Wf researchers want to find out more about the exact structure of individual molecules, they often use modern scanning probe microscopes. With these instruments, a fine tip scans the surface of a sample, providing an image of its three-dimensional geometry. The scanned molecule remains intact. However, it must be strongly cooled and fixed on a base so that any movement is frozen. It is more difficult for those who want to photograph molecules in the gas phase, because there is not much time to get a snapshot of the volatile compounds. In most cases, only a more ruthless approach can help here: With an intense short laser flash, the molecule is broken down into its individual parts in fractions of a second during the flight. The researchers then reconstruct the original molecular structure from the fragments.
As a result, a molecule is irretrievably destroyed. But you get additional information, for example about its dynamics and the distribution of the charge. So far, the process has only worked for small molecules. The larger the connections become, the more difficult the reconstruction becomes. Researchers have now succeeded for the first time in photographing two relatively complex organic compounds consisting of ten and eleven atoms, respectively.
Coulomb explosion of iodopyridine (C5H4IN) showing the carbon (red) and nitrogen (green) atoms. The ring appears distorted because the detector registers the momentum of the fragments of the explosion rather than a direct image. The iodine atom and the hydrogen atoms are not shown for geometric reasons.
:
Photo: European XFEL / Rebecca Boll, To Jahnke
The experiments were carried out at the world’s largest X-ray laser, the “European free-electron laser in the X-ray range”, the European XFEL. For around five years, the facility in Hamburg has been delivering brilliant X-ray flashes that are extremely short, lasting a billionths of a second (femtoseconds). The wavelength of X-rays is roughly the size of an atom. This allows small crystals and individual molecules to be studied in detail in the gas phase.
The physicists working with Rebecca Boll from the XFEL and Till Jahnke from the Goethe University in Frankfurt used iodine pyridine (C5H4IN) and iodopyrazine (C4H3IN2), two aromatic hydrocarbon compounds of a size comparable to that of nucleobases in hereditary molecules. The researchers vaporized the substances and used them to form fine molecular beams. They directed each one into a zone where the X-ray flashes from the XFEL crossed the path of the molecules. The intensity of the radiation pulses was so high that many electrons were snatched from a molecule that was hit. Positively charged atoms remained.
As a result of electrostatic repulsion, the molecule immediately broke apart before the atoms within it could rearrange themselves spatially. Its components – ions of carbon, iodine, nitrogen and hydrogen – were now flying off in all directions. Using the so-called COLTRIMS reaction microscope, a special camera developed by physicists at the University of Frankfurt, Boll and her colleagues detected the ions with detector plates. From the place and time of the impact, the researchers were able to determine the trajectory and momentum of each individual particle. Finally, the original geometry and structure of both the iodopyridine and iodopyrazine molecules could be reconstructed from the data. As the researchers report in the journal “Nature Physics”, they were able to learn a lot about the processes that take place during the departure. They were even able to make the light hydrogen atoms visible and examine their reaction behavior.
The quality of the snapshots obtained in this way surprised the researchers. Something comparable would hardly be possible with a conventional femtosecond laser. Now Rebecca Boll’s team wants to combine the individual images into a film sequence in order to be able to study the entire dynamics of the molecular explosion. “Our major goal is to film photochemical reactions at the molecular level,” says Till Jahnke. To do this, the physicists first want to stimulate the molecule with a laser pulse and only then break it down into its components and analyze them.
With the help of the Frankfurt reaction microscope, individual electrons can be detected in addition to the atoms, which are released by an ionized molecule, for example. Physicists around Jahnke recently used this fact to investigate the dynamics of elementary physical processes such as the photoelectric effect. The researchers wanted to know how long it takes after absorbing a photon for an electron to be emitted from a molecule. To do this, they irradiated a carbon monoxide molecule with the intense light from the Bessy II synchrotron source in Berlin. As a result of the excitation, the molecule broke apart. The photo effect released an electron, but only after a certain time delay. This averaged 130 attoseconds (trillionths of a second), as the researchers report in “Nature Communications”. However, the timing of the electron emission depended on the angle at which, i.e. in which direction, the particle left the molecule.