The slow positron beam apparatus used to fire positrons at a lithium fluoride crystal. Image: Professor Yasuyuki Nagashima from Tokyo University of Science.
The slow positron beam apparatus used to fire positrons at a lithium fluoride crystal. Image: Professor Yasuyuki Nagashima from Tokyo University of Science.

A positron is the antiparticle of an electron, with the same mass and spin as an electron but the opposite charge. It is an attractive particle for scientists, having led to important insights and developments in the fields of elementary particle physics, atomic physics, materials science, astrophysics and medicine. For example, positrons are known to be components of antimatter. They can also be used to detect lattice defects in solids and semiconductors, and for the structural analysis of the topmost surface of crystals.

Positronic compounds, namely bound states of positrons with regular atoms, molecules or ions, represent an intriguing aspect of positron–matter interactions and have been studied experimentally via observation of positron annihilation in gases. It may even be possible to generate new molecules and ions via the formation of positron compounds, but no research has ever been done from such a perspective.

Now, a research team that includes Yasuyuki Nagashima from Tokyo University of Science (TUS), Japan, has discovered an innovative way to explore the interactions between positrons and ionic crystals. This work, reported in a paper in Physical Review Letters, involved collaborative efforts from Takayuki Tachibana, a former assistant professor at TUS who is currently affiliated with Rikkyo University in Japan, and Daiki Hoshi, a former graduate student at TUS.

The researchers used a technique based on a well-explored phenomenon that occurs when a solid is bombarded with an electron beam. “It has long been known that when electrons are injected into a solid surface, atoms that make up the surface are ejected as monoatomic positive ions,” explains Tachibana. This process, known as electron-stimulated desorption, motivated the team to explore what would happen if a crystal was bombarded with positrons rather than electrons.

In their experiments, the researchers fired a beam of either positrons or electrons at the (110) surface of a lithium fluoride (LiF) crystal. Using carefully placed electric fields generated by deflectors, they could control the incident energies of the charged particles. Moreover, the deflectors allowed them to redirect any ions desorbed from the crystal towards an ion detector. The detected signals were then used to conduct a spectroscopic analysis to identify the precise composition of the desorbed ions.

They found that when the LiF crystal was bombarded with electrons, only the expected monoatomic ions, namely Li+, F+ and H+ (due to residual gases in the experimental chamber) were detected. However, bombarding the crystal with positrons led to the detection of positive molecular fluorine ions (F2+) and positive hydrogen fluoride ions (FH+). Notably, this is the first-ever report of molecular ions being ejected on bombardment with positrons.

After further analysis and experimentation, the researchers developed a desorption model to explain their observations. According to this model, as positrons are injected into a solid, some of them return to the surface after losing their energy. In the case of LiF crystals, these positrons may attract two neighboring negative fluorine ions on the surface to form a positronic compound. If the bound positron annihilates one of the fluorine ion’s core electrons, a special type of electron, known as an Auger electron, is emitted, resulting in a charge swap and the generation of a F2+ ion. This ion is pushed out of the crystal by the repulsing forces of the nearby Li+ ions.

The findings of this study could further scientists’ understanding of matter–antimatter interactions. “The stability and binding properties of positronic compounds provide unique perspectives on the interaction of antiparticles with ordinary substances, paving the way for novel investigations in the field of quantum chemistry,” says Tachibana. “The proposed method could thus pave the way for the generation of new molecular ions and molecules in the future.”

Notably, this approach could be leveraged in many applied fields. In materials science, it could be used to modify the surface of materials and study their properties with unprecedented precision. Other potential applications include cancer therapy, quantum computing, energy storage and next-generation electronic devices.

This story is adapted from material from Tokyo University of Science, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.