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Cold Chemistry

Molecular Cooling

For many years the Jun Ye group has been working with a technique known as Stark deceleration to slow down molecules and investigate their collision and cooling behaviors. One seminal experiment in 2012 (using this and other techniques) resulted in the first-ever evaporative cooling of hydroxyl radical molecules  (*OH) down to temperatures of no more than 5 mK.  The final temperature of this landmark experiment may well have been even lower because the group’s temperature measurement system stopped working at 5 mK. This seminal work occurred after nearly two decades of unsuccessful efforts in other laboratories to prepare “real-world” molecules at ultracold temperatures.

Earlier attempts to cool molecules to ultraslow temperatures failed because the target molecules had too few “elastic” collisions, the kind of collisions in which molecules bounce off one another. Thanks to insightful theory work by the John Bohn group, the Ye group opted to cool the *OH molecule, which had been predicted to undergo elastic collisions more than 90% of the time. The high collision rate meant the molecules had time to exchange energy, leading to some molecules with more energy than average and some with less. This variation is critical for evaporative cooling.

To cool the molecules, researchers first used Stark deceleration to bring the molecules to a complete stop in the center of a magnetic trap. This step cooled the molecules to ~ 50 mK.  Then the researchers initiated an innovative evaporative cooling technique that employed an applied electric field to open up little gateways in the trap to let out the hottest molecules. This process removed the most energetic *OH molecules, which lowered the temperatures of the remaining gas of molecules. Lowering the height of the trap speeded up the concentration of the colder molecules at the bottom of the trap.

As the molecular gas got colder, the evaporative cooling process worked better and better. Not only did the temperature rapidly fall, but also the density of the remaining molecules increased. These results may open the door to cooling a gas of *OH molecules down to the point where every molecule enters its lowest quantum state (in a process similar to that used to make the world’s first Bose-Einstein condensate of atoms).
 
In other recent work, the Ye group combined a Stark decelerator with two other experimental techniques to create an ongoing cold-molecule experiment for studying molecular collisions at temperatures of approximately 5 K. Collisions at this temperature are quite interesting because they occur in a crossover region between the classical and quantum worlds.

In the crossover region, interactions between two molecules are “simple” enough to be modeled quantum mechanically. However, these interactions are far more complex than those between ultracold molecules at much lower temperatures. The desire to investigate these interactions in the laboratory led to the creation of the cold-molecule experiment.

The group used the Stark decelerator and magnetic trap in the experiment to slow and trap a beam of hydroxyl molecules (OH) for collision studies with deuterated ammonia, or ND3. The group used adjustments in external electric fields to increase the number of collisions between these two dipolar molecules. The researchers were able to identify both elastic and inelastic collisions during a series of investigations using the cold-molecule experiment.

The cold-molecule experiment is well suited for studying a variety of molecules found on Earth and in space.

Cold Chemistry

The Heather Lewandowski group investigates molecular dynamics and chemical reactions at temperatures of tens of milliKelvin. Lewandowski and her students want to learn more about the strong correlation between cold chemical reactions in the laboratory and the interstellar chemistry that takes place at temperatures of about 3 K. More importantly, they want to understand how chemical reactions proceed when all of the states of the molecules can be controlled in the quantum regime.

The Lewandowski group has begun studies of the control of cold chemical reactions with the precise manipulation of both the internal states of atoms and molecules and their external states (i.e., position, velocaty, and orientation) during interactions. The control of external states of atoms and molecules at milliKelvin temperatures is much easier than at warmer temperatures. A wide distribution of their position, velocity, and orientation occurs at higher energies, making it nearly impossible to precisely control these states.

Lewandowski and her group are currently developing the tools they need to figure out how to precisely control external states of atoms and molecules at very cold temperatures. This control will allow them to meet the challenge of investigating how a change in a single internal or external parameter of a reactant can alter a chemical reaction. In the long term, this research is expected to lead to a precise and detailed understanding of specific chemical reactions on the quantum mechanical level.

The group uses Stark deceleration to cool and slow packets of molecules until the packets come to a stop at temperatures in the tens of milliKelvin range. At these temperatures, there’s simply not enough energy available to excite the atoms and molecules out of their ground electronic, rotational, or vibrational states. Once they are in this very low-energy state, the researchers can trap molecules and observe them for periods of 1–10 seconds—which is 3–5 orders of magnitude longer than scientists are able to observe chemical reactions in molecular beams.

The long observation times should allow the group to observe collisions and slow chemical reactions. One important goal is to observe and study quantum mechanical tunneling through the activation barrier that can inhibit chemical reactions. At very cold temperatures, quantum mechanical theory suggests that the system may tunnel through such barriers and initiate chemical reactions.

The Lewandowski group has embarked on an experimental project to understand the quantum mechanical nature of a simple chemical reaction, the charge-exchange reaction between NH3+ and Rb that yields NH3 and Rb+. This experiment is closely related to other experiments on cold molecule collisions in the Lewandowski lab.

The Lewandowski group studies collisions between deuterated ammonia molecules (ND3) and ultracold Rb atoms. In this work, the researchers cool and trap Rb atoms at the intersection of laser-cooling beams. A pulsed valve creates a beam of cold ND3 molecules. To combine the cold molecules and ultracold atoms, the researchers physically move the coils that form the atom trap across the table until the atom trap overlays the molecule trap. With the traps superimposed, atom-molecule collisions are likely to occur. The researchers discovered that electric fields made collisions occur faster than expected and increased the likelihood that collisions would change the quantum state of an ND3 molecule.