Extreme Ultraviolet Laser Challenges Einstein

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.