31.10.2016 change 31.10.2016

Hydrogen-helium collisions at high and low rotation speeds

The course of collisions between atoms and molecules depends to some extent on their rotation and orientation in space. This aspect of collisions between molecules of hydrogen and helium atoms, common in the universe, has been examined by scientists, including researchers from Poland.

Chemical reactions, which we encounter in everyday life, occur due to collisions of particles. It might seem that such collisions resemble collisions between billiard balls. But it is not so simple: after all molecules may have very irregular shapes, and on top of that they rotate very fast. The course of reactions is further complicated at extremely low temperatures, where the unusual effects of the quantum world come into play.

Researchers want to figure out and describe all these dependencies, to better predict the course of chemical reactions and correctly interpret the results of experiments.

Scientists from Israel, the Netherlands, Germany and Poland tackled the seemingly simple collision of a helium atom with a molecule of hydrogen. Their work has been published in the prestigious "Nature Physics". Researchers analysed the situation, in which the helium atom in an excited state collides with a hydrogen molecule (composed of two hydrogen atoms) and causes its ionisation - breaking it up into positively charged ion and free electron. In the experiment, the course of such reactions was examined in detail at temperatures close to minus 273 degrees centigrade.

"Helium and hydrogen collisions are the basic collisions that occur in space" - commented in an interview with PAP one of the study authors, Dr. Piotr Żuchowski from the Institute of Physics, Nicolaus Copernicus University in Toruń. The scientist explained that these studies will allow to betted understand what happens in interstellar matter. In addition, understanding of collisions at low temperatures it is crucial for the production of ultracold matter, which will be used in quantum computing and the next generation of time standards, such as optical clocks.

According to Dr. Żuchowski, the interactions between the ideal balls are isotropic - regardless of how we rotate the ball and from which side we hit it, it should move in the same way. "In our study instead of two balls we had a ball - a helium atom - and a dumbbell - a hydrogen molecule. The interaction between them depends on such factors as the angle between the axis of the dumbbell and the ball" - said physicist and added: "These interactions are not isotropic and depend on spatial orientation".

Another study author, Dr. Mariusz Pawlak from the Faculty of Chemistry, Nicolaus Copernicus University in Toruń clarified that in the conditions close to absolute zero so-called resonances can be clearly observed - characteristic values of the impact energy at which the reaction occurs much faster.

He explained that if the hydrogen molecule rotates, there are two resonances: two values of energy, at which there is the best chance that the system will break. If the system is not rotating, there is only one resonance. "By toggling the rotating state of the molecule we change the structure of resonances" - said Pawlak and explained that this was the result of experiments. In the framework of quantum theory, the scientists demonstrated why this happens.

Piotr Żuchowski added: "The theory had to be revised to accurately predict collisions of atoms and molecules in subsequent experiments." Dr. Pawlak commented: "Our work will help those who want to design new experimental systems in ultracold physics. The theory can help predict how atoms will collide with molecules".

Understanding the mechanisms governing the interactions and collisions of molecules and atoms is crucial in a number of different fields of science: physics and chemistry of the atmosphere, astrophysics, chemistry processes of combustion processes and in many biochemical processes.

These studies were made possible by a unique experiment conducted at the Weizmann Institute in Israel, by the group led by Prof. Edvardas Narevicius. Necessary theoretical support, in addition to Dr. Żuchowski and Dr. Mariusz Pawlak, was provided by groups of colleagues at the Technion university in Haifa (Israel), universities in Kassel and Dusseldorf (Germany) and Radboud University in Nijmegen (Netherlands).

PAP - Science and Scholarship in Poland, Ludwika Tomala

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