Structural dynamics during the three-ion Coulomb explosion of highly charged CS2 in intense soft x-ray pulses

Using intense femtosecond soft x-ray pulses above the sulfur 2p edge produced by the Free-electron LASer in Hamburg (FLASH), we investigate the multiple ionization and fragmentation dynamics of CS2. We focus on the three-ion fragmentation pathways of CS2 polycations with total charges of +4 to +10, which can only be reached through absorption of multiple x-ray photons. By coupling three-dimensional velocity map imaging with covariance analysis, we determine the relative momentum distribution of all ions produced in each fragmentation channel at high ion count rates per shot. Deviations between these measured relative ion momentum distributions and those predicted by classical Coulomb explosion simulations, assuming an instantaneous charge buildup, grow with increasing total charge state, indicative of nuclear motion during the multiple ionizations that occur within the x-ray pulse duration-as confirmed by comparison with the main fragmentation channel of the trication, which can be formed following single-photon interaction. These dynamics can be modeled using a gradual charge buildup picture of sequential single-photon single-ionization events occurring during the pulse, following the initial core ionization. Our study underscores the importance of complete and channel-resolved measurements, the advances enabled by covariance analysis in high-count-rate experiments, and the critical role of charge buildup and nuclear dynamics during the x-ray free-electron laser pulse in Coulomb explosion imaging. The impacts of this nuclear motion can be controlled and minimized by utilizing shorter x-ray pulses, as will be of great importance in using time-resolved x-ray Coulomb explosion to probe the fastest dynamics in molecular photochemistry.