Photoelectron Diffraction Imaging of a Molecular Breakup Using an X-Ray Free-Electron Laser

Abstract

A central motivation for the development of x-ray free-electron lasers has been the prospect of timeresolved single-molecule imaging with atomic resolution. Here, we show that x-ray photoelectron diffraction-where a photoelectron emitted after x-ray absorption illuminates the molecular structure from within-can be used to image the increase of the internuclear distance during the x-ray-induced fragmentation of an O 2 molecule. By measuring the molecular-frame photoelectron emission patterns for a two-photon sequential K-shell ionization in coincidence with the fragment ions, and by sorting the data as a function of the measured kinetic energy release, we can resolve the elongation of the molecular bond by approximately 1.2 a.u. within the duration of the x-ray pulse. The experiment paves the road toward timeresolved pump-probe photoelectron diffraction imaging at high-repetition-rate x-ray free-electron lasers.

Electron-ion coincidence measurements of molecular dynamics with intense X-ray pulses

“…Complementing previous studies where nuclear motions were revealed by measuring ion kinetics or Auger electrons, our experiment demonstrates that the energies of the photoelectrons ejected from the inner shell are also sensitive probes of nuclear dynamics. These results extend the concept of a “molecular frame” measurement, which normally refers to the orientation of one or more molecular axis 3 , 4 , 10 , 41 , 42 , to the molecular geometry in a more general sense, which is exemplified here by measuring the core-shell photoelectron energy as a function of evolving bond length.…”

“…We observe a shift in the core binding energy of the dissociating N molecular ion, which can be linked to the internuclear separation of the two resulting ionic fragments by measuring their kinetic energies and performing ab initio calculations. Together with a recent demonstration 10 of photoelectron diffractive imaging of molecular dissociation performed using similar experimental approach but focusing on the changes in photoelectron angular distributions instead of energies, these results demonstrate the general feasibility of coincidence spectroscopy at XFELs and its promising broad perspectives for AMO physics and ultrafast photochemistry.…”

“…However, this method has been implemented in only a few experiments at free-electron laser (FEL) facilities to date [6][7][8][9] , largely hindered by its requirement of low interaction rates (less than one interaction event per laser shot). With several high-repetition rate X-ray freeelectron lasers now coming into operation, electron-ion coincidence measurements are becoming a more feasible approach 10 that can take full advantage of the envisioned MHz repetition rates in order to significantly increase the information content of the experiments.…”

“…Conversely, a large time delay between the pump and probe steps will lead to a significantly smaller Coulomb repulsion if the molecular fragments are further apart when their charge is increased to the final charge state in the second photoionization event. This intuitive relation between the observed KER and the molecular bond length, which provides the conceptual basis for the so-called Coulomb explosion imaging approach 15 , has been recently exploited to image the temporal evolution of molecular orbitals by measuring correlated energies of ions and Auger electrons from HCl dissociation in a synchrotron experiment 16 , to time the sequential absorption of several X-ray photons by N molecules 17 , and to sort the photoelectron diffraction images in the above-mentioned EuXFEL experiment 10 . In our experiment, the single-pulse pump-probe scheme enabled by the coincidence detection of ions and photoelectrons, along with a quantitative simulation of the relation between the internuclear separation and the observed KER, allows us to probe the evolving core binding energy of the dissociating molecular ion.…”

Double Core-Hole Generation in O2 Molecules Using an X-Ray Free-Electron Laser: Molecular-Frame Photoelectron Angular Distributions

“…The SSDCH occurs as a small feature at the expected electron energy of 30 eV. A peak at approximately 95 eV results from sequential K-electron emission via the PAP scheme [41], and the main line (the first K emission) is visible at an energy of 127 eV. The TSDCH intensity is located between the PAP feature and the main K line at an energy of about 110 eV.…”