Electron-cloud alignment dynamics induced by an intense X-ray free-electron laser pulse: a case study on atomic argon

In an intense X-ray free-electron laser (XFEL) pulse, atoms are sequentially ionised by multiple X-ray photons. Photoionisation generally induces an alignment of the electron cloud of the produced atomic ion regarding its orbital-angular-momentum projections. However, how the alignment evolves during sequential X-ray multi-photon ionisation accompanied by decay processes has been unexplored. Here we present a theoretical prediction of the time evolution of the electron-cloud alignment of argon ions induced by XFEL pulses. To this end, we calculate state-resolved ionisation dynamics of atomic argon interacting with an intense linearly polarised X-ray pulse, which generates ions in a wide range of charge states with non-zero orbital- and spin-angular momenta. Employing time-resolved alignment parameters, we predict the existence of non-trivial alignment dynamics during intense XFEL pulses. This implies that even if initially the atomic electron cloud is perfectly spherically symmetric, X-ray multi-photon ionisation can lead to noticeable reshaping of the electron cloud. Single photoionisation can align an atomic electron cloud, yet it is unexplored how the alignment evolves during sequential multi-photon multiple ionisation induced by intense X-ray pulses. In their paper, the authors predict the existence of non-trivial electron-cloud alignment dynamics in quantum-state-resolved X-ray multi-photon ionisation.