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Electromagnetic radiation is one of the most powerful tools for studying the structure and dynamics of matter. A major milestone was the development of spectral analysis by Bunsen and Kirchhoff (1861) in Heidelberg: Since each element emits light only at characteristic wavelengths, it can be identified by analysing the light spectrum. Quantum mechanics explains this behaviour by linking each wavelength to a transition between discrete atomic energy levels. This correspondence forms the basis for experiments that use light to measure and control the quantum dynamics of matter, as conducted at the MPIK.
Not all transitions between energy levels behave the same way. Some respond to a broad range of wavelengths, while others react only to a very narrow one – a property known as spectral width. Extremely narrow transitions are especially valuable because they enable ultra‑precise measurements, for example in the development of atomic clocks or in the search for new physics beyond the Standard Model. The disadvantage: they require extremely intense light sources. It’s a bit like target practice — a large goal is easy to hit but gives little precision; a small goal is precise but rarely struck.
Among the narrowest known transitions are the Mössbauer nuclear transitions, discovered in Heidelberg in 1958 by Rudolf Mössbauer (Nobel Prize 1961). These transitions involve hard x‑ray radiation and are now used across the sciences — from analysing pigments in ancient artworks and searching for water on Mars to testing fundamental physical principles.
Today’s most powerful x‑ray sources are x‑ray free‑electron lasers (XFELs). In kilometer‑long facilities, electrons are accelerated to near the speed of light and forced to oscillate in magnetic fields, producing intense ultrashort x‑ray flashes. XFELs allow scientists to observe atomic and molecular motion in real time. However, their light covers a spectral width about a billion times broader than that of the well‑studied Mössbauer transition in iron‑57. So, only a tiny fraction of the radiation interacts effectively with the nuclei, restricting the experimental progress.
To overcome this challenge, scientists in Heidelberg have developed a new concept to generate intense and spectrally narrow x‑ray radiation suitable for probing Mössbauer nuclei. The principle is inspired by the operation of the XFELs, in which electrons emit X-rays as described above. XFELs achieve their enormous intensity through the spatial bunching of electrons, which causes their emitted waves to interfere constructively, in a mechanism known as superradiance.
In the new approach, the researchers propose to recycle the spatially structured electron bunches produced by XFELs, which are normally discarded after the XFEL light is produced. But instead of wiggling the electrons to generate radiation as in the XFEL, the spectrally narrow transitions in Mössbauer nuclei themselves are used to produce x-rays. When directed along the surface of a crystal made of Mössbauer nuclei, the moving electrons’ electric field can excite the nuclei to emit coherent, narrow‑band x‑ray radiation. “The spatial bunching of the electron beam again leads to an enormous interferometric enhancement of the emitted intensity”, explains the first author Ze-An Peng. By systematically analysing all possible scattering geometries, the team identified the optimal configuration, outperforming earlier methods to generate Mössbauer radiation using crystals by many orders of magnitude. “Since the crystal is only centimeter-sized and reuses existing XFEL electron beams, the source could operate in parallel without building a new large-scale facility,” Peng explains.
Looking ahead, this concept opens access to previously unexplored excitation regimes in Mössbauer science and holds potential to extend the capabilities of current XFELs significantly — enabling new precision experiments without additional major infrastructure.
Original publications:
Superradiant parametric Mössbauer radiation source
Ze-an Peng, Christoph H. Keitel and Jörg Evers
Physical Review Letters 137, 035001, DOI: https://doi.org/10.1103/ydpq-zy7k
Parametric Mössbauer radiation in the grazing-diffraction geometry: Specular diffraction and superradiant amplification
Ze-an Peng, Christoph H. Keitel and Jörg Evers
Physical Review A 114, 013115, DOI: https://doi.org/10.1103/s93l-3hr5
Weblinks:
Division ‘Theoretical Quantum Dynamics and Quantum Electrodynamics’ at MPIK
