400 nm-induced Br<sub>2</sub> dissociation dynamics probed by an 800 nm laser pulse

<p><strong>Figure 3.</strong> 400 nm-induced Br<sub>2</sub> dissociation dynamics probed by an 800 nm laser pulse. (a)–(c) Experimentally recorded Br<sup>+</sup> 2D momentum distributions for selected pump–probe delays; (a) when the 800 nm pulse precedes the 400 nm pulse by 0.35 ps, (b) when the pulses overlap and (c) when the 400 nm pulse precedes the 800 nm pulse by 0.63 ps. (d) The Br<sup>+</sup> kinetic energy distribution obtained after integrating the time-dependent Br<sup>+</sup> kinetic energy distribution measured at various delays between the two optical laser pulses. (e) The evolution of the Br<sup>+</sup> kinetic energy distribution with respect to the delay between the 400 and 800 nm pulses. (f) The ionization yield (red) and the degree of alignment (blue) for the component at 0.65 eV. The black dotted line is a Gauss error function fitted to the ionization yield with a rise time of 200 fs.</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>