Autoionizing states in two-photon double ionization of beryllium: a computational study of electron correlation and exchange symmetry*

X-ray and XUV-pump-probe experiments enabled by new intense XFEL and XUV sources [1-3] will allow us to directly observe correlated electron dynamics in atoms and molecules with attosecond time resolution. Two-photon double ionization (TPDI) is of particular interest, as the coincidence detection of the parent-ion and two photoelectrons directly maps, with attosecond time resolution, the correlated motion of electron pairs prior to photoemission. While explicit numerical solvers for the time-dependent Schrödinger equation (TDSE) have successfully reproduced TPDI processes for two-electron systems, such as helium, they did so at considerable numerical cost with no scalability to larger systems. Much of the TPDI dynamics, however, is captured by the recently developed finite-pulse virtual sequential model (FPVSM) [4,5], which we have implemented for atoms and molecules. FPVSM reproduces angularly integrated TPDI observables at a fraction of the cost of TDSE solvers. Here we present a theoretical study of the TPDI in Beryllium, an ideal system due to the presence of autoionizing series in the correlation-dominated non-sequential region. Our FPVSM simulations resolve the interplay between sequential and non-sequential TPDI pathways and observe a clear signature of autoionizing states in the joint energy distribution for the two photoelectrons, accessible with current experiments. This work serves as a benchmark prediction for experiments exploring TPDI of complex atoms and paves the way to time-resolved study and control of concerted electronic motion in atoms and molecules.