We developed a single-shot X-ray spectrometer for wide-range high-resolution measurements of Self-Amplified Spontaneous Emission (SASE) X-ray Free Electron Laser (XFEL) pulses. The spectrometer consists of a multi-layer elliptical mirror for producing a large divergence of 22 mrad around 9070 eV and a silicon (553) analyzer crystal. We achieved a wide energy range of 55 eV with a fine spectral resolution of 80 meV, which enabled the observation of a whole SASE-XFEL spectrum with fully-resolved spike structures. We found that a SASE-XFEL pulse has around 60 longitudinal modes with a pulse duration of 7.7 ± 1.1 fs.

Ultrafast changes of charge density distribution in diamond after irradiation with an intense x-ray pulse (photon energy, 7.8 keV; pulse duration, 6 fs; intensity, $3\times {10}^{19}\mathrm{W}/{\mathrm{cm}}^2$) have been visualized with the x-ray pump–x-ray probe technique. The measurement reveals that covalent bonds in diamond are broken and the electron distribution around each atom becomes almost isotropic within $\sim 5\mathrm{fs}$ after the intensity maximum of the x-ray pump pulse. The 15 fs time delay observed between the bond breaking and atomic disordering indicates nonisothermality of electron and lattice subsystems on this timescale. From these observations and simulation results, we interpret that the x-ray-induced change of the interatomic potential drives the ultrafast atomic disordering underway to the following nonthermal melting.

An x-ray Hanbury Brown-Twiss interferometry to diagnose a temporal profile of a femtosecond electron bunch (e-bunch) is presented. We show that intensity interference of spontaneous x-ray radiation from the e-bunch reflects the e-bunch profile. Based on this relationship, a temporal profile of the 8.1-GeV e-bunch at SPring-8 Angstrom Compact free-electron LAser (SACLA) that generates x-ray free-electron laser (XFEL) light is characterized through the intensity interference measurement. Combining this e-bunch profile with a numerical simulation, the XFEL pulse duration generated by the e-bunch is estimated to be less than 10 fs.

Temporal coherence is one of the most fundamental characteristics of light, connecting to spectral information through the Fourier transform relationship between time and frequency. Interferometers with a variable path-length difference (PLD) between the two branches have widely been employed to characterize temporal coherence properties for broad spectral regimes. Hard X-ray interferometers reported previously, however, have strict limitations in their operational photon energies, due to the specific optical layouts utilized to satisfy the stringent requirement for extreme stability of the PLD at sub-ångström scales. The work presented here characterizes the temporal coherence of hard X-ray free-electron laser (XFEL) pulses by capturing single-shot interferograms. Since the stability requirement is drastically relieved with this approach, it was possible to build a versatile hard X-ray interferometer composed of six separate optical elements to cover a wide photon energy range from 6.5 to 11.5 keV while providing a large variable delay time of up to 47 ps at 10 keV. A high visibility of up to 0.55 was observed at a photon energy of 10 keV. The visibility measurement as a function of time delay reveals a mean coherence time of 5.9 ± 0.7 fs, which agrees with that expected from the single-shot spectral information. This is the first result of characterizing the temporal coherence of XFEL pulses in the hard X-ray regime and is an important milestone towards ultra-high energy resolutions at micro-electronvolt levels in time-domain X-ray spectroscopy, which will open up new opportunities for revealing dynamic properties in diverse systems on timescales from femtoseconds to nanoseconds, associated with fluctuations from ångström to nanometre spatial scales.