Abstract. Multipole radiofrequency ion traps are a highly versatile tool to study molecular ions and their interactions in a well-controllable environment. In particular the cryogenic 22-pole ion trap configuration is used to study ion-molecule reactions and complex molecular spectroscopy at temperatures between few Kelvin and room temperatures. This article presents a tutorial on radiofrequency ion trapping in multipole electrode configurations. Stable trapping conditions and buffer gas cooling, as well as important heating mechanisms, are discussed. In addition, selected experimental studies on cation and anion-molecule reactions and on spectroscopy of trapped ions are reviewed. Starting from these studies an outlook on the future of multipole ion trap research is given. Submitted to: J. Phys. B: At. Mol. Phys. Radiofrequency multipole traps 2 1.
Abstract. Multipole radiofrequency ion traps are a highly versatile tool to study molecular ions and their interactions in a well-controllable environment. In particular the cryogenic 22-pole ion trap configuration is used to study ion-molecule reactions and complex molecular spectroscopy at temperatures between few Kelvin and room temperatures. This article presents a tutorial on radiofrequency ion trapping in multipole electrode configurations. Stable trapping conditions and buffer gas cooling, as well as important heating mechanisms, are discussed. In addition, selected experimental studies on cation and anion-molecule reactions and on spectroscopy of trapped ions are reviewed. Starting from these studies an outlook on the future of multipole ion trap research is given. Submitted to: J. Phys. B: At. Mol. Phys. Radiofrequency multipole traps 2 1.
Multipole radiofrequency ion traps are a highly versatile tool to study molecular ions and their interactions in a well-controllable environment.In particular the cryogenic 22-pole ion trap configuration is used to study ion-molecule reactions and complex molecular spectroscopy at temperatures between a few kelvin and room temperatures.This paper presents a tutorial on radiofrequency ion trapping in multipole electrode configurations.Stable trapping conditions and buffer gas cooling, as well as important heating mechanisms, are discussed.In addition, selected experimental studies on cation and anion-molecule reactions and on spectroscopy of trapped ions are reviewed.Starting from these studies an outlook on the future of multipole ion trap research is given.
Over the past few decades, 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.
The study of ion-molecule reactions plays a vital role in cold chemistry, thus implying the need of well-controlled ion ensembles in a cold environment. The internal and external degrees of freedom of molecular ions, trapped in multipole radio frequency ion traps, can be cooled via collisions with neutral atoms [1]. Usually the coolant undergoes collisions with a thermal shield mounted on a cryostat attaining temperatures of about 4 K. To overcome this lower temperature limit two approaches have been proposed. Firstly, using a laser-cooled buffer-gas localized at the center of the ion cloud [2]. Secondly, sympathetic cooling via a second laser cooled molecular anion like C2− [3]. This, in addition to its potential role in forming carbon chain anions in the interstellar medium, makes C2− an important molecule in anion research. In this contribution we present a newly developed 16-pole cryogenic radio frequency wire trap for spectroscopic and kinetic studies of molecular anions. First results of quenching rates of the first excited vibrational level of C2− due to collisions with H2 are presented. Furthermore, possible reaction pathways with H2 are discussed and recent spectroscopic measurements of the second electronic excited state of C2− are presented. References: [1] R. Wester , J. Phys. B 42, 154001 (2009) [2] B. Höltkemeier et al., Phys. Rev. A 94, 062703, (2016) [3] P. Yzombard et al., Phys. Rev. Lett. 114, 213001 (2015)
Cooling all degrees of freedom of a molecule, a cluster, or even a nanoparticle which is suspended in a vacuum, is an experimental challenge. Without suitable schemes, cold or ultracold chemical reactions are not feasible. Methods such as laser based preparation of very slow atoms, decelerating molecules to low velocities with electric fields or freezing molecular ions into Coulomb crystals, are generally not suitable to cool the vibrational or rotational motion of molecules. This contribution describes a new method in which a beam of slow atoms or molecules (H, He, H2, or D2) is used for cooling charged particles confined in a multi-electrode rf trap. For reaching sub-K temperatures, the fast part of a cold effusive beam is removed with a shutter before the slow remaining neutrals interact with the ion cloud. The development of a pulsed cold beam source is discussed as well as suitable methods for determining the ion temperature. A challenging application is to prepare internally cold CH5+ for spectroscopy or chemistry. New experimental results for hydrogen abstraction in collisions with slow H atoms are reported at energies of a few meV. For evaluating these measurements and for predicting effective rate coefficients at lower energies, the kinematic conditions of the slow neutral beam-ion trap arrangement have been analyzed in detail. The potential of cooling ions such as protonated methane or H3+ with slow energy selected H atoms is briefly mentioned. An interesting process is the formation of weakly bound ions such as H4+ or CH6+ via radiative or ternary association. Such ions are ideal candidates for preparing the corresponding collision complexes very close (microeV) to the dissociation continuum using infrared transitions.
We have recently shown [J. Mikosch et al., Phys. Rev. Lett. 98, 223001 (2007)] that the storage time of a thermal ensemble of ions in a multipole radiofrequency (rf) ion trap is ultimately limited by ion evaporation. An analysis of the evaporation rate was used to determine an effective trap depth as a function of the rf amplitude. This revealed that already for moderate rf amplitudes and low average stability parameter $\ensuremath{\eta}$ the loss of ions is driven by nonadiabatic motion of the fastest ions in the rf field. Here we present the details of the experimental procedure together with our experimental apparatus, which combines a 22-pole rf ion trap with tandem time-of-flight mass spectrometry for ion selection and product analysis. We furthermore describe in detail the numerical and analytical models used in the interpretation of our results, which allows one to predict the most favorable trapping conditions in multipole rf ion traps. From a fit to our data we obtain a maximal value of the multipole stability parameter of ${\ensuremath{\eta}}_{\mathit{max}}=0.36\ifmmode\pm\else\textpm\fi{}0.02$ for adiabatic ion motion.
We describe the setup and characterization of a cryogenic multipole radio-frequency (RF) ion trap that enables the accumulation and cooling of mass-selected ions before injection into the SAPHIRA storage ring. To characterize the RF trap setup, we use ${\mathrm{OH}}^{\ensuremath{-}}$ anions and explore the threshold photodetachment cross section measured after storage in SAPHIRA as a probe of the rotational temperature. Beyond the temperature of the ion trap assembly, cooled to 6 K, the final rotational temperature of the ${\mathrm{OH}}^{\ensuremath{-}}$ ions is strongly influenced by the density of cooled He and the actual number of trapped ions while much less affected (possibly unaffected) by the time-varying field of the trap. To obtain rotationally cold ${\mathrm{OH}}^{\ensuremath{-}}$ ions, the RF trap must be operated with low He density and a low number of ions. High He densities, corresponding to a strong coupling of the trapped ions and He gas, lead to a significant rotational heating of the trapped ion ensemble, and the He density seems to limit the actual reachable rotational temperature. We demonstrate that cold ions in the RF trap remain cold for at least 30 ms in the SAPHIRA (300 K) storage ring at a base pressure of $\ensuremath{\sim}8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}\phantom{\rule{0.16em}{0ex}}\mathrm{mbar}$.