(a) Comparison of the photoelectron kinetic energy spectra of CO₂ molecules with (red) and without (black) alignment laser, recorded along the laser polarization axis

Figure 8. (a) Comparison of the photoelectron kinetic energy spectra of CO₂ molecules with (red) and without (black) alignment laser, recorded along the laser polarization axis. (b) PAD of the X²Π_g, (c) PAD of the A²Π_u+B^2\Sigma _u^+ channel and (d) PAD of the C^2\Sigma _g^+. For the three channels, the red and black curves correspond to the PAD measured with and without alignment, respectively. The blue curve represents the differential PAD obtained from taking the difference between the red and black curves (see text).

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.