Correlated Electron Dynamics in the X-Ray Regime

Free-electron lasers can now produce x-ray pulses with sub-femtosecond duration. This gives the opportunity to drive highly non-equilibrium states of matter and observe the electronic response on the attosecond timescale. We used attosecond angular streaking to explore the ultrafast motion of unstable, core-excited molecules generated by x-ray ionization. We measured the attosecond photoemission delay in the core ionization of nitric oxide. Electron correlation imparts rich structure to the photoemission delay, through interchannel coupling and post-collisional interaction. We also observed ultrafast electron motion in core-ionized 1,1-difluoroethylene, driven by partial coherence between core-level vacancies at nonequivalent carbon sites. In this system, the electronic coherence manifested in the nonlocal quantum correlation between atomic sites and, notably, occurred in the near-absence of charge density motion. This unexplored regime of coherent electronic motion is now experimentally accessible thanks to attosecond x-ray technology.