The single shot based coherence properties of hard x-ray pulses from the Linac Coherent Light Source (LCLS) were measured by analyzing coherent diffraction patterns from nano-particles and gold nanopowder. The intensity histogram of the small angle x-ray scattering ring from nano-particles reveals the fully transversely coherent nature of the LCLS beam with a number of transverse mode 〈M_s〉 = 1.1. On the other hand, the speckle contrasts measured at a large wavevector yields information about the longitudinal coherence of the LCLS radiation after a silicon (111) monochromator. The quantitative agreement between our data and the simulation confirms a mean coherence time of 2.2 fs and a x-ray pulse duration of 29 fs. Finally the observed reduction of the speckle contrast generated by x-rays with pulse duration longer than 30 fs indicates ultrafast dynamics taking place at an atomic length scale prior to the permanent sample damage.

Ruthenium (Ru)‐based electrocatalysts have shown promise for anion exchange membrane water electrolysis (AEMWE) due to their ability to facilitate water dissociation in the hydrogen evolution reaction (HER). However, their performance is limited by strong hydrogen binding, which hinders hydrogen desorption and water re‐adsorption. This study reports the development of RuNi nanoalloys supported on MoO₂, which optimize the hydrogen binding strength at Ru sites through modulation by adjacent Ni atoms. Theoretical simulations reveal that substituting Ni atoms for adjacent Ru atoms reduces the high hydrogen adsorption Gibbs free energy on Ru while maintaining a low energy barrier for water dissociation. As a result, the RuNi/MoO₂ catalyst shows excellent HER performance with a low overpotential of 51 mV at a current density of 100 mA cm⁻², outperforming commercial Pt/C. Furthermore, RuNi/MoO₂ demonstrates high turnover frequency (7.06 s⁻¹), mass activity (13.4 A mg⁻¹), and price activity (1030.77 A dollar⁻¹). In an AEMWE cell, RuNi/MoO₂ as the cathode catalyst achieves a current density of 1 A cm⁻² at 60 °C with just 1.7 V using 1 m KOH. This work highlights the potential of RuNi/MoO₂ for ultra‐high mass activity in efficient AEMWE applications.

Nonlinear optical phenomena in semiconductors present several fundamental problems in modern optics that are of great importance for the development of optoelectronic devices. In particular, the details of photo-induced lattice dynamics at early time-scales prior to carrier recombination remain poorly understood. We demonstrate the first integrated measurements of both optical and structural, material-dependent quantities while also inferring the bulk impulsive strain profile by using high spatial-resolution time-resolved x-ray scattering (TRXS) on bulk crystalline gallium arsenide. Our findings reveal distinctive laser-fluence dependent crystal lattice responses, which are not described by previous TRXS experiments or models. The initial linear expansion of the crystal upon laser excitation stagnates at a laser fluence corresponding to the saturation of the free carrier density before resuming expansion in a third regime at higher fluences where two-photon absorption becomes dominant. Our interpretations of the lattice dynamics as nonlinear optical effects are confirmed by numerical simulations and by additional measurements in an n-type semiconductor that allows higher-order nonlinear optical processes to be directly observed as modulations of x-ray diffraction lineshapes.

The ability to deliver two coherent X-ray pulses with precise time-delays ranging from a few femtoseconds to nanoseconds enables critical capabilities of probing ultra-fast phenomena in condensed matter systems at X-ray free electron laser (FEL) sources. Recent progress made in the hard X-ray split-and-delay optics developments now brings a very promising prospect for resolving atomic-scale motions that were not accessible by previous time-resolved techniques. Here, we report on characterizing the spatial and temporal coherence properties of the hard X-ray FEL beam after propagating through split-and-delay optics. Speckle contrast analysis of small-angle scattering measurements from nanoparticles reveals well-preserved transverse coherence of the beam. Measuring intensity fluctuations from successive X-ray pulses also reveals that only single or double temporal modes remain in the transmitted beam, corresponding to nearly Fourier transform limited pulses.

Using a strain-rosette, we demonstrate the existence of transverse strain using time-resolved x-ray diffraction from multiple Bragg reflections in laser-excited bulk gallium arsenide. We find that anisotropic strain is responsible for a considerable fraction of the total lattice motion at early times before thermal equilibrium is achieved. Our measurements are described by a new model where the Poisson ratio drives transverse motion, resulting in the creation of shear waves without the need for an indirect process such as mode conversion at an interface. Using the same excitation geometry with the narrow-gap semiconductor indium antimonide, we detected coherent transverse acoustic oscillations at frequencies of several GHz.

We investigate the effect of free carrier dynamics on heat transport in bulk crystalline Silicon following femtosecond optical excitation of varying fluences. By taking advantage of the dense 500 MHz standard fill pattern in the PLS-II storage ring, we perform high angular-resolution X-ray diffraction measurements on nanosecond-to-microsecond time-scales with femtometer spatial sensitivity. We find noticeably slowed lattice recovery at increasingly high excitation intensities. Modeling the temporal evolution of lattice displacements due to the migration of the near surface generated heat into the bulk requires reduced thermal diffusion coefficients. We attribute this pump-fluence dependent thermal transport behavior to two separate effects: first, the enhanced nonradiative recombination of free carriers, and, second, reduced size of the effective heat source in the material. These results demonstrate the capability of time-resolved X-ray scattering as an effective means to explore the connection between charge carrier dynamics and macroscopic transport properties.