Fig. 2. Experimental setup, raw data, and structural refinement.

a Experimental setup for time resolved powder X-ray diffraction in quasi grazing angle geometry. The Debye-Scherrer rings are collected on a 2D Jungfrau detector with single photon sensitivity³⁷. b Difference patterns (laser_on-laser_off) showing up to 30% variations of the signal (pump fluence 0.85 mJ mm²) on the ps time scale (<35 ps). Gray curve shows the negative −4.5 ps for the non-excited delay for reference (shifted by −0.1 along y for clarity). c, d Rietveld refinement of reference pattern (pattern with laser off, and pattern at t = 7.5 ps respectively). Measured powder patterns are plotted in light gray circles and result of Rietveld refinement in black, solid line. Orange and blue patterns are contributions to the refinement of the λ- and β- phases respectively; the residual curve is shown in gray, dashed lines.