Realizing split-pulse x-ray photon correlation spectroscopy to measure ultrafast dynamics in complex matter

Split-pulse x-ray photon correlation spectroscopy has been proposed as one of the unique capabilities made possible with x-ray free electron lasers. It enables characterization of atomic-scale structural dynamics that dictates the macroscopic properties of various disordered material systems. Central to the experimental concept are x-ray optics that are capable of splitting an individual coherent femtosecond x-ray pulse into two distinct pulses, introduce an adjustable time delay between them, and then recombine the two pulses at the sample position such that they generate two coherent scattering patterns in rapid succession. Recent developments in such optics showed that, while true amplitude-splitting optics at hard x-ray wavelengths remains a technical challenge, wavefront and wavelength splitting are both feasible, able to deliver two micron-sized focused beams to the sample with sufficient relative stability. Here we show, however, that the conventional approach to speckle visibility spectroscopy using these beam-splitting techniques can be problematic, even leading to a decoupling of speckle visibility and material dynamics. In response, we discuss the details of the experimental approaches and data analysis protocols for addressing issues caused by subtle beam dissimilarities for both wavefront- and wavelength-splitting setups. We also show that in some scattering geometries, the Q-space mismatch can be resolved by using two beams of slightly different incidence angles and slightly different wavelengths at the same time. Instead of measuring the visibility of weak speckle patterns, the time correlation in sample structure is encoded in the side band of the spatial autocorrelation of the summed speckle patterns and can be retrieved straightforwardly from the experimental data. We demonstrate this with a numerical simulation.