Logo: to the web site of Uppsala University

We introduce a general approach for probing nonlinear x-ray propagation by imaging secondary fluorescence emitted transverse to the driving field. When a short, intense x-ray pulse excites a deep 1s core orbital, subsequent K alpha emission from spin-orbit-split 2p states can undergo stimulated amplification. This nonlinear process reshapes the relative populations of the 2p1/2 and 2p3/2 levels along the propagation path, leaving distinct signatures in the delayed L-edge fluorescence. By solving the coupled density-matrix and Maxwell equations, we show that these fluorescence signals provide a direct and experimentally accessible probe of x-ray amplification dynamics. We demonstrate the concept for argon atoms and extend it to molecular systems containing third-row elements, where competing effects of lifetimes, transition intensities, and nonresonant absorption determine the efficiency of stimulated emission. Our results establish L-edge fluorescence as a broadly applicable diagnostic of nonlinear x-ray phenomena, opening opportunities for studying light-matter interactions in regimes where direct detection of amplified x-ray signals is technically challenging.

We present X-ray emission (XES) and absorption (XAS) spectra of the F- ion in aqueous solution, along with a theoretical analysis using combined ab initio quantum chemistry spectral calculations and quantum molecular dynamics simulations. The spectra are predicted to have a significantly different dependence on the distance between the anion and the oxygen atoms of water molecules in the first solvation shell. The XES spectrum shows a strong reconfiguration of emission line intensities, while the excitation energy of a resonance in the XAS spectrum is highly sensitive to this distance. Comparisons with the recorded XES and XAS spectra yield predicted F-O distances that agree with 2.7 Å. Two weak features in the XES spectrum are identified as being due to interatomic radiative decay (IRD), thus proving a negative ion counterpart to the recently identified IRD process for the Na⁺ and Mg²⁺ cations in aqueous solution, with the difference that the IRD feature appears on opposite sides of the main ion emission in the two cases. Arguments are provided that the combination of X-ray emission and absorption, together with quantum spectral and molecular dynamics calculations, has much to offer for future studies of the structure and electronic properties of anionic solutions.

We report resonant inelastic x-ray scattering (RIXS) spectra of neon atoms interacting with intense x-ray pulses generated using an x-ray free-electron laser (XFEL). We find that an unexpected peak emerges near the K alpha line of Ne, which does not coincide with any physical resonances of neon ions. We perform theoretical calculations based on a quantum-state-resolved rate-equation approach with x-ray-induced processes including possible resonant excitations. Our dynamics simulations demonstrate that a sequence of multiple resonant photoabsorption events are involved and the interplay of those multiple resonances in combination with the relatively large spectral bandwidth of XFEL radiation leads to the emergent resonance-like structure at a position where no resonances exist. Our finding offers critical guidance for future applications of high-intensity RIXS at XFEL facilities.

We describe an experimental setup for non-linear interaction and propagation of ultrashort and intense x-ray free-electron laser (XFEL) pulses in a dense gas. It allows one to provide high, but adjustable, target-gas pressures of up to 6 bar within a vacuum environment of 3 · 10- ³ mbar or better. The setup enables investigation of intense x-ray propagation in an optically thick medium with minimal absorption loss of the unfocused beam outside of the target. As an application, we demonstrate the amplification of spectrally-resolved stimulated x-ray Raman scattering (SXRS) in dense neon gas, where the most intense inner part of the beam is almost completely absorbed. As a result, in the inner part of the beam, the SXRS signal exceeds the residual XFEL pulse by a factor of around two. In principle, this reshaping effect allows for a spatial separation of the two spectral components, i.e., the driving pulse and the SXRS signal.

Interference between scattering channels is observed in resonant inelastic x-ray scattering (RIXS) at the Ne K threshold. Final states with 2p-1np) as the main configuration are populated via 1s-1n'p) resonances, where large-amplitude one-electron (n' = n) channels interfere with small-amplitude two-electron (n' not equal n) channels. The interference is manifested in an asymmetric line profile where the two-electron resonance occurs in the tail region of the one-electron resonance. In RIXS, this slowly varying tail plays the role of the continuum for the Fano effect. The asymmetric profile is well modeled by means of a single asymmetry parameter, determined by the amplitudes of the scattering channels and the energy separation between the resonances.

Dissociation upon sulfur K-shell excitation or ionization of SF6 is studied by sulfur L-shell emission spectroscopy using synchrotron radiation and multiconfiguration Dirac-Hartree-Fock calculations of emission energies and transition rates. The decay path involves in particular Auger emission with the ejection of one or more electrons, leading to singly or multiply charged intermediate states. Nevertheless, the results of the study show that the observed photon emission at 151-152 eV following excitation at 2485-2489 eV originates dominantly from transitions in neutral sulfur. This clearly indicates that the central atom retains its electrons in a dissociation process where all fluorine atoms detach before the S 2p decay.

Aqueous solutions are crucial in chemistry, biology, environmental science, and technology. The chemistry of solutes is influenced by the surrounding solvation shell of water molecules, which have different chemical properties than bulk water due to their different electronic and geometric structure. It is experimentally challenging to selectively investigate this property-determining electronic and geometric structure. Here, we report experimental results on the non-local X-ray emission process Intermolecular Radiative Decay, for the prototypical ions Na+ and Mg2+ in water. We show that, in Intermolecular Radiative Decay, an electron from the solvation shell fills a core hole in the solute, and the released energy is emitted as an X-ray photon. We interpret the underlying mechanism using theoretical calculations, and show how Intermolecular Radiative Decay will allow us to meet the challenge of selectively probing the solvation shell from within.

The dynamics of chemical reactions in solution are of paramount importance in fields ranging from biology to materials science. Because the hydrogen-bond network and proton dynamics govern the behavior of aqueous solutions, they have been the subject of numerous studies over the years. Here, we report the observation of a previously unknown associative state in the hydroxide ion that forms when a proton from a neighboring water molecule approaches the hydroxide ion, utilizing resonant inelastic soft X-ray scattering (RIXS) and quantum dynamical simulations. State-of-the-art theoretical analysis reveals state mixing in the electronically excited states between aqueous hydroxide ions and the solvent. Our results give new insights into chemical bonding and excited-state dynamics in the aqueous environment. This investigation of associative states opens up new pathways for spectroscopic studies of chemical reaction dynamics and lays the foundation for directly accessing dynamic proton exchange in solution.

Propagation of intense X-ray pulses through dense media has led to the observation of phenomena such as atomic X-ray lasing1,2, self-induced transparency3 and stimulated X-ray Raman scattering (SXRS)4. SXRS has been long predicted as a means to launch and probe valence-electron wavepackets and as a building block for nonlinear X-ray spectroscopies5,6. However, experimental observations of SXRS to date4,7,8 have not provided spectroscopic information, and theoretical modelling has largely implemented hard-to-realize phase-coherent attosecond pulses. Here we demonstrate SXRS with spectroscopic precision, that is, detection of valence-excited states in neon with a near Fourier-limited joint energy-time resolution of 0.1 eV-40 fs. We used a new covariance analysis between statistically spiky broadband incident X-ray and scattered X-ray Raman pulses. Using 18,000 single shots, we beat not only the incident (about 8 eV) bandwidth but also the approximately 0.2 eV instrumental energy resolution, thus creating super-resolution conditions, in analogy to super-resolved fluorescence microscopy9. Our experimental results, supported by ab initio propagation simulations, reveal the competition between lasing in the ion and stimulated Raman scattering in the neutral. We demonstrate enhanced signal collection efficiency and a broad excitation window, surpassing spontaneous Raman efficiencies by orders of magnitude. This stochastic SXRS approach represents a first step towards tracking elementary events that determine chemical outcomes10.

A grazing-incidence Rowland spectrometer with a collimating element for resonant inelastic soft X-ray scattering at the Veritas beamline at MAX IV, Lund, Sweden, is presented. The instrument is designed to operate in the 250-950 eV range, providing > 30000 resolving power in first order. The inherent temporal resolution of the delay-line detector enables time-resolved experiments on the 270 ps time-scale and also allows for so-called time-gating which significantly improves the signal-to-noise ratio. The instrument is able to rotate around the sample position in the horizontal plane, which enables measurements of the momentum transfer in the X-ray scattering event. The instrument is prepared for installation of a polarimeter to provide full characterization of energy, momentum and polarization of the scattered photons. The design is evaluated using spectral measurements, ray-tracing, vibration and motion measurements.