When a team of physicists focused the full intensity of SLAC’s Linac Coherent Light Source (LCLS) Coherent X-ray Imaging instrument — the world’s most powerful X-ray laser with an intensity of 100 quadrillion kilowatts per sq.cm — on a small molecule, they got a surprise: a short laser pulse stripped all but a few electrons out of the molecule’s biggest atom from the inside out, leaving a void that started pulling in electrons from the rest of the molecule, like a molecular version of a black hole. The research was published this week in the journal Nature.
In this illustration, an ultra-intense X-ray laser pulse from SLAC’s Linac Coherent Light Source knocks so many electrons out of a molecule’s iodine atom (right) that the iodine starts pulling in electrons from the rest of the molecule (lower left), like an electromagnetic version of a black hole. Many of the stolen electrons are also knocked out by the laser pulse; then the molecule explodes. Image credit: DESY / Science Communication Lab.
The researchers shot iodomethane (CH3I) and iodobenzene (C6H5I) molecules with a powerful X-ray beam.
Based on earlier studies with less energetic X-rays, they thought cascades of electrons from the outer parts of the iodine atom would drop down to fill the vacancies, only to be kicked out themselves by subsequent X-rays. That would leave just a few of the most tightly bound electrons. And, in fact, that’s what happened in iodine atoms.
But in the molecules, the process didn’t stop there. The iodine atom, which had a strong positive charge after losing most of its electrons, continued to suck in electrons from neighboring carbon and hydrogen atoms, and those electrons were also ejected, one by one.
Rather than losing 47 electrons, as would be the case for an isolated iodine atom, the iodine in the smaller molecule — iodomethane — lost 54 electrons, including the ones it grabbed from its neighbors.
“As this powerful X-ray light hits a molecule, the heaviest atom, the iodine, absorbs a few hundred times more X-rays than all the other atoms,” explained lead co-author Dr. Artem Rudenko, of Kansas State University.
“Then, most of its electrons are stripped away, creating a large positive charge on the iodine.”
“The positive charge that was created steadily pulls electrons from the other atoms in the molecule, which fills the created vacancies like a short-lived black hole,” he said.
Unlike the real black hole, the molecular version lets the electrons out again.
They are stripped away in a few femtoseconds (a femtosecond is a millionth of a billionth of a second).
“The cycle repeats itself until the molecule explodes,” said co-author Dr. Daniel Rolles, also from Kansas State University.
“In total, 54 of iodomethane’s 62 electrons were ejected in this experiment, far more than we anticipated based on earlier studies using less intense X-rays.”
“In addition, the larger molecule, iodobenzene, loses even more electrons.”
“As far as we are aware, this is the highest level of ionization that has ever been achieved using light,” said co-author Dr. Robin Santra, from the Center for Free-Electron Laser Science at the Deutsches Elektronen-Synchrotron (DESY) in Hamburg, Germany.
Understanding the ultrafast dynamic process is important for many applications of intense X-ray lasers, including X-ray imaging of biomolecules.
“Ultra-intense X-rays give us a new and efficient tool to image biological particles, such as proteins and viruses, with high resolution,” Dr. Rolles said.
“But they also damage and eventually destroy the object we are trying to study. If we can understand the mechanisms that cause the damage, theorists can model how the structure changes during the picture-taking process, allowing researchers to either avoid the damage or to account for its influence.”
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A. Rudenko et al. 2017. Femtosecond response of polyatomic molecules to ultra-intense hard X-rays. Nature 546: 129-132; doi: 10.1038/nature22373
