|An intense beam of high-energy X-ray photons (violet) hits two adjacent iridium atoms (green) in the crystal. This excites electrons in the atoms for a short time. The atoms emit X-ray photons which overlap behind the two iridium atoms (red) and can be analyzed as interference images. (Credit: Markus Grüninger, University of Cologne)|
An international research team led by physicists at the University of Cologne (Köln, Germany) has implemented a new variant of the basic optical double-slit experiment using resonant inelastic x-ray scattering at the European Synchrotron (ESRF; Grenoble, France).1 This new variant offers a deeper understanding of the electronic structure of solids.
The double-slit experiment is of fundamental importance in physics. In 1801, scientist Thomas Young diffracted light at two adjacent slits, thus generating interference patterns at a plane beyond the double slit and demonstrating the wave character of light. During the 20th century, scientists showed that electrons, atoms, or molecules scattered at a double slit show the same interference pattern, which contradicts the classical expectation of particle behavior but can be explained by quantum-mechanical wave-particle dualism.
X-ray scattering from a dimer
Now, the University of Cologne have investigated the phenomenon by means of resonant inelastic x-ray scattering (RIXS) via an iridium oxide (Ba3CeIr2O9) crystal. The crystal is irradiated with strongly collimated, high-energy x-ray photons. The x-rays are scattered by the iridium atoms in the crystal, which take over the role of the slits in Young's classical experiment. Due to the rapid technical development of RIXS and a judicious choice of crystal structure, the physicists were able to observe the scattering on two adjacent iridium atoms -- a so-called dimer.
"The interference pattern tells us a lot about the scattering object, the dimer double slit," says Markus Grüninger, who heads the research group at the University of Cologne. In contrast to the classical double-slit experiment, the inelastically scattered x-ray photons provide information about the excited states of the dimer, in particular their symmetry, and thus about the dynamic physical properties of the solid.
These RIXS experiments require a modern synchrotron as an extremely brilliant x-ray light source and a sophisticated experimental setup. To specifically excite only the iridium atoms, the scientists had to select the very small proportion of photons with the right energy from the broad spectrum of the synchrotron; the scattered photons are selected even more strictly according to energy and direction of scattering. Only a few photons remain as the signal. With the required accuracy, these RIXS experiments are currently only possible at two synchrotrons worldwide, one being the ESRF.
"With our RIXS experiment, we were able to prove a fundamental theoretical prediction from 1994," says Grüninger. "This opens a new door to a whole series of further experiments that will allow us to gain a deeper understanding of the properties and functionalities of solids."
1. A. Revelli et al., Science Advances (2019); doi: 10.1126/sciadv.aav4020.