics.

The dream of physico-chemists to control molecular reactions with light beyond electronic excitations pushes the development of laser pulse shaping capabilities in the mid-infrared (MIR) spectral range. Here, we present a compact optical parametric amplifier platform for the generation and shaping of MIR laser pulses in the wavelength range between 8 µm and 15 µm. Opportunities for judiciously tailoring the electromagnetic waveform are investigated, demonstrating light field control with a spectral resolution of 59 GHz at a total spectral bandwidth of 5 THz. In experiments focusing on spectral amplitude manipulation these parameters result in a time window of 1.8 ps available for shaping the temporal pulse envelope and a phase modulation resolution of 100 mrad for several picosecond delays.

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

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. In these cases, the ability to tailor the phases of electromagnetic waves with high precision is essential. Here 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, that is, 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.

Mapping few-femtosecond slices of ultra-relativistic electron bunches.

Free-electron lasers are unique sources of intense and ultra-short x-ray pulses that led to major scientific breakthroughs across disciplines from matter to materials and life sciences. The essential element of these devices are micrometer-sized electron bunches with high peak currents, low energy spread, and low emittance. Advanced FEL concepts such as seeded amplifiers rely on the capability of analyzing and controlling the electron beam properties with few-femtosecond time resolution. One major challenge is to extract tomographic slice parameters instead of projected electron beam properties. Here, we demonstrate that a radio-frequency deflector in combination with a dipole spectrometer not only allows for single-shot extraction of a seeded FEL pulse profile, but also provides information on the electron slice emittance and energy spread. The seeded FEL power profile can be directly related to the derived slice emittance as a function of intra-bunch coordinate with a resolution down to a few femtoseconds.

Free-electron laser multiplex driven by a superconducting linear accelerator.

Free-electron lasers (FELs) generate femtosecond XUV and X-ray pulses at peak powers in the gigawatt range. The FEL user facility FLASH at DESY (Hamburg, Germany) is driven by a superconducting linear accelerator with up to 8000 pulses per second. Since 2014, two parallel undulator beamlines, FLASH1 and FLASH2, have been in operation. In addition to the main undulator, the FLASH1 beamline is equipped with an undulator section, sFLASH, dedicated to research and development of fully coherent extreme ultraviolet photon pulses using external seed lasers. In this contribution, the first simultaneous lasing of the three FELs at 13.4 nm, 20 nm and 38.8 nm is presented.

High-repetition-rate and high-photon-flux 70 eV high-harmonic source for coincidence ion imaging of gas-phase molecules.

Unraveling and controlling chemical dynamics requires techniques to image structural changes of molecules with femtosecond temporal and picometer spatial resolution. Ultrashort-pulse x-ray free-electron lasers have significantly advanced the field by enabling advanced pump-probe schemes. There is an increasing interest in using table-top photon sources enabled by high-harmonic generation of ultrashort-pulse lasers for such studies. We present a novel high-harmonic source driven by a 100 kHz fiber laser system, which delivers 1011 photons/s in a single 1.3 eV bandwidth harmonic at 68.6 eV. The combination of record-high photon flux and high repetition rate paves the way for time-resolved studies of the dissociation dynamics of inner-shell ionized molecules in a coincidence detection scheme. First coincidence measurements on CH3I are shown and it is outlined how the anticipated advancement of fiber laser technology and improved sample delivery will, in the next step, allow pump-probe studies of ultrafast molecular dynamics with table-top XUV-photon sources. These table-top sources can provide significantly higher repetition rates than the currently operating free-electron lasers and they offer very high temporal resolution due to the intrinsically small timing jitter between pump and probe pulses.

Size and isotope effects of helium clusters and droplets: identification of surface and bulk-volume excitations.

We report a comprehensive investigation of the electronically excited states of helium clusters and droplets of sizes ranging from a few to several 10(7) atoms using time-resolved fluorescence excitation spectroscopy and quantum chemical ab initio calculations. We employ various approaches for our analysis considering the lifetime-dependence of the fluorescence intensity, spectral shifts, intensity scaling with cluster size, isotopic dependence, and density-dependence of the calculated electron wave function radii. A unique feature of helium clusters and droplets is their radially varying particle density. Our results show that short-lived fluorescence is sensitive to regions of increased density and probes excitations located in the bulk volume, whereas long-lived fluorescence is sensitive to regions of reduced density such as for small clusters or for the surface of large droplets. Spectra of (3)He droplets serve as a reference for low density, but are free from contributions of small clusters. This allows us to distinguish regions of reduced density as these can be due to both surface states or small clusters. Our analysis reveals a picture where spectral features are related to regions of different density due to isotopic composition, cluster size, and surface or bulk volume location of the excitations. The 2s and 2p related excitations appear as blue-shifted wings for small clusters or for excited atoms within the surface layer, whereas in the bulk-volume of large droplets, they appear as distinct bands with large intensities, dominating the entire spectrum. Excitations at energies higher than 23 eV are unambiguously assigned to regions of low and medium density location within the deeper parts of the surface layer but show no relation to the bulk volume. Our findings support the idea that in liquid helium high-lying states and, in particular, Rydberg states are quenched in favor of the 2s and 2p excitations.

Ultrafast energy redistribution in C(60) fullerenes: a real time study by two-color femtosecond spectroscopy.

Strong-field excitation and energy redistribution dynamics of C(60) fullerenes are studied by means of time-resolved mass spectrometry in a two-color femtosecond pump-probe setup. Resonant pre-excitation of the electronic system via the first dipole-allowed HOMO-->LUMO+1(t(1g)) (HOMO denotes highest occupied molecular orbital and LUMO denotes lowest unoccupied molecular orbital) transition with ultrashort 25 fs pulses at 399 nm of some 10(12) W cm(-2) results in a highly nonequilibrium distribution of excited electrons and vibrational modes in the neutral species. The subsequent coupling among the electronic and nuclear degrees of freedom is monitored by probing the system with time-delayed 27 fs pulses at 797 nm of some 10(13) W cm(-2). Direct information on the characteristic relaxation time is derived from the analysis of transient singly and multiply charged parent and fragment ion signals as a function of pump-probe delay and laser pulse intensity. The observed relaxation times tau(el) approximately 60-400 fs are attributed to different microcanonical ensembles prepared in the pre-excitation process and correspond to different total energy contents and energy sharing between electronic and vibrational degrees. The characteristic differences and trends allow one to extract a consistent picture for the formation dynamics of ions in different charge states and their fullerenelike fragments and give evidence to collective effects in multiple ionization such as plasmon-enhanced energy deposition.

C60 in intense short pulse laser fields down to 9 fs: excitation on time scales below e-e and e-phonon coupling.

The interaction of C60 fullerenes with 765-797 nm laser pulses as short as 9 fs at intensities of up to 3.7 x 10(14) W cm(-2) is investigated with photoion spectroscopy. The excitation time thus addressed lies well below the characteristic time scales for electron-electron and electron-phonon couplings. Thus, energy deposition into the system is separated from energy redistribution among the various electronic and nuclear degrees of freedom. Insight into fundamental photoinduced processes such as ionization and fragmentation is obtained from the analysis of the resulting mass spectra as a function of pulse duration, laser intensity, and time delay between pump and probe pulses, the latter revealing a memory effect for storing electronic energy in the system with a relaxation time of about 50 fs. Saturation intensities and relative abundances of (multiply charged) parent and fragment ions (C60(q+), q=1-6) are fingerprints for the ionization and fragmentation mechanisms. The observations indicate that for final charge states q>1 the well known C60 giant plasmon resonance is involved in creating ions and a significant amount of large fragments even with 9 fs pulses through a nonadiabatic multielectron dynamics. In contrast, for energetic reasons singly charged ions are generated by an essentially adiabatic single active electron mechanism and negligible fragmentation is found when 9 fs pulses are used. These findings promise to unravel a long standing puzzle in understanding C60 mass spectra generated by intense femtosecond laser pulses.