Strong Electron-Electron-Nuclei Correlations in Two-Photon Double Ionization of H2

Two-photon double ionization is a paradigmatic example of how electron correlation manifests. In molecular targets, its coupling with the slower nuclear motion introduces an additional complication and induces electron-electron-nuclei correlations. Experimentally, momentum-coincident measurements can provide a complete kinematical image of the molecular full Coulomb breakup. Previous theoretical studies have described this process by ignoring nuclear motion and the subsequent Coulomb explosion of the dication. Here we show, by means of a full-dimensional treatment of two-photon double ionization of the H_{2} molecule, that nuclear motion plays a decisive role even for pulses as short as 1.5 fs, a time during which the nuclei are not expected to move significantly. We find strong correlations between nuclear and electronic degrees of freedom, giving access to different electronic processes as a function of nuclear kinetic energy. In particular, we observe unexpectedly strong back-to-back asymmetry in the photoelectron angular distributions, as well as novel interferences resulting from the coherent contributions from two-photon sequential absorption paths via different molecular cationic states.