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Physical Chemistry

X-rays induce electron-gobbling ‘black holes’

Ultra-intense pulses ionize heavy atoms to extremes, causing them to strip electrons from other atoms in molecules

by Jyllian Kemsley
June 5, 2017 | APPEARED IN VOLUME 95, ISSUE 23

High-intensity, hard X-ray pulses induce extreme ionization of heavy atoms, turning them into a kind of “black hole” within molecules, according to new research. The ionized atoms suck electrons in from neighboring atoms before the molecule falls apart (Nature 2017, DOI: 10.1038/nature22373).

To solve macromolecular structures such as those of proteins, crystallographers frequently incorporate transition metals and other heavy atoms into crystals to help with data analysis. But as researchers develop free-electron lasers to use ultra intense, femtosecond X-ray pulses to improve crystal structure data, questions remain about how those X-ray pulses interact with heavy atoms to cause radiation damage that alters or destroys the sample.

Using SLAC National Accelerator Laboratory’s Linac Coherent Light Source and theoretical analysis, a team led by Artem Rudenko of Kansas State University and Sang-Kil Son of Germany’s Deutsches Elektronen Synchrotron particle accelerator studied the effects of ultra intense X-ray pulses on xenon atoms and iodine-containing molecules. The 30-femtosecond laser pulses packed a walloping 2 × 1019 Watts/cm2 with photon energies of 8.3 keV.

The pulses ionized the xenon atoms (n = 54) to a charge state of +48 and, surprisingly, the iodine atoms (n = 53) to a state of +47. An iodomethane molecule gained an overall charge state of +54. This result contrasts with soft or less-intense hard X-ray experiments, in which isolated heavy atoms were ionized more than those bound within a molecule.

According to the researchers’ simulations, as the higher-intensity, hard X-ray pulses cause iodine to lose more electrons, intramolecular electron transfer also occurs to effectively shift negative charge from the rest of the molecule to the iodine. But because the electron transfer occurs faster than the X-ray pulse duration, the transferred electrons are also stripped from the iodine site.

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