Wide-angle x-ray scattering experiment

The experiment was carried out in the transmission WAXS (wide-angle x-ray scattering) geometry. The x-rays were tuned to a maximum flux (about ~2 × 10¹¹ photons per pulse) at 12 keV (bandwidth, FWHM 0.4%; pulse duration, ~25 fs; and repetition rate, 10 Hz), and the spot size at the sample position was ~20-μm diameter (FWHM) focused using a beryllium CRL (44), which gave the position stability of ~4.6 μm in horizontal and ~ 2.3 μm in vertical (45). This size is ca. 1/2 of the optical laser spot size, suited for observing the sample response to the laser shock. To ensure the spatial overlap of the x-ray and optical laser on the sample, a long-working distance microscope was placed near the sample and used for monitoring both beam spots. The overlap point was marked on the microscope camera and was checked periodically during the experimental campaign. The sample plane positioning was reproducible to within the depth of focus of the objective microscope, which was about 2 μm. The near colinear optical alignment to the x-ray beam geometry (5°) and the large Rayleigh length of the focused laser beam (about 3 cm) allowed us to have negligible errors in the longitudinal overlap of the beams. To check the alignment of the optical laser and XFEL positions, test shots were made on an Si wafer, which is located on the same plane as the surface of the iron sample before each shot. The lateral overlap was checked at every shot by using a microscope objective of ×20 magnification, giving a spatial resolution of less than 1 μm, which is much smaller than both laser and x-ray spot sizes.

The sample stage consists of sample holder cartridges (3-mm diameter holes in a 4 × 4 grid), linear motor stages, and a hexapod. This allows for mounting multiple samples and translating each of them to the spatial overlap point without disturbing the x-ray and laser alignments. The time delay between x-ray and optical laser pulses was set by a linear stage position that changes the path length of the laser. The single-shot x-ray image was obtained using a 2D area detector, Rayonix MX225-HS (effective area of 22.5 cm by 22.5 cm). The detector frame rate was 33 Hz, matching the repetition rates of the x-ray and optical laser pulse. A single frame with a time window of 33 ms (exposure time. 32 ms; frame transfer time, 1 ms) captures a single shot diffraction image. The detector was positioned to cover arcs of the six peaks of (110), (200), (211), (220), (310), and (222) in α-phase of iron. Debye-Scherrer rings from the sample were calibrated by fitting diffraction peaks of a CeO₂ (SRM 674b) standard sample using Dioptas (33). It was operated in the 4 × 4 binning mode (number of pixels, 1440 × 1440 effective pixel size, 156 μm× 156 μm), which provides enough momentum resolution to observe the three-wave states in shock-compressed iron. Two or three single shots at the same laser/x-ray delay time were repeatedly measured to check the reproducibility of the compression and relaxation effects.

In addition, to ensure statistically meaningful measurements, separate runs were made under slightly different sample and/or pump-probe conditions. Figure S5 demonstrates that the overall lattice dynamics of iron during shock compression and relaxation stages are consistent between different measurements, showing a similar sequence of phase evolution up to 2000-ps delay time.