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19.004 The duration of a quantum leap


For the first time measured: A quantum leap lasts 45 attoseconds
Der Standard, 20.09.2018
derStandard.at The duration of a quantum leap.

Ultra short processes

For the first time measured: a quantum leap lasts 45 attoseconds

20. September 2018, 06:00

Scientists measured the duration of the photoelectric effect

More than a century ago Einstein has described it; today it supplies the foundation for solar energy and global communication: the photoelectric effect. With this phenomenon light removes electrons from the surface of a solid. Behind it is the fact that particles can only receive and emit energy in stages – that is the famous "quantum leap". A German Austrian research team has now measured the duration of this process: Such a quantum leap therefore lasts 45 attoseconds.

1905 Albert Einstein has explained the photoelectric effect for the first time and was awarded for it with the physics Nobel Prize. With the phenomenon an electron absorbs a photon and "leaps" on that occasion onto a higher energy level – a state, in which it can move freely. This effect plays an important role in many technical uses, for example in solar cells or in light sensitive chips of digital cameras.

Extremely short processes

The change of state of the electron passes off within attoseconds – therefore in the inconceivable short area of billionths of a billionth of a second. Because in the past it was successful to produce extreme short laser pulses on this time scale, it has become possible, to observe the course of such processes and measure their length exactly. For the actual study in the specialist magazine "Nature" German researchers have carried out the experiments building up on calculations and computer simulations of theoretical physicists of the Technical University (TU) Vienna and calculated the length of the quantum leaps of electrons of a tungsten surface.

For it the scientists needed first reference values. In a first step the photoelectric effect was measured with helium atoms, which are simply structured. The result served as reference clock for the calculation of the effect with iodine atoms. The so calibrated "iodine clock" was used for the investigation of a tungsten surface, in the third and last step.

Iodine atoms as stopwatches

For it the iodine atoms were coated on tungsten and the surface fired at with ultra-short laser pulses with high energy. The light is the go-ahead for the process: The electrons are stimulated, break away from their atoms and become free movable. Many electrons reach the surface of the material and escape from the tungsten. "With tungsten the duration of this process gets particularly well investigated, because there the border area of the material gets especially exactly defined", explained Florian Libisch from the Institute for Theoretical Physics of the TU Vienna, who has contributed together with his colleagues Joachim Burgdörfer and Christoph Lemell to the theoretical works and computer simulations.

How long the process lasts, depends on the starting point of the state of the electrons. Considering the leading electrons, therefore those, which can relatively freely move in the metal and conduct the current, when a tension is applied, one can measure exactly the duration of a quantum leap: it is 45 attoseconds, until they escape from the surface of the tungsten. With electrons from the inner shells of the tungsten atoms, therefore with lower energy level, it lasts longer, until they reach the surface, explained Libisch. In this case it is exactly 100 attoseconds, until they leave the metal. (red, APA, 20.9.2018)



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