“To begin the experiment, Krohn and other members of the Lewandowski group packed an ion trap in an ultra-high vacuum chamber with different ions.”Whether utilizing calcium or CCl+ ions, the experimental setup allowed the researchers to filter out undesirable ions utilizing resonant excitation, leaving the preferred chemical reactants behind.”You can shake the trap at a frequency resonant with a particular ions mass-to-charge ratio, and this ejects them from the trap,” states Krohn.Cooling by means of Laser to Create Coulomb CrystalsAfter filtering, the scientists cooled their ions using a procedure understood as Doppler cooling. The resulting Coulomb crystal was an ellipsoid shape with much heavier molecules sitting in a shell outside the calcium ions, pressed out of the traps center by the lighter particles due to the distinctions in their mass-to-charge ratios.Thanks to the deep trap that contains the ions, the Coulomb crystals can remain trapped for hours, and Krohn and the team can image them in this trap. In analyzing the images, the researchers might recognize and keep an eye on the response in genuine time, seeing the ions organize themselves based on mass-to-charge ratios.The team likewise determined the quantum-state reliance of the reaction of calcium ions with nitric oxide by fine-tuning the cooling lasers, which helped produce particular relative populations of quantum states of the trapped calcium ions.
Coulomb crystals are surrounded by particles used in the Lewandowski lab to study astrochemical responses. Credit: Steven Burrows/Olivia Krohn and the Lewandowski groupResearchers at the University of Colorado Boulder have developed experiments to duplicate the chemical responses of the Interstellar Medium, utilizing strategies like laser cooling and mass spectrometry to observe interactions in between ions and molecules.While it might not look like it, the interstellar space between stars is far from empty. Atoms, ions, particles, and more live in this ethereal environment called the Interstellar Medium (ISM). The ISM has actually amazed scientists for decades, as a minimum of 200 distinct particles form in its cold, low-pressure environment. Its a topic that loops the fields of astronomy, chemistry, and physics, as scientists from each field work to identify what kinds of chain reactions occur there.Now, in the just recently published cover post of the Journal of Physical Chemistry A, JILA 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 utilizing Coulomb crystals, a cold pseudo-crystalline structure, to watch ions and neutral particles engage with each other.From their experiments, the scientists resolved chemical characteristics in ion-neutral reactions by utilizing precise laser cooling and mass spectrometry to control quantum states, thereby enabling them to imitate ISM chain reactions effectively. Their work brings researchers closer to responding to some of the most profound questions about the chemical advancement of the cosmos.Filtering through Energy”The field has long been thinking of which chain reactions are going to be the most crucial to inform us about the makeup of the interstellar medium,” describes Krohn, the papers first author. “An actually crucial group of those is the ion-neutral particle reactions. Thats precisely what this experimental apparatus in the Lewandowski group is fit for, to study not just ion-neutral chain reaction but also at reasonably cold temperature levels.”To begin the experiment, Krohn and other members of the Lewandowski group packed an ion trap in an ultra-high vacuum chamber with different ions. Neutral particles were introduced separately. While they knew the reactants entering into the ISM-type chemical experiment, the scientists werent always certain what items would be produced. Depending on their test, the scientists used different kinds of ions and neutral molecules similar to those in the ISM. This included CCl+ ions fragmented from tetrachloroethylene.”CCl+ has been anticipated to be in various areas of space. No ones been able to effectively evaluate its reactivity with experiments on Earth because its so tough to make,” Krohn adds. “You need to simplify from tetrachloroethylene using UV lasers. This creates all kinds of ion fragments, not just CCl+, which can complicate things.”Whether utilizing calcium or CCl+ ions, the experimental setup permitted the researchers to filter out undesirable ions using resonant excitation, leaving the wanted chemical reactants behind.”You can shake the trap at a frequency resonant with a particular ions mass-to-charge ratio, and this ejects them from the trap,” says Krohn.Cooling by means of Laser to Create Coulomb CrystalsAfter filtering, the researchers cooled their ions using a process referred to as Doppler cooling. This technique utilizes laser light to minimize the movement of atoms or ions, effectively cooling them by making use of the Doppler effect to preferentially slow particles approaching the cooling laser. As the Doppler cooling decreased the particles temperatures to millikelvin levels, the ions arranged themselves into a pseudo-crystalline structure, the Coulomb crystal, held in location by the electrical fields within the vacuum chamber. The resulting Coulomb crystal was an ellipsoid shape with heavier particles being in a shell outside the calcium ions, pressed out of the traps center by the lighter particles due to the differences in their mass-to-charge ratios.Thanks to the deep trap that includes the ions, the Coulomb crystals can remain trapped for hours, and Krohn and the group can image them in this trap. In evaluating the images, the researchers might monitor the reaction and identify in genuine time, seeing the ions organize themselves based upon mass-to-charge ratios.The team likewise figured out the quantum-state dependence of the response of calcium ions with nitric oxide by fine-tuning the cooling lasers, which helped produce specific relative populations of quantum states of the trapped calcium ions.”Whats fun about that is it leverages among these more particular atomic physics methods to look at quantum fixed responses, which is a little bit more, I think, of the physics essence of the three fields, astronomy, chemistry, and physics, despite the fact that all 3 are still involved,” adds Krohn.Timing Is EverythingBesides trap filtration and Doppler cooling, the scientists 3rd experimental method assisted them replicate the ISM reactions: their time-of-flight mass spectrometry (TOF-MS) setup. In this part of the experiment, a high-voltage pulse sped up the ions down a flight tube, where they clashed with a microchannel plate detector. The scientists could determine which particles were present in the trap based upon the time it took for the ions to strike the plate and their imaging techniques.”Because of this, weve been able to do a number of various research studies where we can fix surrounding masses of our reactant and item ions,” adds Krohn.This 3rd arm of the ISM-chemistry speculative device improved the resolution even further as the scientists now had multiple ways to determine which items were created in the ISM-type reactions and their particular masses.Calculating the mass of the possible items was particularly crucial as the group could then change out their preliminary reactants with isotopologues with various masses and see what happened.As Krohn elaborates, “That permits us to play cool techniques like replacing hydrogens with deuterium atoms or substituting various atoms with heavier isotopes. When we do that, we can see from the time-of-flight mass spectrometry how our products have changed, which gives us more confidence in our understanding of how to assign what those items are.”As astrochemists have observed more deuterium-containing molecules in the ISM than is gotten out of the observed atomic deuterium-to-hydrogen ratio, switching isotopes in experiments like this allows scientists to get one step more detailed to determining why this may be.”I believe, in this case, it allows us to have good detection of what were seeing,” Krohn states. “And that opens more doors.”Reference: “Cold Ion– Molecule Reactions in the Extreme Environment of a Coulomb Crystal” by O. A. Krohn and H. J. Lewandowski, 15 February 2024, The Journal of Physical Chemistry A.DOI: 10.1021/ acs.jpca.3 c07546This work was supported by the National Science Foundation and the Air Force Office of Scientific Research.