Shaking the Fundamentals of Physics: At the Limits of the Photoelectric Effect

April 24, 2009
Photoionization of xenon: (a) classical photoelectric effect in the outer shell at low photon intensity, (b) strong-field single ionization in the outer shell by long-wave radiation at high intensity, (c) strong-field multiple ionization in the inner 4d shell by short-wave X-rays at high intensity. (Image: PTB)

With extremely short wavelengths and very high intensities, light-matter interaction seems to be different than previously accepted.

By way of the classical photoeffect, Einstein proved in 1905 that light also has particle character. However, with extremely high light intensities, remarkable things happen in the process. Scientists of the Physikalisch-Technische Bundesanstalt (Germany) have found this out with colleagues at FLASH in Hamburg, the first free-electron laser (FEL) for soft X-rays worldwide.

The current models based on Einstein's idea are simply described in such a way: A photon knocks an external electron out of an atom, provided that the photon energy is high enough. However, with wavelengths of only 13 nanometers and high radiation intensities of several petawatt per square centimeter something else - at least with some atoms - happens: With xenon, a whole light-wave packet immediately seems to knock out a huge number of internal electrons. This effect is strongly dependent on the material and not only on the characteristics of the exciting radiation, as accepted before. The work, which is currently published in the journal , has significance for future experiments of materials research at the new large X-ray laser facilities of the world.

The scientists actually wanted to develop methods for the radiometric characterization of X-ray lasers. They irradiated different gases to derive the laser strength from the ionization effect. The aim: with the laser well characterized was, for example, the testing of EUV lithography mirrors. The EUV lithography (EUV stands for extreme ultraviolet) at wavelengths in the range of 13 nanometers is considered as the future technology for the production of ever smaller computer chips.

However, during their experiments at FLASH, the new free-electron laser (FEL) in Hamburg, which currently allows the generation of EUV radiation and soft X-rays of the highest intensity in the world, they unexpectedly discovered things which concern the fundamentals of physics.

With the classical photoelectric effect (a), a single light particle (photon) of sufficient energy interacts with a single electron of the material. The process is energetically described by the Einstein equation (1905) and demonstrates the quantum structure of light. Only at very high intensities, does the multiphoton ionization occur, a process which is described in the extreme case of highly intensive ultra-short light flashes as emitted by long-wave femtosecond lasers, again, in the wave picture of light (b).

Nevertheless, the suitable theoretical models fail in the short-wave X-ray regime as shown by the experiments in Hamburg in which, for the first time, soft X-ray irradiance levels of several petawatts per square centimeter were achieved by strong beam focusing. The comparative quantitative studies prove that the degree of light-matter interaction and, thereby, the nature of the X-ray light are decisively determined by the structure of the atom and correlations in, above all, inner electron shells.

In the extreme case (xenon), a whole wave packet of photons seems to lead to the simultaneous emission of several inner electrons (c).

More information: Extreme ultraviolet laser excites atomic giant resonance. M. Richter et al., Phys. Rev. Lett. (2009) - online publication expected: April 27, 2009.

Photoelectric effect at ultra-high intensities. A. A. Sorokin et al., Phys. Rev. Lett. 99, 213002 (2007)

Source: Physikalisch-Technische Bundesanstalt

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5 / 5 (1) Apr 24, 2009
just got back from PRL....its expensive to want to know!
5 / 5 (3) Apr 24, 2009
I think there may be a little misunderstanding regarding the pictures b and c as described. For b, the release of the outer shell electron has a fixed energy requirement, and so you may need to add low-wavelength photons up, if that is the only wavelength you are using and it does not have enough energy for single photon ionization. For c, the X-ray wavelength is short enough to individually release one inner shell electron per photon.
3.7 / 5 (3) Apr 24, 2009
Wave/particle duality is well-established but I've always been bothered by the generic quality of the "particle" characteristics. Maybe at high energies the "particle" properties of light are somewhat complicated, and actually behave more like the "particles" observed in high energy collider experiments, with half-lives, decay products, resonances, etc.
5 / 5 (2) Apr 24, 2009
I don't usually comment on grammar, but the grammar and punctuation in this article are horrible.

But in this statement - "This effect is strongly dependent on the material and not only on the characteristics of the exciting radiation, as accepted before" - I find it hard to believe that, as the article implies, the characteristics of the material were not considered in the interaction model. If this is the case, all I can say to this article is "duh" or you gotta be kidding me! We were not considering the material properties in this kind of interaction before? I won't go on with a rant, but considering the properties of the material in any type of material / radiation interaction seems like common sense to me.
1 / 5 (2) Apr 24, 2009
Intense lower frequencies of laser light can ionize atoms. Where does this leave the photoelectric effect? It is just one more piece of proof that electromagnetic radiation is not a particle called the photon. Ask yourself this, how can a photon be a particle when its energy and momentum are directly tied to the length of a man-made unit of time, the second, by way of its frequency? Either Planck's constant is wrong or the photon concept of light is wrong. Both cannot be correct.

not rated yet Apr 24, 2009
One could say that both cannot be correct, when in fact the truth is that in some situations even though it defies belief, it is true that both are indeed correct.

The problem is to understand how that can be possible.
5 / 5 (1) Apr 24, 2009
Ask yourself this, how can a photon be a particle when its energy and momentum are directly tied to the length of a man-made unit of time, the second, by way of its frequency? Either Planck's constant is wrong or the photon concept of light is wrong. Both cannot be correct.

Light is neither a particle nor a wave, this has been known and accepted for almost a century now. This is also true for other particles like the electron, neutron and proton; objects as large as carbon-60 fullerenes have been observed behaving as waves in some respects in the double slit experiment.
5 / 5 (1) Apr 26, 2009
May be this is any nonlinear effect, because of the very strong EM field of the laser.
not rated yet Apr 29, 2009
it would very interesting to study the resulting emission spectra!

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