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Characterizing femtosecond X-ray pulses that vary from shot to shot is important for data interpretation. Here, Behrens et al.measure time-resolved lasing effects on the electron beam and extract the temporal profile of X-ray pulses using an X-band radiofrequency transverse deflector.

Two-colour X-ray pulses from free-electron lasers can be used to probe ultrafast dynamics, but the total power is a fraction of the saturation power. Here, Marinelli et al. use twin electron bunches to reach full saturation power and increase the two-colour intensity by an order of magnitude at hard-X-ray energies.

Two-color X-ray pulses with controlled time delay allow exciting one site of a molecule and then probing a different site of the same molecule with femtosecond resolution. Here, the authors use this hetero-site pump-probe technique to study charge redistribution and dissociation of the xenon difluoride molecule.

Lasing in a hard-X-ray free-electron laser is typically seeded from noise due to the self-amplification of spontaneous emission, which limits temporal coherence and spectral characteristics. Researchers now demonstrate self-seeding using X-rays from the first half of the magnetic undulator to seed the second half via a diamond-based monochromator at ångström wavelengths.

A new scheme for multicolour X-ray free-electron lasers at soft X-ray wavelengths is proposed. The scheme significantly improves two-colour pulse generation and makes possible the first demonstration of three-colour pulse generation.

Tunable polarization control and a two-colour X-ray pump–X-ray probe operating mode are demonstrated at the Linac Coherent Light Source (LCLS).

Researchers demonstrate the FERMI free-electron laser operating in the high-gain harmonic generation regime, allowing high stability, transverse and longitudinal coherence and polarization control.

Strong lasing effects similar to those in the optical regime can occur at 1.5–2.1 Å wavelengths during high-intensity XFEL-driven Kα₁ lasing of copper and manganese.

Diffraction-before-destruction of ultrashort X-ray pulses can visualize non-equilibrium processes at the nanoscale with sub-femtosecond precision. Here, the authors demonstrate how the brightness and the spatial resolution of such snapshots can be substantially increased despite ionization.

The local X-ray-induced dynamics that occur in protein crystals during serial femtosecond crystallography (SFX) measurements at XFELs are not well understood. Here the authors performed a time-resolved X-ray pump X-ray probe SFX experiment, and they observe distinct structural changes in the disulfide bridges and peptide backbone of proteins; complementing theoretical approaches allow them to further characterize the details of the X-ray induced ionization and local structural dynamics.

Methods to characterize the free-electron laser pulses are evolving and their performances are also improving. Here the authors demonstrate a method based on the artificial neural networks to predict the output pulses of the X-ray free-electron laser by considering the electron beam parameters as input.

X-ray fee-electron lasers (XFELs) enable time-resolved crystallography experiments and the structure determination of proteins with little or no radiation damage. However currently it is unknown whether the designated 4.5 MHz maximum pulse rate for the European XFEL could lead to sample damage caused by shock waves from preceding pulses. Here, the authors address this question by performing a X-ray pump X-ray probe experiment on haemoglobin microcrystals at the Stanford XFEL facility that mimics the 4.5 MHz data collection mode and observe structural changes and a drop in diffraction data quality of the crystals.

The generation of ultrashort X-ray pulses with a peak power exceeding 100 GW offers new opportunities for studying electron dynamics with nonlinear spectroscopy and single-particle imaging.

Pump-probe spectroscopy is routinely used to interrogate ultrafast valence electronic and vibrational dynamics in complex systems. Here, the authors extend this technique to the X-ray regime using a sequence of femtosecond X-ray pulses to understand core-valence interactions in a solvated molecular complex.

Generation of intense attosecond waveforms with independently controllable amplitude and phase is performed by using a seeded free-electron laser.

X-ray photoelectron spectroscopy probes the chemical environment in a molecule at a specific atomic site. Here the authors extend this concept with a site selective trigger to follow chemical bond changes as they occur on the femtosecond time scale.

X-ray free-electron lasers, important light sources for materials research, suffer from shot-to-shot fluctuations that necessitate complex diagnostics. Here, the authors apply machine learning to accurately predict pulse properties, using parameters that can be acquired at high-repetition rates.

X-ray pump–probe experiments reveal that the molecular structure of C₆₀ molecules substantially delays their fragmentation following photoionization. This may help to understand X-ray laser-induced radiation damage on molecules.

X-ray free electron lasers allow for studying the interaction of magnetic materials with intense X-rays beyond a linear response regime. Here, the authors demonstrate the onset of X-ray induced ultrafast demagnetization in Co/Pd multilayers via a redistribution of valence electrons on timescales shorter than 40 fs.

Attosecond time–energy characterization of pulses generated by the Linac Coherent Light Source (LCLS) X-ray free-electron laser is enabled by angular streaking measurements.

A cascaded hard X-ray self-seeding system is demonstrated at the European X-ray free-electron laser. The setup enables millijoule-level pulses in the photon energy range of 6–14 keV at the rate of ten trains per second, with each train including hundreds of pulses arriving at a megahertz repetition rate.

A hard X-ray self-seeded X-ray free-electron laser at the Pohang Accelerator Laboratory provides X-ray pulses with peak brightness of 3.2 × 10³⁵ photons s^–1 mm^–2 mrad^–2 0.1%BW^–1 at 9.7 keV and a very small shot-to-shot electron energy jitter of 0.012%.

Ultrashort circularly polarised light pulses from free-electron lasers offer a route for exploring chiral molecules and their dynamics, but remain challenging to harness. Here, X-ray pump-probe experiments enable a site-specific photoelectron circular dichroism measurement on a dissociating chiral molecule.