Control of chemical reactions in radiofrequency ion traps

TL;DR

RF ion traps provide a versatile platform for studying ion--molecule chemistry with exquisite control over internal and external degrees of freedom. The review surveys trapping and cooling methods (Doppler, sympathetic, resolved-sideband, cryogenic buffer gas), state-preparation techniques (optical pumping, REMPI, quantum-logic spectroscopy), and methods to tune collision energy and molecular structure, enabling state-resolved reaction studies. It highlights observations of quantum-state dependent kinetics, resonant and non-statistical effects, and isomer- or conformer-specific reactivities, illustrating the rich physics accessible in trapped-ion systems. Looking ahead, the field aims for full state-to-state mapping, ultracold regimes free of micromotion, and exploration of complex and chiral systems, with advances driven by both experimental innovation and theory for open-shell reactions.

Abstract

Over the past years, radiofrequency ion traps have become an attractive platform for studying chemical reactions as they enable a high degree of control over ion-molecule dynamics. In this review, we summarize techniques for the trapping and cooling of atomic and molecular ions in radiofrequency traps including Doppler and resolved-sideband laser cooling, sympathetic cooling, and cryogenic buffer-gas methods. We discuss strategies for controlling key reaction parameters: the preparation of specific internal quantum states by internal cooling, optical pumping, state-selective photoionization and quantum-logic spectroscopy; the manipulation of collision energies through micromotion control, dynamic trapping and combination with molecular beams; and the selection of molecular structure via isotopic substitution, conformational separation and isomer-specific ion generation. We illustrate applications of these approaches by discussing studies on quantum-state-dependent kinetics, quantum-resonance effects and structure-sensitive reactivity in ion-neutral collisions. We conclude by outlining future challenges, including full state-to-state reaction mapping, reaching the ultracold quantum regime free of micromotion, and the exploration of complex and chiral systems.

Figures (6)

• Figure 1: Geometries of typical linear RF ion traps with their effective radial trapping potentials: (a) Linear quadrupole trap, (b) 22-pole trap. The colored circles depict a cross section of the electrodes configuration perpendicular to the longitudinal axis of the trap. See text for details.
• Figure 2: Methods for the cooling of ions in RF traps: (a) Doppler laser cooling (inset: false-color fluorescence image of a single laser-cooled Ca$^+$ ion). (b) Sympathetic cooling of molecular ions by laser-cooled atomic ions (inset: a Coulomb crystal of laser cooled Ca$^+$ ions containing a single dark sympathetically cooled N$_2^+$ ion whose position is indicated by the yellow circle). (c) Resolved-sideband cooling on a red motional sideband of an optical transition between two levels $|g\rangle$ and $|e\rangle$ in the quantum regime of ion motion. (d) Collisional cooling with a cryogenic buffer gas.
• Figure 3: Methods for the internal-state preparation of molecular ions: (a) Optical pumping into selected states. (b) Buffer-gas cooling of the internal degrees of freedom. (c) State-selective destruction of molecular ions by removing specific quantum states from the trap. (d) Resonance-enhanced multiphoton threshold photoionization for generating molecular ions in specific rovibrational states. (e) Projective state preparation of single molecular ions through state measurement (symbolized by the magnifying glass) using quantum-logic spectroscopy.
• Figure 4: Representative experimental setups for studying collisions and reactions of neutral species with trapped ions: (a) Hybrid trapping experiment combining a "blade" linear-quadrupole RF trap with an optical-dipole trap (ODT) for confining ultracold atoms weckesser21a. (b) Cryogenic 22-pole trap setup from Ref. Jusko24a consisting of a storage ion source (SIS) as well as quadrupole guides (QP) and benders (B) connecting the trap (22pt) with the source and particle detector (CEM). Panel (a) is reproduced from Reference weckesser21a with permission from Springer Nature and P. Weckesser. Panel (b) is reproduced from Reference Jusko24a (CC BY 4.0).
• Figure 5: Schematic summary of different types of isomers with examples. See text for details.