A brake for spinning molecules

The precise control of the rotational temperature of molecular ions opens up new possibilities for laboratory-based astrochemistry

Cold does not equal cold for physicists. This is because in physics, there is a different temperature associated with each type of motion that a particle can have. How fast molecules move through space determines the translational temperature, which comes closest to our everyday notion of temperature. However, there is also a temperature for the internal vibrations of a molecule, as well as for the rotational motion around their own axes. Similar to a stationary car with its engine running, the internal rotation (the engine, in this case) does not translate into motion before the clutch is released. In the case of molecules, the many microscopic collisions between the particles which constitute gases, fluids, and solids couple the various forms of motion with each other.

The different temperatures thus approach each other over time. Physicists then say that a thermal equilibrium has been established. However, how fast this equilibrium is reached depends on the collision rate, as well as on any external influences working against this equilibration. For example, the infrared radiation emanating from the contraction of an interstellar gas cloud can cause the rotation of molecules to quicken, even without changing the speed at which the molecules are travelling. These kinds of processes take a very long time in the emptiness of space, as there are very few collisions there.

The cooling method for the rotational temperature is quick and versatile

Time is totally irrelevant at cosmic dimensions but with physical experiments it is crucial. Indeed, physicists can nowadays reduce the flight speed of molecules relatively quickly to almost absolute zero at -273.15 °C. However, it takes several minutes or hours for the rotation of non-colliding particles to cool to a similar level, making some experiments almost impossible. This may be about to change.

“We have managed to cool down the rotation of molecular ions in milliseconds, and down to lower temperatures than previously possible,” says José R. Crespo López-Urrutia, Group Leader at the Max Planck Institute for Nuclear Physics. The researchers from the Max Planck Institute in Heidelberg and the group led by Michael Drewsen at Aarhus University froze molecular rotational motion at 7.5 K (or -265.65 °C). And not only that, as Oscar Versolato from the Max Planck Institute in Heidelberg, who played an important role in the experiments, explains: “With our methods we can choose and set a rotational temperature between about seven and 60 Kelvin, and are able to accurately measure this temperature in our experiments.” Unlike other methods, this cooling principle is very versatile, being applicable to many different molecular ions.

In their experiments, the team used a cloud of magnesium ions and magnesium hydride ions using methods pioneered in Aarhus. This ensemble was “confined” in an ion trap known as CryPTEx, which was developed by researchers at the Max Planck Institute for Nuclear Physics (see Background). The trap consists of four rod-shaped electrodes that are arranged in parallel, in pairs aligned one above the other and having opposite electrical polarities. A high-frequency alternating voltage is applied to the electrodes to confine the ions in the centre close to the longitudinal axis of the trap. The trap is cooled to a few degrees above absolute zero, and there is an excellent vacuum so that adverse collisions are very rare.

Collisions with cold helium atoms slow down the rotation of the molecular ions

In the trap, the physicists cooled the magnesium ions using laser beams which, to put it simply, slow down the ions with their photon pressure. The magnesium hydride ions in turn cool because of their interaction with the magnesium ions. This allowed the researchers to cool the translational temperature of the cloud to minus 273 degrees Celsius until several hundred particles solidify to form a regular crystal. In such crystals, the distances between the particles are very large, in contrast to the situation in crystals familiar from minerals. The particles which the cold laser causes to emit light can thus be seen at their fixed positions under the optical microscope.

To apply a brake to the rotation of the molecular ions, and thus to reduce their rotational temperature, the team injected an extremely tenuous, cold helium gas into the trap. In the ion crystal, the helium atoms flying at a leisurely speed collide with the magnesium hydride ions rotating about their own axis trillions of times per second. The collisions cause the helium atoms to gradually slow down the molecular ions. “This process is similar to the tides,” explains José Crespo: "The rotating ion polarizing the neutral helium atom is a little bit like the moon producing the tidal bulges.” A dipole is thus induced in the helium atom, which tugs at the rotating molecular ion such that it rotates a little slower.

The helium atoms in the experiment mediate between the various temperatures as they transfer translational kinetic energy to the molecular ions in some collisions and remove rotational energy in others. This effect is also exploited by the team to heat the rotational motion of the molecular ions through the amplification of the regular micro-motion of trapped particles.