Charge Transfer Reactions between Water Isotopologues and Kr⁺ ions

Abstract

Astrochemical models often adopt capture theories to predict the behavior of experimentally unmeasured ion–molecule reactions. Here, reaction rate coefficients are reported for the charge transfer reactions of H2O and D2O molecules with cold, trapped Kr+ ions. Classical capture theory predictions are found to be in excellent agreement with the experimental findings. A crossing point identified between the reactant and product potential energy surfaces, constructed from high-level ab initio calculations, further supports a capture-driven mechanism of charge transfer. However, ion–molecule reactions do not always agree with predictions from capture theory models. The appropriateness of using capture theory-based models in the absence of detailed experimental or theoretical studies is discussed, alongside an analysis of why capture theory is appropriate for describing the likelihood of charge transfer between Kr+ and the two water isotopologues.

Spatial and Temporal Detection of Ions Ejected from Coulomb Crystals

“…Coulomb crystals are formed within a linear Paul ion trap, following the same approach as outlined in previous work. − A Ca oven is resistively heated, emitting Ca atoms that are subsequently nonresonantly ionized at the trap center by the 355 nm output of a frequency-tripled Nd:YAG laser. The Ca + ions are Doppler cooled using 397 and 866 nm diodes addressing the 4s 2 S 1/2 → 4p 2 P 1/2 and 3d 2 D 3/2 → 4p 2 P 1/2 transitions, respectively.…”

“…When Kr + ions are introduced to the crystal, they form an outer shell around the fluorescing Ca + ions, with the Ca + ions adopting positions closer to the trap axis and the crystal appearing to elongate. Once formed, Kr + ions can undergo charge transfer reactions with background H 2 O in the chamber, forming H 2 O + product ions . As the H 2 O + ions have a lower mass-to-charge ( m / z ) ratio than both Kr + and Ca + , they form a dark core along the central trap axis that can be observed in the CCD images.…”

“…The use of trapping and laser cooling methods to produce ensembles of translationally cold ions is an area of active research interest. Much of this interest is inspired by the diverse range of applications for cold, confined ionsincluding their use as qubits, , in the determination of fundamental constants, , for precision measurements, − and for the study of chemical reaction dynamics and kinetics. − As many ionic species of interest cannot be easily or efficiently laser-cooled, Coulomb crystals have emerged as a useful tool for the study of processes involving sympathetically cooled trapped ions.…”

“…Instead, reactivity studies typically employ either in situ monitoring of crystal evolution through the comparison of fluorescence images with high-level molecular dynamics simulations − or the destructive determination of ion numbers from time-of-flight mass spectrometry (ToF-MS) measurements. − These methods, used both independently and in combination, have facilitated the successful study of a range of reactions in Coulomb crystals. Fluorescence image comparisons rely on assumptions about the identities of the dark (nonfluorescing) ions, as these species cannot be directly observed by a CCD camera.…”

“…Coulomb crystals are formed once sufficient kinetic energy has been removed from the ions, resulting in a 3D lattice-like ellipsoid structure where Multicomponent Coulomb crystals are formed by admitting Kr gas into the chamber through a high-precision leak valve, with Kr + formed via a [2 + 1] resonance-enhanced multi-photon ionization (REMPI) scheme using the 212 nm output of a Nd:YAG-pumped frequency-tripled dye laser. 10,11 Kr + ions are sympathetically cooled by elastic collisions with cotrapped laser-cooled Ca + ions, yielding bicomponent Coulomb crystals. While only the Ca + ions can be directly observed with the CCD camera, as fluorescence is only emitted by the lasercooled species, the presence of other cotrapped species can be inferred from changes to the positions of Ca + ions within the crystal structure.…”