Imaging Molecular Structure with Photoelectron Diffraction

The possibility to study the structure of polyatomic gas-phase molecules by photoelectron difraction is investigated with the goal of developing a method capable of imaging ultrafast photochemical reactions with femtosecond temporal and sub-Ångström spatial resolution. The fluorine 1s-level of adiabatically laser-aligned 1-ethynyl-4-fluorobenzene (C₈H₅F) molecules was ionized by X-ray pulses from the Linac Coherent Light Source Free-Electron Laser, and the angular distributions ofphotoelectrons with kinetic energies between 30 and 60 eV were recorded by velocity map imaging. Comparison with density functional theory calculations allows relating the measured distributions to the molecular structure. The results of an IR-pump, X-ray-probe experiment on aligned 1,4-dibromobenzene (C₆H₄Br₂) molecules are presented to explore the potential of photoelectron difraction fortime-resolved imaging. The influence of the alignment laser pulse on the pumping and probing step is discussed. Laser-alignment is contrasted with determination of the molecular orientation by photoelectron-photoion coincidences for an exemplary data set on 1-ethynyl-4-fluorobenzene molecules recorded at the PETRA III synchrotron. Both methods are evaluated with respect to their applicability to record time-dependent snapshots of molecular structure. The results obtained in this work indicate possible future avenues for investigating ultrafast molecular dynamics using X-ray Free-Electron Lasers