Planets host a whole spectrum of extreme conditions. From incredibly high pressures to temperatures that are nearly sufficient to vaporize steel, the centers of many planets are host to exotic states of matter that can only exist in these violent environments.
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Such extreme conditions mean that a great deal of new physics is occurring at the hearts of planets. However, finding ways to explore and study this is incredibly challenging. Even on Earth, the core temperature is over 5000 °C and the pressures exceed 3 million atmospheres. As the core is located more than 6000 km below the surface, reaching the Earth’s core is still something of a futile task. There are few materials that can withstand such temperatures before even considering the vast amount of energy consumption that such a feat would require.
This means we need innovative, indirect methods to study the composition and size of planetary cores. Another approach is to try and simulate the same kind of conditions that exist in these environments in a laboratory setting. However, that means finding ways to generate similar extreme environments and developing techniques that can be used to monitor and probe them.
Faces of Carbon
Recent research at the European X-ray Free-Electron Laser (XFEL) facility has shown that it is possible to achieve conditions in an experiment similar to those at the center of planets.1 The European XFEL project is a unique facility capable of producing incredibly intense pulses of high-energy X-ray radiation.
Achieving this means accelerating bunches of electrons over three kilometers and using the radiation that individual electrons in the bunch emit as a way of generating X-rays. This is a process known as self-amplified spontaneous emission and can be used to generate some of the brightest high-energy X-rays in the world.
The team investigated several different forms of carbon at the Helmholtz International Beamline for Extreme Fields (HIBEF) – a specially designed experimental end station that could make use of the high-energy X-rays generated by the XFEL on samples that would behave like warm, dense matter.
The warm dense matter is an unusual state of matter that is found in environments such as planet cores. It is sometimes thought as an intermediate state between plasmas, a highly charged collection of particles, and solids.
Such states are thought to be formed during events such as comet collisions with the Earth’s surface. Comets, and the chemical species found on them, are thought to hold the answers to how life began on Earth.
Being able to understand what processes might occur in warm dense matter and whether new chemical species could be formed could help us find the mechanisms and reactions that kickstarted the long process of life on Earth as we know it.
While all the carbon samples the team used started as solid, thin films, under a tightly focused beam of less than 100 µm diameter of very intense X-rays, the samples were in a state that resembled warm dense matter.
Understanding what was happening with the different forms of carbon and how this phase change had affected them required a technique known as X-ray Raman scattering. Raman scattering is a commonly used technique in chemical analysis and imaging, where photons scattered by the sample are detected following laser irradiation and the amount of energy lost or gained relative to the incident laser energy is measured.
X-ray Raman scattering relies on the same approach as standard Raman scattering methods but instead of removing a high-lying valence orbital that is only weakly bound to the molecule or atom, the initial excitation involves a core level electron that is very tightly bound to the nucleus.2
When this core level electron is promoted, several processes can take place. One possibility is to scatter another photon from the sample and look at the energy loss, which, when using X-ray photons, can recover the electronic spectrum of the molecule. The other key advantage of using X-rays for this technique is that the core level transitions have energies that are highly element-specific. By performing an X-ray Raman scattering experiment, it is possible to look at very localized information, even in incredibly complex systems.
Despite the resolving power and flexibility of X-ray Raman scattering, it is an incredibly experimentally demanding technique. Raman transitions generally have low probabilities and need very bright excitation sources for the Raman signal to be observable. While there are several suitable laser sources that cover much of the visible spectrum, achieving this in the X-ray regime requires special facilities such as the European XFEL as they are one of the few places capable of generating such intensities.
X-rays are ideal for probing warm dense matter as high-energy X-rays have good penetration depths so can look right through the clouds of carbon atoms created by the intense laser fields. This is just one example of the new types of experiments that are now feasible thanks to new-generation X-ray sources, which have been a boon for non-linear spectroscopies.
This proof-of-concept experiment is the first step in developing new ways to probe new physics and is one example of how X-ray free-electron lasers have driven a wealth of scientific development, not just for new experimental techniques but for the creation of new detectors, electronics, and high-performance computing infrastructure.3
References and Further Reading
- K. Voigt, M. Zhang, K. Ramakrishna, A. Amouretti, K. Appel, E. Brambrink, V. Cerantola, D. Chekrygina, T. Döppner, R. W. Falcone, K. Falk, L. B. Fletcher, D. O. Gericke, S. Göde, M. Harmand, N. J. Hartley, S. P. Hau-Riege, L. G. Huang, O. S. Humphries, M. Lokamani, M. Makita, A. Pelka, C. Prescher, A. K. Schuster, M. Šmíd, T. Toncian, J. Vorberger, U. Zastrau, T. R. Preston and D. Kraus, (2021) Demonstration of an x-ray Raman spectroscopy setup to study warm dense carbon at the high energy density instrument of European XFEL. Phys. Plasmas, 28, 082701, https://doi.org/10.1063/5.0048150
- N. Rohringer (2019) X-ray Raman scattering: a building block for nonlinear spectroscopy. Phil. Trans. R. Soc. A. 377201704712017047, https://doi.org/10.1098/rsta.2017.0471
- C. Pellegrini (2016) X-ray free-electron lasers: from dreams to reality. Phys. Scr., T169, 014004, http://dx.doi.org/10.1088/1402-4896/aa5281