Mass-selective removal of ions from Paul traps using parametric excitation
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Mass‑selective removal of ions from Paul traps using parametric excitation Julian Schmidt1,2,3 · Daniel Hönig1 · Pascal Weckesser1 · Fabian Thielemann1 · Tobias Schaetz1 · Leon Karpa1 Received: 4 May 2020 / Accepted: 14 July 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract We study a method for mass-selective removal of ions from a Paul trap by parametric excitation. This can be achieved by applying an oscillating electric quadrupole field at twice the secular frequency 𝜔sec using pairs of opposing electrodes. While excitation near the resonance with the secular frequency 𝜔sec only leads to a linear increase of the amplitude with excitation duration, parametric excitation near 2 𝜔sec results in an exponential increase of the amplitude. This enables efficient removal of ions from the trap with modest excitation voltages and narrow bandwidth, therefore, substantially reducing the disturbance of ions with other charge-to-mass ratios. We numerically study and compare the mass selectivity of the two methods. In addition, we experimentally show that the barium isotopes with 136 and 137 nucleons can be removed from small ion crystals and ejected out of the trap while keeping 138 Ba+ ions Doppler cooled, corresponding to a mass selectivity of better than Δm∕m = 1∕138 . This method can be widely applied to ion trapping experiments without major modifications since it only requires modulating the potential of the ion trap.
1 Introduction Radiofrequency (rf) Paul traps [1–3] can store ions for a wide range of charge-to-mass ratios Q/m and thus represent a versatile and powerful tool with applications such as quantum information processing [4, 5] or cold chemistry [6, 7]. Many experiments involve trapping one or two atomic or molecular ion species. Over the course of the experiment, other (parasitic) isotopes or ion species may appear in the trap, e.g. due to chemical reactions of the ion of interest with background gas particles, and create a disturbance [8]. The parasitic ions can be embedded into the ion crystal after sympathetic cooling [9] or remain on large orbits [10]. * Leon Karpa [email protected]‑freiburg.de Julian Schmidt [email protected] 1
Physikalisches Institut, Albert-Ludwigs-Universität Freiburg, Hermann‑Herder‑Straße 3, 79104 Freiburg, Germany
2
Laboratoire Kastler Brossel, Sorbonne Université, CNRS, ENS-PSL Research University, Collège de France, 4 place Jussieu, Paris, France
3
National Institute of Standards and Technology, Boulder, CO, USA
In principle, it is possible to operate any quadrupole mass filter (including rf ion traps) in a parameter regime for which the trajectories of the desired species are stable and those of other species are unstable due to their specific charge-to-mass ratios. The dynamics are given by the solutions of the Mathieu equation and regions of stability can be described with the a and q parameters (see below) which are related to the electrostatic and radiofrequency voltages [1]. Due to technical limitations, it can be cha
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