collisions
collisions

Electric field makes ultracold molecules stick together

When molecules at room temperature collide, e.g. in the atmosphere, they form a complex that falls apart again within picoseconds. In ultracold collisions, at temperatures of only one millionth of a degree Celsius above absolute zero, the so-called “sticking time” can be a million times longer, in the order of microseconds. In a theoretical study published in Physical Review Letters, researchers from the Institute for Molecules and Materials (IMM) of Radboud University show that a small electric field of around 10 V/cm, comparable to the field between the connectors of a household battery, increases the sticking time by several orders of magnitude. Such an electric field is a hundred times too weak to polarize the molecules. So, how can such a small electric field lead to such a huge increase in the sticking time?

Density of states

The sticking time of ultracold collisions is determined by just one property: its density of states. The closer the energy levels of the collision complex are, the longer the sticking time. A special feature of ultracold collision complexes is that they do not rotate at all. In the language of quantum mechanics this means that only states with total angular momentum J=0 are accessible, and only those states should be counted when computing the density of states. However, when there is an external electric field present and the collision complex has an electric dipole moment, the field will exert a torque on the complex and it starts rotating trying to align with the field. Hence, when computing the lifetime of a collision complex in a field, also states with J>0 must be counted and the resulting lifetime becomes orders of magnitude longer.

Illustration of the interaction between two ultracold molecules

Illustration of the interaction between two ultracold molecules (surface) and the interaction between a molecular dipole and the electric field.

These principles were already known, but it was not clear how strong the electric field must be for this effect to occur. The research team found that the effect of the field is much stronger at low temperatures, because sticking times are longer and there is more time for the complex to start rotating and reach the initially inaccessible rotating states. “We computed the effect in several ways, using classical simulations and quantum mechanics, and found similar results. This gave us the confidence to make predictions that can be tested experimentally”, first author Marijn Man says.

Loss prevention and stability

So why is this study of these ultracold collisions important and relevant? Collisions between molecules often form the dominant loss mechanism in ultracold molecular gases, limiting the lifetime of the gas. By better understanding these collisions we might find a convenient way to prevent losses and make ultracold gases more stable. These ultracold gases can be used in many exciting applications, one of these applications is quantum simulation. Here the ultracold gas is used to mimic the behavior of interesting but poorly understood materials.

Theoretical Chemistry

Tijs Karman is Assistant Professor in the Theoretical and Computational Chemistry department. The group is part of IMM. They aim to explain and predict properties of molecules, clusters, and molecular solids. Part of the research is to study quantum phenomena in molecular collisions. Karman focuses on theoretical research into collisions between ultracold molecules, which are promising for quantum computing and simulation. Ultimately realizing these applications requires understanding and controlling collisions and interactions between molecules.

Article information

Symmetry breaking in sticky collisions between ultracold molecules
Marijn P. Man, Gerrit C. Groenenboom, and Tijs Karman
Physical Review Letters 129, 243401 (2022)
DOI 10.1103/PhysRevLett.129.243401

More information?

For more information, please contact
Tijs Karman: t.karman [at] science.ru.nl
IMM Communications: imm-communication [at] ru.nl (imm-communication[at]ru[dot]nl)

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