(a) Relevant potential curves of neutral Br2, including the energy curve for the Coulomb explosion channel Br+ + Br+ that can be reached in the dissociative ionization by the FEL pulse
Figure 2. (a) Relevant potential curves of neutral Br2, including the energy curve for the Coulomb explosion channel Br+ + Br+ 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 C(1Πu) state to the different Coulomb explosion channels at different internuclear distances (solid lines). The dashed line shows the fragment energy during dissociation along the C(1Πu) potential. The curves have been obtained using classical calculations which are described in more detail in the text.
The dissociation dynamics induced by a 100 fs, 400 nm laser pulse in a rotationally cold Br2 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 Br2 molecules were transiently 'fixed in space' using laser-induced alignment. In addition, similar alignment techniques were used on CO2 molecules to allow the measurement of the photoelectron angular distribution (PAD) directly in the molecular frame (MF). Our results on MFPADs in aligned CO2 molecules, together with our investigation of the dissociation dynamics of the Br2 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.