Calculation of effective pump dose in X-ray-pump/X-ray-probe experiments.

In pump-probe experiments on solid materials performed within ultrafast X-ray science, the energy deposited by an X-ray pump pulse in the sample has a non-uniform spatial distribution. The following X-ray probe pulse then measures a volume-integrated average of contributions from the differently irradiated regions of the sample. Here we propose a scheme to calculate an effective fluence of the pump pulse such that the observable of interest calculated with the effective fluence is very close to the volume-integrated observable. This approach simplifies computational simulations of X-ray irradiated solids, which typically use periodic boundary conditions and assume a uniformly irradiated simulation box. Obtaining a prediction on a volume-integrated observable requires a significant computational effort, as it is necessary to run multiple simulations for the different exposure conditions and then perform their volume integration. The proposed scheme reduces this effort to a single calculation with the effective fluence.