Albert Einstein’s photo effect re-measured

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For his explanation, Albert Einstein once received the Nobel Prize – now researchers have measured the photoelectric effect more precisely than ever before. The phenomenon is happening faster than expected.

The theory of relativity is the main work of the exceptional physicist Albert Einstein. But his Nobel Prize in 1921 he got for another job. It was about the so-called photoelectric effect. This is the release of electrons from a semiconductor or metal surface when it is irradiated. The problem with this is that the observed phenomena could not be explained quite well when light was regarded as a wave as usual.

Instead, Einstein’s explanations, based on Max Planck’s work for example, must assume that light can also consist of particles. This so-called wave-particle dualism marked the birth of modern quantum physics.

Scientists from the Technical University of Munich, the Max Planck Institute for Quantum Optics in Garching and the Vienna University of Technology have now been able to measure the photoelectric effect with unprecedented precision. It was about the question of how much time passes between the light absorption by the solid and the resulting release of the electron from its interior.

So far researchers have only been able to determine the direction and energy of the electrons. The path could not be observed because of the tiny dimensions and the extremely short duration. A German-Austrian team reports in the journal “Nature” now of a new method of measurement. The researchers attached individual iodine atoms to a tungsten crystal and then irradiated it with X-ray flashes.

The flashes started the photo effect. Since the iodine atoms reacted extremely quickly to incident X-rays, the researchers said, these could have been used as light and electron stopwatches. It was thus determined the time until the release of the photoelectrons from the crystal with an accuracy of a few attoseconds, said Reinhard Kienberger, one of the researchers involved.

An attosecond is a billionth of a billionth of a second. The measurement showed that photoelectrons from the tungsten crystal can be generated in about 40 attoseconds – about twice as fast as expected. Even faster, electrons have been released from atoms on the surface of the crystal. After irradiation with X-rays, they would have released electrons without any measurable delay.

This is interesting for the production of particularly fast photocathodes for use in free-electron lasers, the researchers said. The new findings should also help in the development of novel solar cells.

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