We demonstrate an experimental method to record snapshot diffraction images of polyatomic gas-phase molecules, which can, in a next step, be used to probe time-dependent changes in the molecular geometry during photochemical reactions with femtosecond temporal and angstrom spatial resolution. Adiabatically laser-aligned 1-ethynyl-4-fluorobenzene (C₈H₅F) molecules were imaged by diffraction of photoelectrons with kinetic energies between 31 and 62 eV, created from core ionization of the fluorine (1s) level by ≈80 fs x-ray free-electron-laser pulses. Comparison of the experimental photoelectron angular distributions with density functional theory calculations allows relating the diffraction images to the molecular structure

The sequential inner-shell multiple ionization of krypton was investigated at the Linac Coherent Light Source using ion time-of-flight spectroscopy at photon energies above (2 keV) and below (1.5 keV) the L edge with two x-ray pulse lengths (5 and 80 fs, nominally) and various pulse energies. At 2.5 mJ pulse energy, charge states up to Kr 17+ were recorded for M-shell ionization and charge states up to Kr 21+ for L-shell ionization. Comparing the experimental charge state distribution to Monte Carlo rate-equation calculations, we find a strong enhancement of higher charge states at 2 keV photon energy as compared to the theoretical predictions. This enhancement is explained with a resonant ionization pathway where multiple excitations into outer valence and Rydberg orbitals are followed by autoionization. These resonant pathways play an important role for the photoionization of ions with charge higher than Kr 12+ , for which direct one-photon L-shell photoionization is energetically impossible at 2 keV photon energy. Only a small pulse-length dependence of the charge state yield is observed at an x-ray pulse energy of 0.4 mJ.

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