A rigorous determination of the error bars would require the acquisition of several data in identical conditions, which has not been possible in the present case because of the long acquisition time for a single waveform. However, a reasonable estimate can be provided by analyzing the scans where more than one signal image has been collected at each Δt step and those repeated more than one time in similar conditions, even if in a limited Δt range. This analysis is proposed in fig. S1E, from which one can infer an overall uncertainty of about 15%. This value is larger than the statistical error expected from the total ADU counts (typically >103) observed within ROIsig and above the noise level (typically <0.1 photons/shot) estimated from the data at Δt < 0 (e.g., see Fig. 2D). This uncertainty could be due to jitters and drifts in the FEL pointing and/or other subtle parameters, for instance, the intensity distribution across the wavefront of the FEL beams, which cannot be accounted for by the data reduction procedure. However, it is worth noting that the “quality” of the present FEL pump/FEL probe data surpasses those from the few time-resolved experiments carried out on condensed matter by exclusively using FEL pulses. This highlights the great advantage of the “background-free conditions,” inherently related to the noncollinear wave-mixing approach used in the present work (and generally applicable also in other contexts).

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