“Quantum boiling” reveals relativity in atoms

In a way of “quantum boiling” with intense X-ray flashes, scientists have stripped xenon atoms of most of their electrons. The experiments reveal the impact of Albert Einstein's theory of Special Relativity on the quantum structure of atoms. The international team around Sang-Kil Son and Robin Santra from CFEL and Daniel Rolles and Artem Rudenko from Kansas State University report their study in the current issue of Nature Communications.

Quantum boiling evaporates electrons from the atom in a similar way conventional boiling evaporates water molecules. Illustration: Sherin Santra

“Understanding atomic structure is fundamentally important,” explains Son. “Quantum mechanics tells us how electrons are placed in different atomic shells.” The atomic shell structure is the base for Mendeleev’s periodic table and determines chemical properties of atomic elements. “Electrons in the outermost atomic shells typically move already at about one percent of the speed of light,” adds Santra. “The electrons in inner atomic shells, however, move even faster, particularly in heavier atoms. Then, quantum mechanics must be complemented by the theory of special relativity to accurately describe the atomic structure.”

Strong X-ray light can evaporate electrons from atoms in a process that is similar to the boiling of water: The X-rays only knock out a small number of electrons directly, dislodging them from the inner shells. The holes left by the missing electrons quickly “bubble up” towards the outer shells, transferring energy to other electrons, ultimately kicking them out of the atom, too. “When boiling water, the collisions between molecules play a central role, and in analogy the collisions of electrons play the central role in quantum boiling of atoms,” explains Son.

Producing the strongest ever X-ray flashes, accelerator driven devices called free-electron lasers have made quantum boiling a reality, creating conditions corresponding to a temperature of a few hundred million degrees Celsius, higher than the temperature at the center of the Sun. Quantum boiling of atoms lays bare the atomic inner shells and, thus, makes the strong relativistic effects visible in a new way.

In the experiment, xenon atoms were exposed to intense X-ray pulses generated by the X-ray free-electron laser LCLS at the US National Accelerator Laboratory SLAC in California. This created xenon ions with high electric charges, depending on the number of electrons that were knocked out of the atoms. When counting how many atoms had lost how many electrons, the scientists noted three distinctive bumps at telltale charge states. The theory of quantum mechanics, together with the theory of special relativity, confirmed the experimental observation and explains the mechanism how to lose electrons. 

“We could show that the bumps correspond to a relativistic splitting of the second shell into certain distinct energy levels,” says Santra. This relativistic splitting of energy levels is a very well-known effect in atomic physics, called fine structure. However, while the energy differences in the fine structure of the lightest element, hydrogen, is around 50 microelectronvolts (50 millionths of an electron volt), the observed energy difference in xenon is almost a ten million times as large, amounting to 300 electronvolts.

Predicting the dynamical behavior of atomic charge states was extremely challenging and took several months on high-performance computers, but ultimately led to success: Not only did the calculations provide a successful description of quantum boiling and the accompanying manifestation of relativity, but they also demonstrate the predictive power of newly developed computational tools for future experiments with strong X-ray free-electron lasers.