X-Ray Lasers Make Atoms Act Like “Black Holes” in Molecules

When zapped with the world’s strongest x-ray laser beam, big atoms within some molecules do something very strange: They behave a bit like minuscule “black holes,” sucking in electrons from the molecules around them, a new study has found. But rather than just teaching us more about the cosmos, these findings may help with something a lot closer to home. Researchers think this tactic could let scientists better analyze viruses, bacteria and other tiny complex structures here on Earth.

X-rays have long been used not only to help doctors peer into people but also to help scientists probe the structures of molecules and other microscopic objects. The more powerful the x-ray beams, the higher the resolution of the images researchers can get.

In the new study scientists at Kansas State University (K.S.U.) and their colleagues experimented with the world’s most powerful x-ray laser, the Linac Coherent Light Source (LCLS) at the SLAC National Accelerator Laboratory—a machine that can generate pulses delivering nearly 100 quintillion watts per square centimeter. That is roughly 100 times more intense than if all the sunlight hitting Earth’s surface were focused onto one thumbnail, says study co-author Sébastien Boutet, an LCLS staff scientist. The findings were published this week in Nature.

The researchers fired x-ray laser pulses—each lasting about 30 femtoseconds (quadrillionths of a second)—at the side of a narrow beam of molecules streaming through a vacuum chamber. On average, “each x-ray pulse ‘sees’ just one molecule—and blasts it into pieces,” says study co-author Artem Rudenko, a physicist at K.S.U.’s Atomic, Molecular and Optical Physics Group. “Then we repeat this experiment a few hundred thousand times.” These are the most intense x-ray pulses ever used to examine such molecules, the researchers say.

The scientists analyzed two kinds of molecules found in biological processes: iodomethane and iodobenzene, which both consist of carbon, hydrogen and iodine. Atoms of iodine are much heavier than those of carbon and hydrogen, and possess more electrons; an iodine atom has 53, compared with carbon’s six and hydrogen’s one. Such large atoms are often found in vital biological molecules that scientists want to observe—for instance, humans use iodine to synthesize thyroid hormones—so researchers want to know more about how these atoms behave when scanned. Until now no one had investigated what impact such intense x-ray pulses would have on molecules with atoms this heavy, Rudenko says. Previous research used weaker beams or focused on simpler molecules composed of much fewer atoms.

In the new study special mirrors focused the x-rays onto spots about 200 nanometers or billionths of a meter wide, Rudenko says. Carbon and hydrogen are essentially transparent to the x-ray frequencies in the laser pulses, so the iodine absorbed the brunt of the blasts. The scientists recorded data from the molecules in the instant before the laser pulses destroyed them.

Based on previous research, the scientists had expected the laser pulses to first strip the innermost electrons from the iodine atoms. They had anticipated that electrons from the outer parts of the atoms would then cascade in to fill these vacancies—only to get kicked out of the atoms altogether after soaking up subsequent x-rays, leaving the atoms with just a few of their most tightly-bound electrons.

But the flow of electrons did not stop there. Each iodine atom, having strong positive charges after losing most of its electrons, continued to yank negatively charged electrons from neighboring carbon and hydrogen atoms. “The iodine atom gobbles all the electrons it can get from its neighbors like a black hole gobbles matter around it,” Rudenko says. “However, unlike a black hole, it does let the electrons out again when the next photon is absorbed.”

Instead of losing 47 electrons when zapped, as solitary iodine atoms would, each of those in the iodomethane molecules lost 54—including electrons it had stolen from its neighbors. In the larger iodobenzene molecule each iodine atom lost even more electrons—exactly how many remains uncertain. “We don’t know where our chain reaction stops,” Rudenko says.

This was far more damage to the molecules than expected, based on prior experiments with simpler molecules or weaker beams. The researchers had thought electrons from relatively distant parts of the rest of the molecules would not have had time to get pulled into iodine atoms within the span of the 30-femtosecond x-ray pulses, says study co-author Daniel Rolles, a physicist at the K.S.U. group. “We could have guessed it before, if we had just wrapped out minds around having a nearly infinite photon density, and realized how efficient charge transfer could be,” he adds.

Physicist Nora Berrah at the University of Connecticut, who did not take part in this study, notes she and her colleagues “observed this effect recently in a large molecule, but we didn’t yet publish our work. Thus we confirm that this is a general effect.”

These findings will likely help scientists better plan and interpret experiments conducted with LCLS and other powerful x-ray lasers, such as the European XFEL. “Our results demonstrate that when interpreting x-ray imaging data—in particular, from biological objects—one needs to give particular attention to the regions containing heavy atoms, like iron clusters in proteins,” Rudenko says, adding that most current models simulating the effects of radiation damage on molecules treat their atoms as relatively similar.

Future research could explore how electrons move in even bigger molecules, eventually approaching very large ones such as proteins. Rudenko adds research analyzing how electric charges flow within molecules hit by x-rays will leap forward with the development of more rapid-firing lasers. Whereas the LCLS fires 120 shots per second, the European XFEL will fire 27,000 times per second when it begins operations in September. “And the real breakthrough,” Rudenko says, “can be expected a few years later, around 2020, when LCLS II—with up to a million shots per second—will come online.”


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