X-ray pulses create 'molecular black hole'

The extremely intense X-ray flash knocks so many electrons out of the iodine atom (right) such that it pulls in the electrons of the methyl group (left) like an elecetromagnetic version of a black hole, before finally spitting them out.

Scientists have used an ultra-bright pulse of X-ray light to turn an atom in a molecule briefly into a sort of electromagnetic black hole. Unlike a black hole in space, the X-rayed atom does not draw in matter from its surroundings through the force of gravity, but electrons with its electrical charge -- causing the molecule to explode within the tiniest fraction of a second. The study provides important information for analysing biomolecules using X-ray lasers, as the scientists report in the journal Nature.

The researchers used the free-electron laser LCLS at the SLAC National Accelerator Laboratory in the US to bath iodomethane (CH3I) molecules in intense X-ray light. The pulses reached intensities of 100 quadrillion kilowatts per square centimetre. The high-energy X-rays knocked 54 of the 62 electrons out of the molecule, creating a molecule carrying a positive charge 54 times the elementary charge. "As far as we are aware, this is the highest level of ionisation that has ever been achieved using light," explains the co-author Robin Santra from the research team, who is a leading DESY scientist at the Center for Free-Electron Laser Science (CFEL).

This ionisation does not take place all at once, however. "The methyl group CH3 is in a sense blind to X-rays," says Santra, who is also a professor of physics at the University of Hamburg. "The X-ray pulse initially strips the iodine atom of five or six of its electrons. The resulting strong positive charge means that the iodine atom then sucks electrons away from the methyl group, like a sort of atomic black hole." In fact, the force exerted on the electrons is considerably larger than that occurring around a typical astrophysical black hole of ten solar masses. "The gravitational field due to a real black hole of this type would be unable to exert a similarly large force on an electron, no matter how close you brought the electron to the black hole," says Santra.

The process happens so quickly that the electrons that are sucked in are then catapulted away by the same X-ray pulse. The result is a chain reaction in the course of which up to 54 of iodomethane's 62 electrons are torn away -- all within less than a trillionth of a second. "This leads to an extremely high positive charge building up within the space of a ten-billionth of a metre. That rips the molecule apart," says co-author Daniel Rolles of DESY and Kansas State University.

Observing this ultra-fast dynamic process is highly significant to the analysis of complex molecules in so-called X-ray free-electron lasers (XFEL) such as the LCLS in California and the European XFEL, which is now going into service on the outskirts of Hamburg. These facilities produce extremely high-intensity X-rays, which can be used, among other things, to determine the spatial structure of complex molecules down to the level of individual atoms. This structural information can be used by biologists, for example, to determine the precise mechanism by which biomolecules work. Other scientists have already shown that the molecules reveal their atomic structure before exploding. However, to study the dynamics of biomolecules, during photosynthesis for example, it is important to understand how X-rays affect the electrons.

In this study, iodomethane serves as a model system. "Iodomethane is a comparatively simple molecule for understanding the processes taking place when organic compounds are damaged by radiation," says co-author Artem Rudenko from Kansas State University. "If more neighbours than a single methyl group are present, even more electrons can be sucked in."

Santra's group at CFEL has for the first time managed to describe these ultra-high-speed dynamics in theoretical terms, too. This was made possible by a new computer program, the first of its kind in the world. "This is not only the first time that this experiment has been successfully carried out; we even have a numerical description of the process," points out co-author Sang-Kil Son from Santra's group, who was in charge of the team that developed the computer program. "The data are highly relevant to studies using free-electron lasers, because they show in detail what happens when radiation damage is produced."

Apart from DESY, Kansas State University and SLAC, Tohoku University in Japan, the Max Planck Institute for Nuclear Physics in Germany, the University of Science and Technology Beijing in China, the University of Århus in Denmark, Germany's national metrology institute Physikalisch-Technische Bundesanstalt, the Max Planck Institute for Medical Research in Germany, the Argonne National Laboratory in the US, Sorbonne University in France, the Brookhaven National Laboratory in the US, the University of Chicago in the US, Northwestern University in the US and the University of Hamburg in Germany were also involved in the study.

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