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

A central motivation for the development of x-ray free-electron lasers has been the prospect of time-resolved 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 ${\mathrm{O}}_2$ 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 time-resolved pump-probe photoelectron diffraction imaging at high-repetition-rate x-ray free-electron lasers.

Popular Summary

The ability to see structural changes in single molecules during a chemical reaction has long been a dream of physicists and chemists. One suggested approach is to use finely tuned x-ray lasers to unleash electrons that illuminate a molecule from within. Using just such a setup, we create a movie that records the breakup of an oxygen molecule.

We use intense light pulses from an x-ray free-electron laser to emit an electron from an oxygen molecule. This photoelectron can be thought of as a wave, thanks to the wave-particle duality of quantum physics. As the electron wave propagates away from its host atom, it illuminates the geometrical features of the molecule, much like a radar or sonar measurement images the topology of a terrain. With this technique, we are able to obtain electron diffraction patterns that effectively “see” the nuclei separate within the 25-fs duration of the x-ray pulse.

Our measurement is a first of its kind, finally demonstrating that this experimental scheme is viable, employing novel x-ray free-electron laser sources and multicoincidence particle detection. In particular, the results suggest that time-resolved imaging of molecular rearrangement during photoreactions will be possible in the near future.

Figure 1

Scheme of photoelectron diffraction imaging during Coulomb explosion of ${\mathrm{O}}_2$. (a) A $K$-shell electron is ionized upon irradiation with XFEL light. (b) After the innershell vacancy has been filled after a first Auger decay, the molecule is doubly charged and fragments in a Coulomb explosion. (c) During the fragmentation a second photon triggers the emission of another $K$-shell electron illuminating the molecule from within. (d) A further, subsequent Auger decay yields the observed final ${\mathrm{O}}^{+}/{\mathrm{O}}^{3+}$ state in which the Coulomb explosion continues until the ions are well separated and finally detected. Steps (a) to (c) occur during a single XFEL light pulse, i.e., within approximately 25 fs.

Figure 2

Schematic of the modeled internuclear distance extraction. After initial $K$-shell ionization of the neutral molecule at the mean internuclear distance $R_{\mathrm{mean}}$, an Auger decay of the singly charged molecular ion triggers a Coulomb explosion along the repulsive ${\mathrm{O}}^{+}/{\mathrm{O}}^{+}$ curve. The second $K$-shell electron is ionized at internuclear distance $R_2$ after a delay $t$. The molecule is now in an ${\mathrm{O}}^{+}/{\mathrm{O}}^{2+}$ state, which—within our simplified model—it immediately leaves after emission of a second Auger electron. The Coulomb explosion continues along the ${\mathrm{O}}^{+}/{\mathrm{O}}^{3+}$ repulsive curve. The total kinetic energy of the ions $\mathrm{KER}={\mathrm{KER}}_A+{\mathrm{KER}}_B$ is measured in our experiment.

Figure 3

Polarization-averaged molecular-frame angular distributions of the second photoelectron emitted after the first Auger decay for different internuclear distances as obtained from our theoretical calculations. Distributions ranging from (a) $R=2.28\mathrm{a}.\mathrm{u}.$ (mean ground state internuclear distance) to (d) $R=3.50\mathrm{a}.\mathrm{u}.$, which is the internuclear distance that can be reached within the duration of the XFEL pulse, are shown. The second photon has been absorbed by the ion on the right. An electron energy of 93 eV is used for the simulations. The labels $m1$ to $m4$ depict the different maxima mentioned in the text. The distributions are symmetric by definition with respect to the molecular axis.

Figure 4

Polarization-averaged molecular-frame angular distribution of the first photoelectron emitted in the overall process. The molecule is aligned horizontally with the triply charged ${\mathrm{O}}^{3+}$ ion located on the right. The XFEL photon energy is set to $h\nu =670\mathrm{eV}$, yielding a $K$-shell electron of approximately 127 eV kinetic energy. The red line is the result of our full theoretical calculations.

Figure 5

Examples of diffraction patterns in polar representation. A wave with wavelength $\lambda$ is emitted from the right center (emitter, labeled “$E$”) and diffracted at the left center (scatterer, labeled “$S$”). The red line depicts the angular distribution of the superposition of the direct and the scattered wave. (a) to (e) show the angular distributions for different distances between $E$ and $S$. (a) At shortest distances no diffraction lobes are visible. (b) As the distance increases, a first maximum $m1$ forms toward the emitter side. (c) Further increase yields the first minimum in the direction of $E$ and the maximum $m1$ moves toward the main lobe in the direction of $S$. As the distance is increased, a new lobe $m2$ appears (d) and moves toward $S$, as well (e).

Figure 6

Polarization-averaged molecular-frame angular distributions of the second photoelectron (emitted during the ${\mathrm{O}}^{+}/{\mathrm{O}}^{+}$ Coulomb explosion) for several KERs. The theoretical predictions shown in Fig. 3 are overlayed as red lines (see text for details). The internuclear distances $R$ and the absorption times $t$ of the second photon have been obtained using a simple Coulomb explosion model (see Appendix pp2). The ion, which is triply charged in the final state, is located on the right.

Figure 7

Measured kinetic energy release distribution of molecules ending in (a) the doubly charged ${\mathrm{O}}^{+}/{\mathrm{O}}^{+}$ final state and (b) the quadruply charged ${\mathrm{O}}^{+}/{\mathrm{O}}^{3+}$ final state. The red-marked area in (b) depicts the region of KER which is used to generate the angular distributions shown in Fig. 6.