A schematic of the experimental setup showing the two optical paths that can be chosen with the flip mirror (FP): (1) optical pulses cross the FEL pulse with a 10° angle at the centre of the VMI; (2) optical pulses propagate collinearly with the FEL pulse (see the text)

<p><strong>Figure 1.</strong> A schematic of the experimental setup showing the two optical paths that can be chosen with the flip mirror (FP): (1) optical pulses cross the FEL pulse with a 10° angle at the centre of the VMI; (2) optical pulses propagate collinearly with the FEL pulse (see the text). DM: dichroic mirror; SHG: second harmonic generation; WP: waveplate; DL: delay line; L: lens; FP: flip mirror; MFS: multilayer focusing mirror; HM: mirror with central hole; PV: pulsed valve.</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>