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Physical Review Letters

Quantum-State Controlled Formation of Cosmically Relevant Metallic Molecular Ions

G. Stephen Kocheril, Chase Zagorec-Marks, Heather J. Lewandowski

Published May 20, 2025• 1 citation
Publisher

Abstract

Metal-containing molecular ions are fundamentally important in both terrestrial and interstellar chemistry, yet their formation mechanisms have been largely unexplored experimentally. To address this lack of fundamental understanding of how these ions are created in space, we conducted an experimental study of a quantum-state-controlled reaction between Ca^{+} and C_{2}H_{2} to investigate a potential unifying mechanism for the formation of metallic molecules. By tuning the quantum-state populations of laser-cooled Ca^{+}, we demonstrate precise control over reactivity. Our results reveal a single ionic product, CaCCH^{+}, providing valuable insights into the mechanisms underlying the formation of metal-containing ions in space.

DOI
10.1103/physrevlett.134.203401

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Metal-containing molecular ions are fundamentally important in both terrestrial and interstellar chemistry, yet their formation mechanisms have been largely unexplored experimentally. To address this lack of fundamental understanding of how these ions are created in space, we conducted an experimental study of a quantum-state-controlled reaction between Ca^{+} and C_{2}H_{2} to investigate a potential unifying mechanism for the formation of metallic molecules. By tuning the quantum-state populations of laser-cooled Ca^{+}, we demonstrate precise control over reactivity. Our results reveal a single ionic product, CaCCH^{+}, providing valuable insights into the mechanisms underlying the formation of metal-containing ions in space.
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Isomer- and state-dependent ion-molecule reactions between Coulomb-crystallised Ca+^++ ions and 1,2-dichloroethene
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We report a systematic investigation of isomer- and state-dependent reactions between Coulomb-crystallised laser-cooled Ca+ ions and cis/trans-1,2-dichloroethene (DCE) isomers. By manipulating the electronic state populations of Ca+ through tuning of laser cooling parameters, we observed distinct reactivities in its ground and excited states, as well as with the geometric isomers of DCE. Our experiments revealed two primary reaction channels, formation of CaCl+ and C2H2CaCl+, followed by secondary reaction pathways. While excited-state reactions proceed at rate coefficients consistent with capture theory predictions, ground-state reactions show a systematically lower reactivity. Ab initio calculations of reaction pathways suggest that this suppression stems from the formation of long-lived reaction complexes. The cis isomer was found to exhibit a higher reactivity with all electronic states of Ca+ than its trans counterpart. The present study provides insights into the combined effects of molecular structure and quantum states influencing ion–molecule reaction dynamics.
We report a systematic investigation of isomer- and state-dependent reactions between Coulomb-crystallised laser-cooled Ca+ ions and cis/trans-1,2-dichloroethene (DCE) isomers. By manipulating the electronic state populations of Ca+ through tuning of laser cooling parameters, we observed distinct reactivities in its ground and excited states, as well as with the geometric isomers of DCE. Our experiments revealed two primary reaction channels, formation of CaCl+ and C2H2CaCl+, followed by secondary reaction pathways. While excited-state reactions proceed at rate coefficients consistent with capture theory predictions, ground-state reactions show a systematically lower reactivity. Ab initio calculations of reaction pathways suggest that this suppression stems from the formation of long-lived reaction complexes. The cis isomer was found to exhibit a higher reactivity with all electronic states of Ca+ than its trans counterpart. The present study provides insights into the combined effects of molecular structure and quantum states influencing ion–molecule reaction dynamics.
We have produced large samples of ultracold (<20 mK) ArH + , ArD + , N 2 H + , N 2 D + , H 3 + , D 3 + , D 2 + , H 2 D + and D 2 D + molecular ions, by sympathetic cooling and crystallization via laser-cooled Be + ions in a linear radio-frequency trap. As technique, we used chemical reactions with sympathetically cooled noble gas atomic ions or N 2 + and O 2 + molecular ions. These ultracold molecules are interesting targets for high-precision measurements in fundamental physics and may open new routes for the study of state-selective chemical reactions relevant to interstellar chemistry.