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Coulomb-explosion imaging tackles an 11-atom molecule

25 March 2022

Until now, the technique was thought to work only on molecules with approximately five or fewer atoms. A powerful x-ray source leaves that limit in the dust.

Rebecca Boll works on the Small Quantum Systems scientific instrument at the European X-Ray Free-Electron Laser
Rebecca Boll works on the Small Quantum Systems scientific instrument at the European X-Ray Free-Electron Laser. Credit: European XFEL/Jan Hosan

Sometimes the only way to get a good look at something is to destroy it. When archaeologists excavate a site, they forever disrupt the context of how the artifacts they find were arranged in the ground. For gas-phase chemists, the molecules of interest are so small and move so fast that they can’t be directly imaged—unless they’re blown to bits.

In recent years, two complementary techniques have shown their power to capture still images of gas-phase molecules and even movies of molecules in motion. Both involve blasting the target molecule with an x-ray pulse intense enough to strip it of several electrons and break many or all of its chemical bonds. In x-ray diffraction imaging—related to the less destructive x-ray crystallography of ordered solid samples—researchers derive molecular structure from the small fraction of x-ray photons that scatter elastically from the molecule before it’s destroyed. In Coulomb-explosion imaging, on the other hand, researchers collect the positively charged atomic fragments that fly apart under the force of electrostatic repulsion. By measuring the fragments’ momentum, they can deduce the molecule’s starting structure.

Each technique has its strengths and weaknesses. X-ray diffraction can image large molecules and even viruses, but it struggles to detect hydrogen atoms, which scatter few photons. Coulomb-explosion imaging is equally sensitive to light and heavy atoms. But until recently, it’s been limited (at least in its purest form) to molecules of three–five atoms. Researchers assumed that it wouldn’t be possible to simultaneously ionize all the atoms of a larger molecule. And they assumed that they could reconstruct a molecule’s structure only if they detected all of its ionic fragments—a daunting task for larger molecules when the detection efficiency for each ion hovers around 60%.

Now, working at the European X-Ray Free-Electron Laser, or EuXFEL, Rebecca Boll (seen in the photo), Till Jahnke, and colleagues show that neither of those assumptions is true—at least for some molecules. When struck by one of the EuXFEL’s intense x-ray pulses, 2-iodopyridine (shown in the figure on the right) easily expels enough electrons to ionize all 11 of its atoms. And even when the researchers detected as few as three of the atoms from any given molecule, the momentum distributions they collected from repeated Coulomb explosions were crisp enough to distinguish all five of the molecule’s carbon atoms, as shown in the figure on the left, and all four of its hydrogens. A theory team led by Robin Santra performed the simulations that connected the momentum distribution back to the molecular structure.

Graph of XFEL data shows peaks for carbon atoms and a nitrogen atom
Credit: Adapted from R. Boll et al., Nat. Phys. (2022), doi:10.1038/s41567-022-01507-0

Admittedly, 2-iodopyridine was a fortuitous choice of molecule. Its iodine atom has a large x-ray-absorption cross section, and its rigid hexagonal ring lacks the floppiness of other similarly sized organic molecules. Among the researchers’ next steps is to extend their experiments to complex molecules without those advantages. Their eventual goal is to image molecules not in equilibrium but in the middle of a chemical reaction. (R. Boll et al., Nat. Phys., 2022, doi:10.1038/s41567-022-01507-0.)

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