(a)–(c) 2D projections of the momentum distribution of Br^{n +}, n = 1–3 fragments resulting from XUV-only ionization of Br₂ at 90.6 eV photon energy and, (d) the corresponding fragment kinetic energy distributions

Figure 4. (a)–(c) 2D projections of the momentum distribution of Br^{n +}, n = 1–3 fragments resulting from XUV-only ionization of Br₂ at 90.6 eV photon energy and, (d) the corresponding fragment kinetic energy distributions. The dashed lines in panel (d) correspond to the calculated fragment kinetic energies for the various Coulomb explosion channels observed in our measurement, starting at the equilibrium internuclear distance. The inset highlights the nonlinearity of the yields for the different charge states by plotting these yields against the He⁺ yield, which is expected to depend only on the FEL pulse energy. (e), (f) The 2D photoelectron momentum distribution and the photoelectron spectrum of Br₂ molecules seeded in helium.

Abstract

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.