In the pump-probe method the reaction is first triggered (pump) by photolysis (most often laser light) and then a diffraction pattern is collected by an X-ray pulse (probe) at a specific time delay.
Therefore, the reaction that one wishes to study with pump-probe must be able to relax back to its original conformation after triggering, enabling many measurements on the same sample.
Diffusion-trapping methods utilizes diffusion techniques to get the substrates into the crystal and thereafter different trapping techniques are applied to get the intermediate of interest to accumulate in the crystal prior to collection of the diffraction pattern.
These trapping methods could involve changes in pH,[2] use of inhibitor[3] or lowering the temperature in order to slow down the turnover rate or maybe even stop the reaction completely at a specific step.
Just starting the reaction and then flash-freeze it,[4] thereby quenching it at a specific time step, is also a possible method.