Two-electron interference in two-photon attosecond double ionization

Advances in X-ray free-electron lasers (XFEL) and high-harmonic-generation technologies have enabled XUV-pump XUV-probe schemes to directly observe correlated electronic motion in two-photon double ionization (TPDI) processes with attosecond time resolution by detecting photoelectron pairs in coincidence. Of particular interest is the interference between distinct TPDI pathways leading to the same final two-electron state, a feature well-described in helium by the finite-pulse virtual-sequential model (FPVSM). However, much less is known about the TPDI process in more complex atoms and molecules. Recently, we extended FPVSM to arbitrary atomic targets and successfully reproduced angularly integrated signals measured at FLASH in 2009 for atomic neon. Neon is an interesting target because TPDI produces both singlet $(^1D^e,\,^1S^e)$ and triplet $(^3P^e)$ $2s^22p^4$ states of the $\mathrm{Ne}^{2+}$ ion, associated with photoelectron pairs of singlet and triplet multiplicity, respectively. Consequently, the spatial part of the photoelectron-pair state functions in these two channels has opposite permutational parity, expected to be reflected in their two-particle interference patterns. Our model confirms that in the $(^3P^e)$ channel, interference peaks and troughs are inverted compared to those in the singlet channels or in TPDI of helium. Building on these findings, we are now implementing FPVSM for molecular targets using \textsc{astra}. We will present some preliminary results demonstrating the formation of double-hole states in molecular nitrogen, a phenomenon crucial for understanding coherent molecular state control and the interaction of extreme light with matter.