(a) Relevant potential curves of neutral Br<sub>2</sub>, including the energy curve for the Coulomb explosion channel Br<sup>+</sup> + Br<sup>+</sup> that can be reached in the dissociative ionization by the FEL pulse

<p><strong>Figure 2.</strong> (a) Relevant potential curves of neutral Br<sub>2</sub>, including the energy curve for the Coulomb explosion channel Br<sup>+</sup> + Br<sup>+</sup> that can be reached in the dissociative ionization by the FEL pulse. (b) Expected final kinetic energy for a single fragment resulting from ionization from the dissociative <em>C</em>(<sup>1</sup>Π<sub><em>u</em></sub>) state to the different Coulomb explosion channels at different internuclear distances (solid lines). The dashed line shows the fragment energy during dissociation along the <em>C</em>(<sup>1</sup>Π<sub><em>u</em></sub>) potential. The curves have been obtained using classical calculations which are described in more detail in the text.</p> <p><strong>Abstract</strong></p> <p>The dissociation dynamics induced by a 100 fs, 400 nm laser pulse in a rotationally cold Br<sub>2</sub> sample was characterized by Coulomb explosion imaging (CEI) using a time-delayed extreme ultra-violet (XUV) FEL pulse, obtained from the Free electron LASer in Hamburg (FLASH). The momentum distribution of atomic fragments resulting from the 400 nm-induced dissociation was measured with a velocity map imaging spectrometer and used to monitor the internuclear distance as the molecule dissociated. By employing the simultaneously recorded in-house timing electro-optical sampling data, the time resolution of the final results could be improved to 300 fs, compared to the inherent 500 fs time-jitter of the FEL pulse. Before dissociation, the Br<sub>2</sub> molecules were transiently 'fixed in space' using laser-induced alignment. In addition, similar alignment techniques were used on CO<sub>2</sub> molecules to allow the measurement of the photoelectron angular distribution (PAD) directly in the molecular frame (MF). Our results on MFPADs in aligned CO<sub>2</sub> molecules, together with our investigation of the dissociation dynamics of the Br<sub>2</sub> molecules with CEI, show that information about the evolving molecular structure and electronic geometry can be retrieved from such experiments, therefore paving the way towards the study of complex non-adiabatic dynamics in molecules through XUV time-resolved photoion and photoelectron spectroscopy.</p>