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arXiv:2509.08791

Isomer- and state-dependent ion-molecule reactions between Coulomb-crystallised Ca$^+$ ions and 1,2-dichloroethene

Lei Xu, Richard Karl, Jutta Toscano, Stefan Willitsch

Published Sep 10, 2025• 0 citations
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Abstract

We report a systematic investigation of isomer- and state-dependent reactions between Coulomb-crystallised laser-cooled Ca$^+$ ions and \emph{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 C$_2$HCaCl$^+$, 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. \emph{Ab initio} calculations of reaction pathways suggest that this suppression stems from the formation of long-lived reaction complexes. The \textit{cis} isomer was found to exhibit a higher reactivity with all electronic states of Ca$^+$ than its \textit{trans} counterpart. The present study provides insights into the combined effects of molecular structure and quantum states influencing ion-molecule reaction dynamics.

DOI
10.48550/arxiv.2509.08791

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Isomer- and state-dependent ion-molecule reactions between Coulomb-crystallised Ca+^++ ions and 1,2-dichloroethene
We report a systematic investigation of isomer- and state-dependent reactions between Coulomb-crystallised laser-cooled Ca+^++ ions and \emph{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 C2_22​HCaCl+^++, 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. \emph{Ab initio} calculations of reaction pathways suggest that this suppression stems from the formation of long-lived reaction complexes. The \textit{cis} isomer was found to exhibit a higher reactivity with all electronic states of Ca+^++ than its \textit{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 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.
Rate constants for chemical reactions of laser-cooled Ca(+) ions and neutral polar molecules (CH(3)F, CH(2)F(2), or CH(3)Cl) have been measured at low collision energies (<E(coll)>/k(B)=5-243 K). Low kinetic energy ensembles of (40)Ca(+) ions are prepared through Doppler laser cooling to form "Coulomb crystals" in which the ions form a latticelike arrangement in the trapping potential. The trapped ions react with translationally cold beams of polar molecules produced by a quadrupole guide velocity selector or with room-temperature gas admitted into the vacuum chamber. Imaging of the Ca(+) ion fluorescence allows the progress of the reaction to be monitored. Product ions are sympathetically cooled into the crystal structure and are unambiguously identified through resonance-excitation mass spectrometry using just two trapped ions. Variations of the laser-cooling parameters are shown to result in different steady-state populations of the electronic states of (40)Ca(+) involved in the laser-cooling cycle, and these are modeled by solving the optical Bloch equations for the eight-level system. Systematic variation of the steady-state populations over a series of reaction experiments allows the extraction of bimolecular rate constants for reactions of the ground state ((2)S(1/2)) and the combined excited states ((2)D(3/2) and (2)P(1/2)) of (40)Ca(+). These results are analyzed in the context of capture theories and ab initio electronic structure calculations of the reaction profiles. In each case, suppression of the ground state rate constant is explained by the presence of a submerged or real barrier on the ground state potential surface. Rate constants for the excited states are generally found to be in line with capture theories.
This thesis describes the study of a range of ion-molecule reactions at very low collision energies using a newly developed experimental technique which involves the reaction of velocity-selected beams of translationally cold neutral molecules with very low kinetic energy ion ensembles. These studies have been enabled by the construction of a new apparatus for trapping and laser-cooling gas phase atomic ions (40Ca⁺). The laser-cooling process results in the formation of ordered, low kinetic energy, lattice-like ion structures, also known as "Coulomb crystals". The properties of single and multicomponent Coulomb crystals (which may also involve molecular ions), and their manipulation via modulation of the applied fields, are explored experimentally and with the use of molecular dynamics simulations. Variations in the laser-cooling parameters are shown to result in different steady-state populations of the electronic states of 40Ca⁺ involved with the laser cooling cycle, and these are modelled within an appropriate theoretical framework. The imaging of 40Ca⁺ fluorescence as a function of time allows the study of various ion-molecule reactions at collision energies around 300 K, with single ion sensitivity. These reaction studies are extended to low-temperature (collision energies close to 1 K), by combination of the ion trap apparatus with a bent quadrupole guide velocity-selector. Ion-molecule collision energies are shown to be variable over a short range through a change in the quadrupole guide voltage, or the ion trapping parameters; the effect of these modulations on the rate constant is explored for Ca⁺ + CH₃F. Bimolecular rate constants for the reactions of 40Ca⁺ with CH₃F, CH₂F₂ and CH₃Cl have been determined for a range of 40Ca⁺ state populations, allowing resolution of the global rate contributions from the ground and combined excited states. These results are analysed in the context of capture theories and ab initio electronic structure calculations. In each case, suppression of the ground state rate constant is explained by the presence of either a submerged or real barrier on the ground state potential surface. Rates of reaction from the combined excited states are generally found to be in line with capture theories, and in some cases variation is found between the high and low collision energy regimes. Molecular product ions generated in these experiments have been shown to be sympathetically-cooled into the crystal structure, and subsequently identified through resonance-excitation mass spectrometry. Molecular ions were also produced by multiphoton laser ionisation of a thermal background gas of OCS molecules. An ion-molecule reaction involving a molecular ion, that of charge transfer between OCS⁺ and ND₃, has been studied at a collision energy near 1 K for the first time using sympathetically-cooled OCS⁺ and velocity-selected ND₃. These experiments illustrate the generality of the techniques described herein, and should lead to many possibilities for future studies.
Cold chemical reactions between laser-cooled Ca+ ions and Rb atoms were studied in an ion-atom hybrid trap. Reaction rate constants were determined in the range of collision energies ⟨E coll⟩/k B=20 mK-20 K. The lowest energies were achieved in experiments using single localised Ca+ ions. Product branching ratios were studied using resonant-excitation mass spectrometry. The dynamics of the reactive processes in this system (non-radiative and radiative charge transfer as well as radiative association leading to the formation of CaRb+ molecular ions) have been analysed using high-level quantum-chemical calculations of the potential energy curves of CaRb+ and quantum-scattering calculations for the radiative channels. For the present low-energy scattering experiments, it is shown that the energy dependence of the reaction rate constants is governed by long-range interactions in line with the classical Langevin model, but their magnitude is determined by short-range non-adiabatic and radiative couplings which only weakly depend on the asymptotic energy. The quantum character of the collisions is predicted to manifest itself in the occurrence of narrow shape resonances at well-defined collision energies. The present results highlight both universal and system-specific phenomena in cold ion-neutral reactive collisions.
Cold chemical reactions between laser-cooled Ca^+ ions and Rb atoms were studied in an ion-atom hybrid trap. Reaction rate constants were determined in the range of collision energies /k_B = 20 mK-20 K. The lowest energies were achieved in experiments using single localized Ca^+ ions. Product branching ratios were studied using resonant-excitation mass spectrometry. The dynamics of the reactive processes in this system (non-radiative and radiative charge transfer as well as radiative association leading to the formation of CaRb^+ molecular ions) have been analyzed using high-level quantum-chemical calculations of the potential energy curves of CaRb^+ and quantum-scattering calculations for the radiative channels. For the present low-energy scattering experiments, it is shown that the energy dependence of the reaction rate constants is governed by long-range interactions in line with the classical Langevin model, but their magnitude is determined by short-range non-adiabatic and radiative couplings which only weakly depend on the asymptotic energy. The quantum character of the collisions is predicted to manifest itself in the occurrence of narrow shape resonances at well-defined collision energies. The present results highlight both universal and system-specific phenomena in cold ion-neutral reactive collisions.