Exploding a photo subject in order to take its picture? An international research team at the world's largest X-ray laser, the European XFEL, used this “extreme” method to take pictures of complex molecules. With the ultra-bright X-ray flashes generated by the facility, the scientists were able to take snapshots of gas-phase iodopyridine molecules with atomic resolution. In the process, the X-ray laser caused the molecules to explode, and the image was reconstructed from the pieces. “Thanks to the extremely intense and particularly short X-ray pulses of the European XFEL, we were able to produce an image that is unprecedentedly clear for this method and the size of the molecule,” reports Rebecca Boll from European XFEL, principal investigator of the experiment and one of the two first authors of the publication in the scientific journal Nature Physics, in which the team describes their results. Such clear images of complex molecules have not been possible with the experimental techniques used up to now.
The images are an important step towards recording molecular movies, which researchers hope to use in the future to observe details of biochemical and chemical reactions or physical changes with high resolution. Such films are expected to stimulate developments in various fields of research. "The method we use is particularly interesting for investigating photochemical processes," explains Till Jahnke from European XFEL, a member of the core team of the study. Such processes in which chemical reactions are triggered by light are of great importance both in the laboratory and in nature, for example in photosynthesis or in visual processes in the eye. "The development of molecular movies is fundamental research, but the knowledge gained from them could help us to better understand such processes in the future and develop new ideas for medicine, sustainable energy production or materials research," hopes Jahnke.
In the method known as Coulomb Explosion Imaging, a high-intensity and ultra-short X-ray laser pulse kicks numerous electrons out of the molecule. Due to the strong electrostatic repulsion between the remaining, positively charged atoms, the molecule explodes within a few femtoseconds – a millionths of a billionth of a second. The individual ionized fragments then fly apart and are registered by a detector.
“Up to now, Coulomb explosion imaging was limited to small molecules consisting of no more than five atoms,” explains Julia Schäfer from the Center for Free-Electron Laser Science (CFEL) at DESY, the other first author of the study. “With our work, we have broken this limit for this method.” Iodopyridine (C5H4IN) consists of eleven atoms.
The film studio for the explosive molecule images is the SQS (Small Quantum Systems) experiment station at European XFEL. In a COLTRIMS reaction microscope (REMI), specially developed for these types of investigations, electric fields are used to direct the charged fragments onto a detector. The location and time of impact of the fragments are measured and can then be used to reconstruct their momentum – the product of mass and velocity – with which the ions hit the detector. “This information can be used to obtain details about the molecule, and with the help of models, we can reconstruct the course of reactions and processes involved,” says DESY researcher Robin Santra, who led the theoretical part of the work.
Coulomb Explosion Imaging is particularly suitable for tracking very light atoms such as hydrogen in chemical reactions. The technique enables detailed investigations of individual molecules in the gas phase, and is thus a complementary method for producing molecular movies, as are also being developed for liquids and solids at other European XFEL experiment stations.
“We want to understand fundamental photochemical processes in detail. In the gas phase, there is no interference from other molecules or the environment. We can therefore use our technique to study individual, isolated molecules,” says Jahnke. Boll adds: “We have already taken the first steps towards investigating molecular dynamics, meaning to combine individual images into a real molecular movie.”