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arXiv:1404.4292 · Physical Review A

Sympathetic cooling of molecular ions in a surface-electrode ion trap

Arezoo Mokhberi, Stefan Willitsch

Published Aug 4, 2014• 10 citations
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Abstract

We report the sympathetic cooling and Coulomb crystallization of molecular ions above the surface of an ion-trap chip. ${\mathrm{N}}_{2}^{+}$ and ${\mathrm{CaH}}^{+}$ ions were confined in a surface-electrode radio-frequency (rf) ion trap and cooled by the interaction with laser-cooled ${\mathrm{Ca}}^{+}$ ions to secular translational temperatures in the millikelvin range. The configuration of trapping potentials generated by the surface electrodes enabled the formation of planar bicomponent Coulomb crystals and the spatial separation of the molecular from the atomic ions on the chip. The structural and thermal properties of the Coulomb crystals were characterized using molecular dynamics simulations. The present study extends chip-based trapping techniques to cold molecular ions with potential applications in mass spectrometry, chemistry, quantum information science, and spectroscopy.

DOI
10.1103/physreva.90.023402

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We report the sympathetic cooling and Coulomb crystallization of molecular ions above the surface of an ion-trap chip. N2+{\mathrm{N}}_{2}^{+}N2+​ and CaH+{\mathrm{CaH}}^{+}CaH+ ions were confined in a surface-electrode radio-frequency (rf) ion trap and cooled by the interaction with laser-cooled Ca+{\mathrm{Ca}}^{+}Ca+ ions to secular translational temperatures in the millikelvin range. The configuration of trapping potentials generated by the surface electrodes enabled the formation of planar bicomponent Coulomb crystals and the spatial separation of the molecular from the atomic ions on the chip. The structural and thermal properties of the Coulomb crystals were characterized using molecular dynamics simulations. The present study extends chip-based trapping techniques to cold molecular ions with potential applications in mass spectrometry, chemistry, quantum information science, and spectroscopy.
Sympathetic cooling and Coulomb crystallisation of molecular ions above the \nsurface of an ion-trap chip were demonstrated. N2+ and CaH+ ions were confined \nin a surface-electrode radiofrequency ion trap and cooled by the interaction \nwith laser-cooled Ca+ ions to secular translational temperatures in the \nmilliKelvin range. The configuration of trapping potentials generated by the surface \nelectrodes enabled the formation of planar bicomponent Coulomb crystals \nand the spatial separation of the molecular from the atomic ions. The structural \nand thermal properties of the crystals were characterized using molecular \ndynamics simulations. The effects of trap anharmoncities on the shape and \nenergy of bicomponent crystals were theoretically investigated. It was shown \nthat the trapping potentials can also deliberately be engineered to spatially \nseparate ion species in bicomponent crystals. Furthermore, a multi-functional \nsurface-electrode radiofrequency ion-trap chip has been developed to enable experiments \nwith cold molecular ions using a monolithic device. The chip was \ndesigned to combine various tasks such as loading and preparation of ions, mass \nspectrometry, spectroscopy, reaction studies, and manipulation of ion crystals \nin a miniaturised device. This chip features carefully engineered ion channel \nintersections that enable transporting sympathetically cooled molecular ions in \nthe form of bicomponent crystals. A detailed description of the fabrication and \nsimulation of the two chips are presented. The present study extends chipbased \ntrapping techniques to Coulomb-crystallised molecular ions with potential \napplications in mass spectrometry, cold chemistry, quantum technology, and \nspectroscopy.
Cold molecular ions are of great interest for applications in cold collision studies, chemistry, precision spectroscopy and quantum technologies. In this context, sympathetic cooling of molecular ions by the interaction with laser-cooled atomic ions is a powerful method to cool their translational motion and achieve translational temperatures in the millikelvin range. Recently, we implemented this method in a surface-electrode ion trap. The flexibility in shaping the trapping potentials offered by the surface-electrode structure enabled us to generate planar bicomponent Coulomb crystals and spatially separate the molecular from the atomic ions. Here, we present a detailed description of the fabrication and simulation of the trap as well as a theoretical and experimental investigation of the structural and energetic properties of the Coulomb crystals obtained in the device. We discuss in more detail the separation of different ion species using static electric fields and explore the effects of trap anharmoncities on the shape of bicomponent crystals.
Ensembles of cold atomic and molecular ions in ion traps prepared at millikelvin temperatures by laser and sympathetic cooling have recently found considerable interest in both physics and chemistry. At very low temperatures the ions form ordered structures in the trap also known as "Coulomb crystals". Ion Coulomb crystals exhibit a range of intriguing properties which render them attractive systems for novel experiments in chemical dynamics, ultrahigh-resolution spectroscopy and quantum-information processing. In this article we review the methods used to prepare atomic and molecular ion Coulomb crystals and discuss some recent studies in mass spectrometry, low-temperature chemistry and precision spectroscopy to illustrate their scientific potential for chemical applications. Finally, we conclude with an outlook on outstanding challenges and prospective further developments in the field.
Translationally cold, spatially localised molecular ions prepared by sympathetic cooling with laser-cooled atomic ions in ion traps have recently found a wide range of applications in both chemistry and physics. The very low temperatures of the ions (down to millikelvins), their tight localisation in the trap and the ability to control and manipulate single isolated molecules on the quantum level offer intriguing possibilities for new experiments in the realms of cold chemistry, precision molecular spectroscopy, mass spectrometry and quantum technology. The present article gives an overview of the basic experimental methods, current topics and recent developments in this field.
We discuss the design and optimisation of two types of junctions between surface-electrode radiofrequency ion-trap arrays that enable the integration of experiments with sympathetically cooled molecular ions on a monolithic chip device. A detailed description of a multi-objective optimisation procedure applicable to an arbitrary planar junction is presented, and the results for a cross junction between four quadrupoles as well as a quadrupole-to-octupole junction are discussed. Based on these optimised functional elements, we propose a multi-functional ion-trap chip for experiments with translationally cold molecular ions at temperatures in the millikelvin range. This study extends complex chip-based trapping techniques to Coulomb-crystallised molecular ions with potential applications in mass spectrometry, spectroscopy, controlled chemistry and quantum technology.
Using molecular dynamics simulations we characterize theoretically Coulomb clusters of laser- and sympathetically-cooled ions in a five-wire surface-electrode ion trap. We show that the asymmetry of the trapping potential leads to significantly different cluster structures and ion energy distributions in comparison to conventionally used linear Paul traps and to an asymmetric segregation of the ions in bi-component Coulomb clusters. We explore the impact of our results on the implementation of sympathetic cooling of molecular ions in surface-electrode traps and discuss possible challenges for the realization of such experiments.