Session K03: Ultrafast X-ray Science

Hybridized states of extreme ultraviolet light and matter revealed by nonlinear x-ray scattering

Nonlinear frequency conversion processes are broadly explored and applied in the visible spectral region. Extending these effects into the x-ray spectral domain is challenging due to their inherently low cross section [1]. Yet, these processes are accessible at high-brilliance storage-ring based x-ray sources and x-ray free electron lasers. The process of x-ray parametric down conversion (XPDC) of x-ray photons in a pair of x-ray (signal) and visible (idler) photons is predominantly mediated by the valence electron density and provides a valence sensitive probe of electronic structure. Recently, we developed a quantitative theory [1,3] that linked the measured nonlinear scattering signal to an electronic current-density density correlation function. We identified a characteristic signature, that allows for an unequivocal experimental identification of this extremely weak process – the XPDC emission cone [1]. In a recent experiment, we successfully measured the XPDC cone resulting of nonlinear x-ray scattering of photons of 10 keV from diamond, for idler energies in the range of 100 eV. The XPDC scattering cone clearly shows the contribution of two distinct branches, manifested by a positive and negative signal with respect to the Compton scattering background. We interpret this finding as inelastic x-ray scattering from a XUV polaritonic excitation in diamond. A simple polariton model of two coupled electronic and photonic excited states concurrent with energy and momentum conservation of the nonlinear scattering process agrees with the data and corroborates our interpretation. Momentum-resolved nonlinear x-ray scattering thus provides a means to determine the microscopic structure of EUV polaritons on the length scale of the x-ray wavelength.

 

[1] C. Boemer, D. Krebs, A. Benediktovitch, E. Rossi, S. Huotari & N. Rohringer, Faraday Discuss. 228, 451 (2021).

[3] D. Krebs and N. Rohringer, Phys. Rev. X (under review), arXiv:2104.05838   

Imaging ultrafast molecular processes with high-repetition-rate X-ray free electron lasers

The availability of high-repetition-rate XFEL facilities like the European XFEL and the upcoming LCLS-II opens up new exciting possibilities for time-resolved molecular imaging. In particular, techniques relying on multi-coincident charged-particle detection like Coulomb explosion imaging or molecular-frame photoelectron measurements now become feasible at XFELs. Recently, XFEL experiments of this type succeeded in obtaining complete momentum-space images of ring molecules, capturing the positions of all constituents including hydrogen atoms, enabled reconstruction of full three-dimensional geometry of smaller polyatomic molecules like halomethanes, and used photoelectron diffraction and spectroscopy to characterize X-ray-induced molecular dissociation. This talk will review recent progress in ion-ion and ion-electron coincident measurements at XFELs, discuss application of these techniques to time-resolved experiments and the relation to more conventional X-ray photoelectron spectroscopy. Specific examples will include XFEL-driven Coulomb explosion imaging of different C₇H₈ isomers and of strong-field driven dynamics in halomethanes, as well as potential applications to light-induced charge transfer reactions.

    

Timing dissociative dynamics of small molecules and dimers with internal clocks applying coincident particle momentum imaging

One of the central goals of modern ultrafast science is to understand the coupled motion of electrons and nuclei in dissociation processes of molecular and cluster targets. The challenges for a comprehensive experimental investigation are to simultaneously enable select electronic transitions that initiate the dynamics, distinguish and isolate the various reaction channels that occur, trace the dissociation pathways on the potential energy surfaces, measure the emission patterns and energies of the reaction products, and time the fragmentation steps.

In this presentation I will show how this demanding task can be accomplished with reaction microscopy while applying different internal clock mechanisms instead of pump-probe interrogation schemes. In particular, the change in charge state during sequential dissociation processes of small molecules and dimers is monitored in real-time in three examples: Upon photo double ionization with synchrotron radiation (a) the electron transfer via spin-orbit coupling in water molecules and (b) the interatomic decay of transient states in NeKr dimers is followed on the potential energy landscape and timed. Moreover, (c) the sequential emission of electrons during structural changes of oxygen molecules during charge-up after two X-ray absorption at the EuXFEL are traced, and the femtosecond dynamics of the dissociating ion is revealed.   

Propagation-induced x-ray phenomena at ultrahigh intensities

Our understanding of x-ray interactions with single isolated particles in the ultraintense multiphoton regime has matured greatly over the past decade as focused, tunable x-ray pulses with intensities up to 10²⁰ W/cm² at wavelengths of 1 Å have become available at x-ray free-electron lasers (XFELs).  A general framework has been developed for ultraintense x-ray interactions with atoms, molecules, clusters and biomolecules – both under resonant and non-resonant conditions.  Much less studied are propagation-based phenomena at high x-ray intensities, where the target-field interaction reshapes the electromagnetic field in the spectral, temporal and spatial domains.  Following the seminal demonstrations of XFEL-pumped atomic lasers and stimulated Raman scattering there have been few investigations in gas-phase systems where theoretical studies are most tractable.  In this talk I will discuss our theoretical study that uses a coupled time-dependent Schroedinger equation/Maxwell wave equation (TDSE/MWE) approach to study propagation in gases [1] and our recent [2] and planned experimental investigations aimed at observing self-induced transparency, stimulated Raman scattering, and four-wave mixing with x-rays.