Unlocking the secrets of space chemistry with cold Coulomb crystals

Coulomb crystals are surrounded by molecules used in the Lewandowski laboratory to study astrochemical reactions. Credit: Steven Burrows/Olivia Krohn and the Lewandowski Group

Researchers at the University of Colorado Boulder have developed experiments to replicate the chemical reactions of the interstellar medium, using techniques such as laser cooling and mass spectrometry to observe interactions between ions and molecules.

Although it may not look like it, the interstellar space between the stars is far from empty. Atoms, ions, molecules and more reside in this ethereal environment known as the Interstellar Medium (ISM). The ISM has fascinated scientists for decades, because at least 200 unique molecules are formed in the cold, low-pressure environment. It’s a topic that links the fields of chemistry, physics and astronomy, as scientists from each field try to determine what types of chemical reactions occur there.

Now, in the recently published cover story of the Journal of Physical Chemistry AJILA Fellow and University of Colorado Boulder Physics Professor Heather Lewandowski and former JILA graduate student Olivia Krohn highlight their work to mimic ISM conditions by using Coulomb crystals, a cold pseudocrystalline structure, to see how ions and neutral molecules interact with each other.

From their experiments, the researchers solved the chemical dynamics in ion-neutral reactions by using precise laser cooling and mass spectrometry to control quantum states, allowing them to successfully mimic ISM chemical reactions. Their work brings scientists closer to answering some of the most profound questions about the chemical evolution of the cosmos.

Filter by Energy

“The field has long thought about which chemical reactions will be most important in telling us more about the composition of the interstellar medium,” explains Krohn, the paper’s first author. “A very important group of these are the ion-neutral molecular reactions. That is exactly what this experimental device from the Lewandowski group is suitable for, to study not only ion-neutral chemical reactions, but also at relatively low temperatures.”

To begin the experiment, Krohn and other members of the Lewandowski group loaded an ion trap into an ultra-high vacuum chamber with different ions. Neutral molecules were introduced separately. Although they knew which reactants would participate in the ISM-type chemical experiment, the researchers were not always sure which products would result. Depending on their test, the researchers used different types of ions and neutral molecules, similar to those in the ISM. This included CCl+ ions fragmented from tetrachloroethylene.

“CCl+ is predicted to be in different parts of space. But no one has been able to effectively test its reactivity with experiments on Earth because it is so difficult to make,” Krohn adds. “You have to break it down from tetrachloroethylene with UV lasers. This creates all kinds of ion fragments, not just CCl+, which can complicate things.”

Whether calcium or CCl+ ions, the experimental setup allowed the researchers to filter out unwanted ions using resonant excitation, leaving behind the desired chemical reactants.
“You can shake the trap at a frequency that resonates with the mass-to-charge ratio of a particular ion, and this ejects them from the trap,” says Krohn.

Laser cooling to create Coulomb crystals

After filtering, the researchers cooled their ions using a process known as Doppler cooling. This technique uses laser light to reduce the movement of atoms or ions, effectively cooling them by using the Doppler effect to preferentially slow down particles moving towards the cooling laser. As the Doppler cooling lowered the temperature of the particles to millikelvin levels, the ions arranged themselves into a pseudo-crystalline structure, the Coulomb crystal, which was held in place by the electric fields in the vacuum chamber. The resulting Coulomb crystal had an ellipsoid shape with heavier molecules sitting in a shell outside the calcium ions and pushed out of the center of the trap by the lighter particles due to the differences in their mass-to-charge ratio.

Thanks to the deep trap containing the ions, the Coulomb crystals can remain trapped for hours, and Krohn and the team are able to capture them in this trap. Analyzing the images, the researchers were able to identify and monitor the reaction in real time, seeing the ions self-organize based on mass-to-charge ratios.

The team also determined the quantum state dependence of the reaction of calcium ions with nitric oxide by fine-tuning the cooling lasers, which helped produce certain relative populations of quantum states of the trapped calcium ions.

“The nice thing about it is that it uses one of these more specific atomic physics techniques to look at quantum solvated reactions, which I think is a little more than the physical essence of the three fields: chemistry, astronomy and physics, even though all three are still always be involved,” Krohn adds.

Time is everything

In addition to trap filtration and Doppler cooling, the researchers’ third experimental technique helped them mimic the ISM reactions: their time-of-flight mass spectrometry (TOF-MS) setup. In this part of the experiment, a high-voltage pulse accelerated the ions through a flight tube, where they collided with a microchannel plate detector. The researchers were able to determine which particles were present in the trap based on the time it took for the ions to hit the plate and their imaging techniques.

“This has allowed us to do a number of different studies where we can resolve adjacent masses of our reactants and product ions,” Krohn adds.

This third branch of the experimental apparatus for ISM chemistry improved the resolution even further, as the researchers now had multiple ways to determine which products were created in the ISM-like reactions and their respective masses.

Calculating the mass of the potential products was especially important because the team could then swap their initial reactants with isotopologues of different masses and see what happened.

As Krohn explains: “That allows us to do cool tricks, like replacing hydrogen atoms with deuterium atoms or replacing different atoms with heavier isotopes. When we do that, we can see from time-of-flight mass spectrometry how our products have changed, which gives us more confidence in our knowledge of how to assign what those products are.”

Because astrochemists have observed more deuterium-containing molecules in the ISM than expected from the observed atomic deuterium-to-hydrogen ratio, swapping isotopes in experiments like this can bring researchers one step closer to determining why this is so.

“I think this allows us to properly detect what we’re seeing in this case,” Krohn says. “And that opens more doors.”

Reference: “Cold ion-molecule reactions in the extreme environment of a Coulomb crystal” by OA Krohn and HJ Lewandowski, February 15, 2024, The Journal of Physical Chemistry A.
DOI: 10.1021/acs.jpca.3c07546

This work was supported by the National Science Foundation and the Air Force Office of Scientific Research.