Data extraction and analysis were performed using home-written scripts. To retrieve and analyze the data from the TOF-MS and electron VMI data, a combination of the BeamtimeDAQAccess 36 and CAMPFancyAnalysis 37 Python libraries was used. For the analysis of the PImMS data, a home-written set of LabView scripts and the CAMPFancyAnalysis library were employed. The inverse Abel transformation for the ion images was performed using the onion peeling method, processing of the photoelectron angularly-integrated images was done using direct Abel transformation. Both routines were used as implemented in the PyAbel package. A detailed description of the recoil-frame covariance analysis for the ion VMI data can be found elsewhere. The photoelectron-photoion covariance analysis (PEPICOV) was performed in the following way: first, each single-shot electron image was converted into the radial distribution function (RDF), and each single-shot TOF-MS was converted into the mass-overcharge (m/z) MS. The centers of the images and the momentum calibration were determined based on the helium II photoelectron data, which was obtained in pre-and post-measurements for the same molecular beam with 41 eV XUV radiation from FLASH, published in Refs. 17,19. Then, the covariance map was obtained as