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

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.