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Journal of Physics B Atomic Molecular and Optical Physics

Kinetic energy measurements of ions stored in a RF trap by use of phase-stopped ejection and retarding potential methods

R.-J. Champeau, A. Crubellier, J O Gaardsted, Dan Marescaux, D Pavolini

Published Mar 14, 1994• 4 citations

Abstract

The kinetic energy distribution of a cloud of ions stored in a RF trap has been investigated by use of two methods based on phase-stopped ejection. The oscillation of the RF voltage is stopped at a phase corresponding to zero field Value inside the trap. After their subsequent ejection from the trap, the ions are detected by an electron multiplier. The instantaneous ion cloud temperature value corresponding to the phase-stopping time can be deduced from the recorded profile of the ion signal. An alternative method consists in submitting the ejected ions to a repulsive field, applied before the detector, which allows us to discriminate the velocity classes. In both cases, the experiments are interpreted thanks to a simple theoretical description of a cloud of ions at thermal equilibrium in a RF trap and to a precise calculation of the ejection process. Both methods are compared, for the various trap working points. In all cases, the ion cloud average temperature is of the order of a tenth of the potential well depth.

DOI: 10.1088/0953-4075/27/5/010

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Abstract The ions trapped, whether in the Paul trap or in the Penning trap, usually have energies of the order of 1 eV, that is, a temperature of about 10 000 K. High temperature means high kinetic energy, and this is un favourable to most trap-based experiments for the following reasons. First, with high kinetic energy, amplitudes of ion oscillations are high and consequently the chances of ion losses are high; this results in loss of sensitivity. Second, high kinetic energy enhances both the first-order and the second-order Doppler effects; so, unless the ions are cooled, all spectroscopic experiments are adversely affected in a major way. While isolation of the ions in the trapped condition does not permit ion energy to be easily dissipated to the surroundings, for the same reason, the trapped ions can be effectively cooled and, once cooled, they can be retained cold in the trap. In recent years, most of the trap-based experiments have used ion cooling of one kind or the other. In this chapter we present an overview of various cooling methods and their applications.

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The kinetic energy distribution of a cloud of ions stored in a RF trap has been investigated by use of two methods based on phase-stopped ejection. The oscillation of the RF voltage is stopped at a phase corresponding to zero field Value inside the trap. After their subsequent ejection from the trap, the ions are detected by an electron multiplier. The instantaneous ion cloud temperature value corresponding to the phase-stopping time can be deduced from the recorded profile of the ion signal. An alternative method consists in submitting the ejected ions to a repulsive field, applied before the detector, which allows us to discriminate the velocity classes. In both cases, the experiments are interpreted thanks to a simple theoretical description of a cloud of ions at thermal equilibrium in a RF trap and to a precise calculation of the ejection process. Both methods are compared, for the various trap working points. In all cases, the ion cloud average temperature is of the order of a tenth of the potential well depth.

We present an experimental method for the characterization of the kinetic energies of ions confined in a 22-pole radio frequency trap by inducing a small potential barrier using the surrounding ring electrodes, allowing the selective extraction of ions. Energy sampling experiments have been performed on buffer-gas thermalized He + ions at trap temperatures between 10 and 180 K, resulting in distinct extraction curves as a function of the potential barrier, and a differentiated behavior depending on the escape time from the trap. The experiments are complemented by Monte Carlo simulations of the ion trajectories inside the calculated trap potential and allow us to investigate the properties of the sampling method, the role of ion motion coupling, and the impact of residual buffer-gas collisions on the observed results. The technique has also been successfully applied to identify energetic H 3 + ions produced in an exothermic reaction inside the trap. Upon calibration, this method can provide relative kinetic energy distributions or be used to filter the maximum desired kinetic energy of the ions inside the trap.

Abstract In a trap operating with a voltage of 10 = 500 V, singly-charged ions have a maximum ‘binding’ energy of order 500eV.3 This trap depth corresponds to the kinetic energy at a temperature of 6 x 106 K. This is more than enough to trap ions, even if the ions do receive a large recoil kick during the ionization process-to load an ion trap experimenters send a weak (neutral) atomic beam through the trapping region where an electron beam ionizes a few of the atoms by knocking an electron off. These ions created by electron bombardment have much greater kinetic energy than the thermal energy of atoms at room temperature (equivalent to only 1 / 40 eV). It would be unwise to try to be more precise about the typical energy of an ion since it depends on the voltage used. In contrast, a magnetic trap for neutrals has a maximum depth of only 0.07K.

Molecular dynamics simulations of the ion clouds stored in a linear quadrupole rf ion trap are performed at low temperatures (0.1 mK--15 K). The introduction of periodic boundary conditions allowed us to study the translationally uniform ion cloud geometry. The time evolution of the kinetic energy of the ion cloud is monitored during the simulation and the increase in the kinetic energy of the cloud (rf heating) is determined; its dependence on temperature, trapping voltage, and the number of ions is studied. The dependence of rf heating rate on temperature shows that rf heating is undetectable until the temperature reaches approximately 0.5 K; it then increases rapidly until it reaches a maximum at around 2 K. It starts to decrease slowly at higher temperatures. The rf heating rate is shown to increase very sharply with the amplitude of the trapping voltage. The dependence of the rf heating rate on the number of ions shows the influence of the ion crystal shell structure at low temperatures, and has a simple linear dependence at higher temperatures.

Characteristics of the Yb+ ion cloud trapped in a rf trap in the presence of He buffer gas were determined by fluorescence detection as well as the rf resonance method. Combining the results obtained by the two methods, we determined the spatial ion density distribution and found that the trapped ions were thermalized by collisions with the He buffer gas. It was confirmed by both methods that the harmonic motion of the trapped ions was excited by the trapping field leaked to the rf resonance detection circuit (subharmonic oscillation), and moreover, by the fluorescence detection method, that the probing rf field used for the rf resonance method was indispensable as “the initial push” for exciting the amplitude.

Abstract The ions trapped, whether in the Paul trap or in the Penning trap, usually have energies of the order of 1 eV, that is, a temperature of about 10 000 K. High temperature means high kinetic energy, and this is un favourable to most trap-based experiments for the following reasons. First, with high kinetic energy, amplitudes of ion oscillations are high and consequently the chances of ion losses are high; this results in loss of sensitivity. Second, high kinetic energy enhances both the first-order and the second-order Doppler effects; so, unless the ions are cooled, all spectroscopic experiments are adversely affected in a major way. While isolation of the ions in the trapped condition does not permit ion energy to be easily dissipated to the surroundings, for the same reason, the trapped ions can be effectively cooled and, once cooled, they can be retained cold in the trap. In recent years, most of the trap-based experiments have used ion cooling of one kind or the other. In this chapter we present an overview of various cooling methods and their applications.