Super-intense lasers can boot bunches of electrons from the inner region of atoms, according to a new study.

This extension of the photoelectric effect, in which one photon knocks one electron off the edge of an atom, could make physicists reconsider when light is a wave and when it's a particle.

"The photoelectric effect was the most famous effect to demonstrate that light can have particle character," said Mathias Richter of the Physikalisch-Technische Bundesansalt in Berlin, and lead author of the study published Monday in Physical Review Letters. "Now we come and say, even the photoelectric effect is better described in the wave picture of light if you apply these high intensities."

Light has been caught kicking electrons out of atoms since the 1830s. The photoelectric effect is responsible for early video cameras, digital cameras, solar cells, night vision goggles and Albert Einstein's Nobel Prize in Physics.

Physicists expected the energy of the electrons would depend on the intensity of the light, or how much energy it transfers to a given area in a certain amount of time. They were startled in 1902 when a German physicist showed that the electrons' energy depended instead on the color (or the frequency) of the light. Einstein solved the puzzle three years later by suggesting that light is both a wave and a particle at the same time. Light particles – called photons – carry a packet of energy that depends on their frequency.

But Einstein didn’t do the experiment with extremely intense light. In the original version of the photoelectric effect, one photon kicks out one electron, like one pool ball smacking into another. The first electrons to go are the outermost ones, because the atom holds them less tightly.

In the new study, the physicists shot xenon atoms with FLASH, an x-ray laser that uses intense photons in the extreme ultraviolet energy range, about forty times the energy of visible light. The xenon atoms lost a whopping 21 electrons at once, which indicates that it was hit by 50 photons simultaneously. Not only that, but the first electrons to pop off were from an inner region of the atom, like if you peeled an onion starting with the second layer.

"What we normally do when we put an atom in one of these intense laser beams is we start stripping the electrons from the outside inward," said Louis DiMauro, a physicist at The Ohio State University working on the Linac Coherent Light Source, a high-energy x-ray laser in California. "If what they’re saying is correct, which I believe it is, things like the light source are going to strip atoms from the inside out."

Richter thinks that rather than acting like a billiard ball, the incoming photons acted like a wave. "This is beyond describing it by individual photons," he said. "It would be better to think about the idea that these photons interact as a collective, that they act together like a good team."

The bundle of light energy made the inner electrons shudder so violently they broke out of their atomic prisons. Their flight left holes for outer electrons to fall into, and the energy they released in moving between layers freed still more electrons.

"This is a nice extension of Einstein’s photoelectric effect," Richter said. "It’s the photoelectric effect under so extreme conditions that it’s better to describe it in the wave picture of light than the particle picture."

"It’s a pretty exciting result," DiMauro said, though he cautioned that the idea needs to be tested more rigorously. "I think their speculation has some legs to it, but these are the first type of experiments that have looked at this fundamental process. There’s a need for some more evidence."

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Image: Deutsches Elektronen-Synchrotron desy.de