New Cooling Technique for Observing Collisions Between Cold Molecules

At very low temperatures, how do chemical reactions proceed? An investigation of molecular samples that are cold, slow and dense at the same time is needed in order to answer this question.

Scientists guided by Dr. Martin Zeppenfeld from the Quantum Dynamics Division of Prof. Gerhard Rempe at the Max Planck Institute of Quantum Optics in Garching have recently taken a vital step in this direction by coming up with a new cooling method: the so-called “cryofuge”, which incorporates cryogenic buffer-gas cooling with a particular kind of centrifuge in which rotating electric fields decelerate the precooled molecules down to velocities of less than 20 m per second.

The team succeeded in observing collisions between the cold molecules because of the high flux densities that were attained. For two chemical compounds possessing a strong electric dipole moment, the collision probability as well as its dependence on flux density and velocity was thus determined (Science, 13^th October 2017). The new technique is considered to be a milestone for the developing field of cold chemistry and could also open the outlook towards controlling and manipulating chemical pathways at very low temperatures.

The production of cold molecules has been considered to be a huge challenge: laser-cooling – an extremely efficient method for atoms – in general does not work for molecules as they showcase rotational and vibrational states besides the electronic states. On the other hand, a huge number of molecules, for example, water (H₂O), comprise of an uneven electric charge distribution. Molecules with such an electric dipole moment are capable of being influenced and hence get decelerated by electric fields.

The MPQ team has frequently experimented with deuterated ammonia (ND₃) and fluoromethane (CH₃F). Originally, the molecules have a velocity of several hundred meters per second and a temperature of around 200 Kelvin. As an initial step, the molecules thermalize with a helium or neon buffer-gas in the cryogenic buffer-gas cell and then get cooled down to 6 Kelvin (helium) and 17 Kelvin (neon) respectively. This is followed by extracting them from the cryogenic environment by a bent electrostatic quadrupole guide. When they leave the buffer-gas cell, their speed gets reduced to 50 to 100 m per second.

However, it is not only the velocity that matters. Regarding the molecular collisions that we aim to observe it is crucial that during this cooling process also the internal states are being cooled. We can prove that only very few and low rotational and vibrational states are excited.

Dr. Martin Zeppenfeld, Leader of the Project

With the help of a straight guide the molecules are transferred to the centrifuge decelerator, which is the second part of the cooling device.

“Varying the guiding voltage on the straight guide we can control the trap depth and thereby the molecular beam densities,” explains Thomas Gantner, Doctoral Candidate at the experiment. “The higher the voltage, the higher the beam density. This kind of control is necessary in order to get a better understanding of the mechanisms behind the cold dipolar collisions that we are going to measure after the deceleration process.”

The molecules, as they enter the centrifuge, first propagate around the periphery in a stationary storage ring with a diameter of 40 cm composed of two rotating and two static electrodes. The molecules are then picked up by a rotating electric quadrupole guide almost at any point around the storage ring and they are pushed along its spiral shape towards the rotation axis. Thus, while the electric fields enable the molecules to move into the center of the disk, they will constantly have to counteract the centrifugal force induced by the quadrupole guide that rotates at 30 Hertz, thus constantly slowing the molecules down.

A final straight guide helps in bringing the molecules to a quadrupole mass spectrometer where they are analyzed with regards to their velocity.

The molecules spend about 25 milliseconds inside the quadrupole guide. This is plenty of time for them to interact, and in these collisions, molecules are being lost. The analysis reveals that the losses increase for decreasing velocities and increasing beam densities. The evaluation of the data relies to a large extent on model calculations that were done by Xing Wu, who is first author of this work and achieved his doctoral degree on this experiment.

Thomas Gantner, Doctoral Candidate

“The observation of cold molecular collisions is a milestone for the field of cold chemistry,” emphasizes Dr. Zeppenfeld. “The generic principle underlying the cryofuge enables its application to a wide range of dipolar compounds. We envision the possibility that in the future chemical reactions with long interaction times can be realized at very low temperatures.”

Additionally, the cryofuge is capable of extending the range of research topics offered by experiments with cold molecules. For example, the slow and cold beam of methanol produced could be preferably suited for measuring differences in the proton-to-electron mass ratio.

Based on theoretical predictions, these could be brought about by interaction with dark matter. The cryofuge is also capable of serving as a perfect source for current experiments with laser-coolable diatomic molecules. The long-range and anisotropic dipole coupling, on the other hand, mediates interactions over micrometer distances. This renders cold polar molecules mainly ideal for applications in quantum computing or quantum simulation.

The very first observation of collisions in a cold gas of naturally occurring molecules brings us closer to the dream of achieving a complex quantum gas such as a Bose Einstein condensate of water molecules.

Professor Gerhard Rempe