DESY, Hamburg

The results of two experiments conducted at the AMO beamline of the LCLS are presented, whose goal was to study the structure of polyatomic molecules by photo electron diffraction. Tracing the dynamics of individual atoms in a molecule in space and time is one of the longstanding dreams in science. The method established here holds the promise of imaging ultrafast photo chemical reactions in gas-phase molecules with femtosecond temp oral and sub-Ångström spatial resolution. The uorine 1 s -level of adiabatically laser-aligned 1-ethynyl-4-uorob enzene (C 8 H 5 F) molecules was ionized by x-ray pulses from the LCLS, and the angular distributions of photo electrons with kinetic energies b etween 30 and 60 eV were recorded by velocity map imaging [ 1 3 ]. Comparison with density functional theory calculations allows relating the measured distributions to the molecular structure. An IR-pump, x-ray-probe experiment on aligned 1,4-dibromob enzene (C 6 H 4 Br 2 ) molecules explored the potential of photo electron diffraction for time-resolved imaging [ 2 4 ]. The evolution of the molecular structure after a given time delay is probed by a femtosecond photo electron pulse, which is initiated by core-level absorption of the LCLS x-ray pulse, thus the achievable temporal resolution is only limited by the pulse durations and the synchronization of the two light pulses. The results obtained in this work indicate possible future avenues for investigating ultrafast molecular dynamics using x-ray Free-Electron Lasers.

[1] R. Boll, et al. , Femtosecond photoelectron diraction on laser-aligned molecules: Towards time-resolved imaging of molecular structure , Physical Review A 88 , 061402(R) (2013) .

[2] R. Boll, et al. , Imaging Molecular Structure through Femtosecond Photoelectron Diraction on Aligned and Oriented Gas- Phase Molecules , Faraday Discussions 171 , in press (2014) .

[3] R. Boll, Imaging Molecular Structure with Photoelectron Diraction , PhD thesis , Heidelb erg (2014).

[4] D. Rolles, et al., Femtosecond x-ray photoelectron diraction on gas-phase dibromobenzene molecules , Journal of Physics B 47 , 124035 (2014) .