Extended Data Fig. 1: CDW diffracted intensity for the reconstruction of rocking curves.

(a) Intensity of first-order NC (left) and IC (right) diffraction spots as a function of temporal delay for the data set shown in Fig. 3c in the main text. The curves are additionally sorted and binned by the k_z-component of the respective reflections. The scattered intensity at the IC spot position before time zero is subtracted for both types of curves as a measure of the inelastic background in the images. The NC curves are normalised with respect to the measured intensity before time zero and show similar behaviour as a function of temporal delay. For the IC curves, we normalise to the measured intensity after 300 ps. During the phase formation, IC spots probed at a k_z -component closer to the ZOLZ display an initial overshoot (blue curve). In contrast, reflections in the FOLZ are characterised by a continuous intensity increase (red curve), leading to the sharpening of the reciprocal lattice rod along the out-of-plane direction described in the main text and Fig. 3. (b) Average intensity of first-order NC (blue), first-order IC (red) and second-order NC spots (green). Curve normalisation as described in (a).