XFEL: Symmetry breakdown of electron emission

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2018/11/07
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Symmetry breakdown of electron emission

Limits of dipole approximation shown for the first time in sequential photoionization

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The SQS instrument at European XFEL. Copyright European XFEL

In a study carried out at FERMI in Italy and published today in Nature Communications, a team of international scientists demonstrate for the first time a significant symmetry breakdown of electron emission in ionic matter. The results have significant implications for a range of experiments, especially at free-electron laser (FEL) facilities.

When matter absorbs light of a sufficiently short wavelength or at high enough intensity, it can emit electrons. This process of electron ejection is known as photoionization and is an important basis for many experiments that investigate the electronic structure of atoms, and the structural and chemical information of molecules. When lots of ‘light-particles’ known as photons, are available during a very short time frame, multiple electrons may be emitted in quick succession, providing even more information about the way light and matter interact with another. In this way, the spectroscopic study of ionic matter has been revolutionized by FELs over the past few years.

Generally, it is assumed that for photons with long wavelengths in relation to the absorbing site, the interplay between photon and electron can be described as dipole interaction. However, for very short wavelengths and several other conditions, this assumption starts to break down since the momentum of the photon can distort electron emission patterns in its propagation direction.

Just like a cork or boat will only bob up and down vertically on the ocean on gentle, shallow waves with a long wavelength, but will then also be moved forward in the direction of the waves once they are of shorter wavelength, so the momentum of the photon can steer the movement of the electrons under certain conditions.

Non-dipole (top) and dipole (bottom) electron emission patterns.

As modern free-electron laser facilities such as European XFEL begin to harness the power of X-rays with ultra-high intensity and very short wavelengths for spectroscopy and scattering experiments, a complete and accurate understanding of electron emission, including the propagation direction of the light, will be essential to be able to interpret results correctly.

Now, for the first time, an international research team led by scientists from European XFEL, DESY and the University of Kassel, have shown experimentally that so-called non-dipole effects can play an important role in sequential electron emission even at wavelengths that are commonly assumed to be symmetrical in distribution. The specially designed study, done using ultra-intense X-ray pulses at the free-electron laser FERMI in Italy, investigated the sequential emission of electrons from gaseous argon around 25 nm photon wavelength. This particular configuration allows for a “zoomed view” into the relevance of non-dipole effects and showed a significant symmetry breakdown of electron emission for neutral and also subsequently created ionic matter.

The findings, published today in Nature Communications, have broad implications for future experiments on the fundamentals of photoemission from atoms and molecules at high intensities, applications in condensed matter physics, as well as astrophysical phenomena. Furthermore, time-resolved experiments with short-pulse light sources such as FELs are capable of monitoring the evolution of matter dissociating into ionic fragments which are subsequently further ionized in order to investigate their chemical and structural property changes on their natural time scale of femtoseconds to picoseconds. The altered symmetry signature of the electrons, as demonstrated in the current study, can therefore be decisive for correct interpretation of several kinds of time-resolved experiments. The Small Quantum Systems (SQS) instrument at European XFEL, which starts operation at the end of 2018, has been designed to routinely capture such non-dipole effects.

Original article:
Symmetry breakdown of electron emission in extreme ultraviolet photoionization of argon.
M. Ilchen, G. Hartmann, E. V. Gryzlova, A. Achner, E. Allaria, A. Beckmann, M. Braune, J. Buck, C. Callegari, R. N. Coffee, R. Cucini, M. Danailov, A. De Fanis, A. Demidovich, E. Ferrari, P. Finetti, L. Glaser, A. Knie, A. O. Lindahl, O. Plekan, N. Mahne, T. Mazza, L. Raimondi, E. Roussel, F. Scholz, J. Seltmann, I. Shevchuk, C. Svetina, P. Walter, M. Zangrando, J. Viefhaus, A. N. Grum-Grzhimailo, M. Meyer Nature Communications 9, 4659 (2018).
https://doi.org/10.1038/s41467-018-07152-7