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arXiv:0902.1695 · Journal of Physics B Atomic Molecular and Optical Physics

How can a 22-pole ion trap exhibit ten local minima in the effective potential?

Rico Otto, P. Hlavenka, Sebastian H. Trippel, Jochen Mikosch, Kilian Singer, Matthias Weidemüller, Roland Wester

Published Jul 15, 2009• 35 citations
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Abstract

The column density distribution of trapped OH − ions in a 22-pole ion trap is measured for different trap parameters. The density is obtained from position-dependent photodetachment rate measurements. Overall, agreement is found with the effective potential of an ideal 22-pole. However, in addition, we observe ten distinct minima in the trapping potential, which indicate a breaking of the 22-fold symmetry. Numerical simulations show that a displacement of a subset of the radiofrequency electrodes can serve as an explanation for this symmetry breaking.

DOI
10.1088/0953-4075/42/15/154007

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Photodetachment spectroscopy is a powerful spectroscopic technique for determining the internal state distribution of a molecular anion. Previously, our group studied the threshold photodetachment spectroscopy of CN⁻ at both 16 Kelvin and 295 Kelvin in a 22-pole ion trap and measured the electron affinity of CN with great precision (EA: 3.864(2) eV). The dicarbon anion, our current molecule of interest, is a well-studied system due to its stable electronic level structure and potential laser cooling transition. Here we present the threshold photodetachment spectroscopy study of C₂⁻ in a radiofrequency 16-pole ion trap at 8 Kelvin. We investigated the behavior of the cross section near the threshold for the ground state transition, C₂X¹Σ⁺g ← C₂⁻ X²Σ⁺g. We see a p-wave behavior for this ground state detachment. And we measured the electron affinity, which is consistent with the previously measured values."

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The column density distribution of trapped OH − ions in a 22-pole ion trap is measured for different trap parameters. The density is obtained from position-dependent photodetachment rate measurements. Overall, agreement is found with the effective potential of an ideal 22-pole. However, in addition, we observe ten distinct minima in the trapping potential, which indicate a breaking of the 22-fold symmetry. Numerical simulations show that a displacement of a subset of the radiofrequency electrodes can serve as an explanation for this symmetry breaking.
The absolute photodetachment cross section of OH- anions at a rotational and translational temperature of 170 K is determined by measuring the detachment-induced decay rate of the anions in a multipole radio-frequency ion trap. In comparison with previous results, the obtained cross section shows the importance of the initial rotational-state distribution. Using a tomography scan of the photodetachment laser through the trapped ion cloud, the derived cross section is model-independent and thus features a small systematic uncertainty. The tomography also yields the column density of the OH- anions in the 22-pole ion trap in good agreement with the expected trapping potential of a large field free region bound by steep potential walls.
Radiofrequency multipole traps have been used for some decades in cold collision experiments and are gaining interest for precision spectroscopy due to their low micromotion contribution and the predicted unusual cold-ion structures. However, the experimental realisation is not yet fully controlled, and open questions in the operation of these devices remain. We present experimental observations of symmetry breaking of the trapping potential in a macroscopic octupole trap with laser-cooled ions. Numerical simulations have been performed in order to explain the appearance of additional local potential minima and be able to control them in a next step. We characterise these additional potential minima, in particular with respect to their position, their potential depth and their probability of population as a function of the radial and angular displacement of the trapping rods.
Abstract Radio-frequency (RF) linear multipole traps have been shown to be very sensitive to mis-positioning of their electrodes, which results in a symmetry breaking and leads to extra local minima in the trapping potential as shown in Pedregosa-Gutierrez et al (2018 J. Mod. Opt. 65 529) disturbing the operation of the trap. In this work, we analytically describe the RF-potential of a realistic octupole trap by including lower order terms to the well-established equation for a perfectly symmetric octupole trap. We describe the geometry by a combination of identified defects, characterized by simple analytical expressions. A complete equation is proposed for a trap with any electrode deviation relying on a combination of the simple cases where the defects are taken individually. Our approach is validated by comparison between analytical and numerical results for defect sizes up to 4% of the trap radius. As described in Pedregosa-Gutierrez et al (2018 Rev. Sci. Instrum. 89 123101), an independent fine-tuning of the amplitude of the RF voltage applied on each electrode can be used to mitigate the geometrical defects of a realistic trap. In a different way than in Pedregosa-Gutierrez et al (2018 Rev. Sci. Instrum. 89 123101), the knowledge of an analytical equation for the potential allows to design the set of RF-voltages required for this compensation, based on the experimental measurement of the ion positions in the trap, without information concerning the exact position of each electrode, and with a small number of iterations. The requirements, performances and limitations of this protocol are discussed via comparison of numerical simulations and analytical results.
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.
Photodetachment spectroscopy is a powerful spectroscopic technique for determining the internal state distribution of a molecular anion. Previously, our group studied the threshold photodetachment spectroscopy of CN⁻ at both 16 Kelvin and 295 Kelvin in a 22-pole ion trap and measured the electron affinity of CN with great precision (EA: 3.864(2) eV). The dicarbon anion, our current molecule of interest, is a well-studied system due to its stable electronic level structure and potential laser cooling transition. Here we present the threshold photodetachment spectroscopy study of C₂⁻ in a radiofrequency 16-pole ion trap at 8 Kelvin. We investigated the behavior of the cross section near the threshold for the ground state transition, C₂X¹Σ⁺g ← C₂⁻ X²Σ⁺g. We see a p-wave behavior for this ground state detachment. And we measured the electron affinity, which is consistent with the previously measured values."