Evaporative Cooling of Anions

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Physicists from Heidelberg and Innsbruck develop technique for cooling negatively charged molecules

Radio frequency trap, in which the negatively charged molecules are trapped and
Radio frequency trap, in which the negatively charged molecules are trapped and evaporated with light. | © Heidelberg University
Molecular anions, negatively charged molecules, are difficult to cool due to their specific electronic configuration. A team of physicists led by Matthias Weidemüller from Heidelberg University’s Institute for Physics and Roland Wester from the Department of Ion Physics and Applied Physics at the University of Innsbruck (Austria) have now developed a new method for cooling molecular anions to below three Kelvin, i.e. approximately minus 270 degrees Celsius, in a short time. According to the researchers, this enables, for example, new investigations of chemical reactions in space.

Cooling atoms and ions to near absolute zero - which is at zero Kelvin or minus 273,15 degrees Celsius - is routine in many laboratories today. As the particles can be very well controlled at these temperatures such systems provide an ideal platform for exploring many scientific questions. They are moreover the basis for precision clocks or quantum bits, which are fundamental to quantum computers, among other things. However, negatively charged ions, known as anions, are difficult to cool and thus to control in their movements. Researchers at Heidelberg University and the University of Innsbruck have now jointly further developed a technique for selectively sorting out the warmest particles from a cloud of molecular ions and thus cooling the remaining molecular ions to below three Kelvin.

In the experiment, the ions are enclosed in a radio frequency trap and spread along the longitudinal axis of the trap. This allows ions with higher energy to move further away from the centre of the trap. "We exploit this to selectively remove these ions from the trap," says Dr Eric Endres of the Department of Ion Physics and Applied Physics at the University of Innsbruck. "Using a focused laser beam positioned at the edge of the ion cloud, we neutralise the warmer ions. The photons from the laser thereby detach an electron from the anion, creating a neutral molecule that drops out of the trap." After the higher-energy ions evaporate, the remaining ions cool to a lower temperature. "By slowly moving the laser light towards the trap centre, the highest energy anions are evaporated one by one, leading to a temperature of 2.2 Kelvin in less than four seconds," explains Saba Hassan, a doctoral candidate in Prof. Weidemüller’s research group at the Institute for Physics of Heidelberg University.

Previously used techniques allow anions to be cooled down to three Kelvin. "With our further developed method, this barrier can now in principle be broken for any kind of negatively charged molecule, allowing many new investigations into the fundamentals of nature or, for example, of chemical reactions in space," state research group leaders Matthias Weidemüller and Roland Wester.

The research results were published in the journal "Nature Physics".

J. Tauch, S. Z. Hassan, M. Nötzold, E. S. Endres, R. Wester and M. Weidemuller: Laser-induced forced evaporative cooling of molecular anions below 4 K. Nature Physics (15 June 2023).