Ion man: diving into depths of plasmas

Catholic University of Louvain

It had seemed impossible but he did it! Matthieu Génévriez observed Rydberg states within molecular ions, the charged particles that are one of the components of plasma. This provides a better understanding of the behaviour of this fourth state of matter, present in the outer layers of atmosphere surrounding planets and in the outermost layers of stars.

Plasma is found in the most distant layers of the atmosphere or the outermost layers of the sun. Observable on Earth in the form of lightning, shooting stars or the aurora borealis, this fourth state of matter, alongside the solid, liquid and gaseous states, is composed mainly of charged particles (positively or negatively). Astrophysicists are interested in understanding its behaviour in order to model it and make predictions. To do this, it’s first necessary to understand the behaviour of plasma’s constituent particles, ions, and more particularly their structure and reactivity, or in other words their ability to collide and/or assemble with each other.

After earning a PhD in atomic and molecular physics at UCLouvain, Dr Génévriez joined ETH Zürich for a postdoctorate. There, he became particularly interested in Rydberg states of molecular ions. “Rydberg states are states in which a molecule or atom has a highly excited electron that spends its time away from the centre of the molecule or atom.” These states are known at the scale of the atom and the neutral molecule, but are still poorly understood for molecular ions, i.e. charged molecules such as those found in plasmas. But Dr Génévriez was not to be outdone: “These molecular ions are indeed difficult to observe and we wanted to know how to observe and characterise them.” To do this, the young researcher developed a new technique. Or rather, he hijacked the use of an existing technique to meet his challenge! “We used laser quantum control. This technique is normally used to control the results of a chemical reaction. Here we used it to observe Rydberg states within molecular ions.”

In a study published in Physical Review, Dr Génévriez and his colleagues demonstrated that:

  • Rydberg states are indeed present in molecular ions;
  • it’s possible to observe and characterise them using the technique the team developed;
  • the states provide information on the reactivity of the ions in the plasma;
  • molecular ions excited into a Rydberg state tend to break very quickly because the positive charge tends to jump from one atom to another.

The excellence of this work hasn’t escaped the attention of the editors of Physics, the magazine of the American Physical Society, which has featured it on its cover. The magazine highlights a few scientific publications per year whose results are judged to be of exceptional quality and which “will change the course of research, inspire a new way of thinking or arouse curiosity.” Very few publications are selected per year and their appearance in Physics allows for broad dissemination of the results throughout the physics community.

Today, back at UCLouvain with this new expertise, Dr Génévriez will use the quantum control technique to detect Rydberg states in molecular ions of interest, such as in the CO2+ – which is found in the outermost layers of the atmosphere -, nitrogen (N2+), oxygen (O2+) or even helium (He2+).

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