Extracting contrast in an X‐ray speckle visibility spectroscopy experiment under imperfect conditions.

Pump–probe experiments at synchrotrons and free‐electron lasers to study ultrafast dynamics in materials far from equilibrium have been well established, but techniques to investigate equilibrium dynamics on the nano‐ and pico‐second timescales remain underdeveloped and experimentally challenging. A promising approach relies on a double‐probe X‐ray speckle visibility spectroscopy setup at split‐and‐delay beamlines of X‐ray free‐electron lasers. However, the logistics in consistently producing two collinear, perfectly overlapping pulses necessary to conduct a faithful experiment is difficult to achieve. In this paper, a method is introduced to extract contrast in the case where an angular misalignment and imperfect overlap exists between the two pulses. Numerical simulations of a dynamical system show that contrast can still be extracted for significant angular misalignments accompanied by partial overlap between the two pulses. [ABSTRACT FROM AUTHOR]