**3. Time-resolved measurements**

In general, there are two ways of time-resolved imaging: The first way is single-shot imaging, where images are acquired on a time scale much shorter than the dynamics that is to be observed. Here, the dynamics is recorded as it is happening, without any need of  reproducibility. However, when temporal resolution in the nanosecond regime or below and simultaneous spatial resolution in the nanometer regime is needed, the intensity of the light required to obtain a single image becomes so large that the sample evaporates on a picosecond time scale. X-ray holography is unique in being capable of recording single shot videos with only a few femtoseconds between the frames [17], that is, before the atoms of the sample start to move, but the approach is limited to a few frames and to sub-picosecond imaging.

For longer time scales or more frames, pump-probe imaging needs to be employed. Here, the intensity in a single shot is so low that the sample remains unperturbed. However, in order to obtain an image, many such shots need to be accumulated and the sample needs to be in an identical state during every shot. Besides a perfect reproducibility of the dynamics in the specimen, two steps are critical to enable such a pump-probe experiment: First, the excitation that triggers the dynamics must be periodic with a repetition that exactly matches the repetition rate of the incident photon pulses. And second, it is crucial to know the time at which the excitation should be applied in order to arrive simultaneously with the photons at the sample. Details of how to realize this precise timing are discussed in the appendix.

Once the excitation is in sync with the probing X-rays, images can be acquired at every delay of interest between pump and probe. That is, videos are recorded in a frame-by-frame mode: The delay between pump and probe is fixed for one image. After the acquisition of each image, the delay is changed and the accumulation of a new frame starts.
