Absolute Duration of the Photoelectric Effect

It provides the basis for solar energy and global communi­cations: the photo­electric effect. Albert Einstein described it over a century ago. For the first time, scientists from the Tech­nical Univer­sity of Munich TUM, the Max-Planck Institute of Quantum Optics MPQ, and the TU Vienna have now measured the absolute duration of the light absorp­tion and of the resulting photo­electron which is released from a solid body.

A laser pulse hits a tungsten surface on which iodine atoms have been deposited. Both the tungsten atoms and the iodine atoms lose electrons, which can then be measured. (Source: TU Vienna)

When a solid body is irra­diated with X-rays, electrons separate from it and move towards the surface. But how long does this take? This question was investigated by the inter­national research team led by Reinhard Kien­berger from the Chair of Laser and X-ray Physics at TUM. This is because in the past, only the direction and energy of the electrons could be deter­mined. Previously, the path of the electrons, e.g. through a crystal, could not be observed due to its micro­scopic dimen­sions and the extremely short duration of the process.

However, the inter­national team developed a new measuring method which now allows the time between the absorp­tion of an X-ray photon and the emission of an electron to be determined. For this purpose, the physi­cists “glued” individual iodine atoms to a tungsten crystal and exposed it to X-ray flashes which triggered the photo­electric effect. Because the iodine atoms react extremely quickly to incident X-rays, they serve as light and electron stop­watches.

In order to increase the precision of the measure­ment, these stopwatches were then calibrated in a further experiment with an only recently developed absolute reference. “This allows the emission of the photo­electrons from a crystal to be determined with an accuracy of a few atto­seconds”, says Reinhard Kien­berger. The measure­ment shows that photo­electrons from the tungsten crystal can be generated in around 40 atto­seconds – around twice as fast as expected. This is due to the fact that light of certain colors inter­acted primarily with the atoms in the upper­most level of the tungsten crystal.

Another interes­ting effect was also observed during the experiment: Electrons from atoms on the surface of a crystal are freed even faster. Upon being irra­diated with X-rays, they imme­diately released electrons without a measurable delay. This could be interes­ting for the manu­facturing of parti­cularly quick photo­cathodes for an appli­cation in a free-electron laser, concluded the TUM researchers, as they now know how to acce­lerate or mani­pulate the photon-electron conver­sion.

Further­more, the new method can also be used to examine the behavior of compli­cated molecules on surfaces – a promising approach to e.g. develop innovative new solar cells. With the knowledge of these hitherto unknown photo­chemical processes, technical appli­cations can now be opti­mized even further. (Source: TUM)

Reference: M. Ossiander et al.: Absolute timing of the photoelectric effect, Nature 561, 374 (2018); DOI: 10.1038/s41586-018-0503-6

Links: Laboratory for Attosecondphysics, Max-Planck-Institute of Quantum Optics, Garching, Germany • Chair for Laser and X-ray Physics E11 (R. Kienberger), Technical University of Munich, Munich, Germany • Institute for Theoretical Physics, Vienna University of Technology, Vienna, Austria

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