Again, not a physicist. But my understanding is that on a femptosecond timescale (the duration of the x-ray pulse generated by LCLS-II), effectively no reactions occur. The pulse destroys the sample, so that means you have to pump through a steady stream of sample particles to image. So for any given experiment, you'll get a stream of what are effectively snapshots of the reaction occurring at a given moment in time, and then it's up to software post-processing to figure out what you're looking at in a given snapshot and how far progressed the chemical reaction is at that point.