Imaging the Ultrafast Coupled Electron-Nuclear Dynamics of Light-Induced Molecular Fragmentation

We employ multi-dimensional imaging to track the ultrafast photodissociation of Br₂ using a pump-probe scheme with femtosecond laser pulses. A weak 400 nm, 38 fs pump pulse initiates the dissociation on the C-state of neutral Br₂, followed by a delayed 800~nm, 25 fs ionizing probe pulse. We measure the 3-dimensional photoion and photoelectron momentum distributions (PMDs) in coincidence using COLd Target Recoil Ion Momentum Spectroscopy (COLTRIMS). Thereby we can directly observe the electronic evolution and the structural transformation during the dissociation process. We support these findings with a semi-classical two-step model, as well as time-dependent density-functional theory (TDDFT) calculations. We observe a discrepancy between the completion of the electronic evolution and that of the structural transformation, highlighting the need for comprehensive measurements to determine the time scale of an electron-nuclear coupled molecular dynamics.