In many experiments utilizing ion traps, the ions must be cooled in order to increase the precision and accuracy of the measurements. Laser cooling is very effective when it can be applied, but it can only be used with a few kinds of ions, since it depends critically on the details of the electronic level structure. Other methods, such as resistive cooling, active-feedback cooling, collisional cooling, radiofrequency side-band cooling, or sympathetic laser cooling, can be applied to many kinds of ions. Progress in cooling of trapped ions has made possible improved measurements of mass ratios and atomic spectra and the observation of new phenomena, such as the formation of ordered Coulomb "crystals" of ions.
In many experiments utilizing ion traps, the ions must be cooled in order to increase the precision and accuracy of the measurements. Laser cooling is very effective when it can be applied, but it can only be used with a few kinds of ions, since it depends critically on the details of the electronic level structure. Other methods, such as resistive cooling, active-feedback cooling, collisional cooling, radiofrequency side-band cooling, or sympathetic laser cooling, can be applied to many kinds of ions. Progress in cooling of trapped ions has made possible improved measurements of mass ratios and atomic spectra and the observation of new phenomena, such as the formation of ordered Coulomb "crystals" of ions.
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.
A detailed investigation of ion crystals produced by laser and sympathetic cooling in a linear RF trap has been conducted.\n\nThe laser cooling methods were examined and applied to the trapped ^24Mg^(positive) ions. The crystals produced by the laser cooling were studied, including the dependence on RF voltage, end cap DC voltage, laser power and laser frequency. By manipulating the different RF voltages and endcap DC voltages, the structure phase transition of the ion crystals was observed.\n\nIn addition, the sympathetic cooling of different ion species with the laser cooled 24Mg^(positive) was carried out. In this process, the mixed Mg^(positive) and He^(positive) crystals were created andidentified, and mixed Mg^(positive) and H2^(positive) crystals were produced. The effect of an unwanted chemical reaction of Mg^(positive) and H2 was observed and minimized. After sympathetic cooling of light ion species, the sympathetic cooling of heavy molecular ions such as fullerene ions was also carried out. The efficiencies and final temperature in both cases are very different. Theoretically to interpret the results, molecular dynamics simulations of the laser cooling and sympathetic cooling were implemented. And the simulations were compared with the experimental results.\n\nIn the process of carrying out this research, the optics were rebuilt to provide reliable UV sources for the photoionization and laser cooling of Mg ions. The imaging\nsystem was reconfigured to take the images of ion crystals. New elements were added tin the ion trap to improve the ability to manipulate ions.
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.
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.
This work presents various novel results in the field of experimental and theoretical trapped ion studies. All investigations involve laser-cooled 138Ba+ ions confined in a linear quadrupole ion trap, which serve as coolant for complex molecular ions, as target for collisions with neutral atoms, or as model system in theoretical analyses. The special feature of the apparatus used in this work, is a molecular ion source based on electrospray ionization that allows for the production of gas-phase molecular ions of almost arbitrary species with a maximum mass-to-charge ratio of 2,000 Da. Trapped together with laser-cooled 138Ba+ ions, the molecular ions can be cooled down, kept cold and investigated for many minutes and in principle for up to hours in a nearly collisionless environment.
In this work, the sympathetic cooling of complex molecular ions to translational temperatures below 1 K is demonstrated for different types of molecules covering a mass range from 182 to 12,400 Da. Using the example of the protein cytochrome c, with a mass of about 12,400 Da the most massive molecular species sympathetically cooled in an ion trap so far, the methods for detection of successful trapping and cooling, and temperature determination are presented in detail. In one case, multiply protonated molecules of the protein cytochrome c have been cooled to less than 0.75 K. Furthermore, two methods for measuring photodestruction rates of cold, trapped complex molecular ions have been developed that can, for example, be applied in destructive spectroscopy schemes. Both techniques are demonstrated using the example of singly protonated glycyrrhetinic acid molecules (GAH+) dissociated by an ultraviolet (UV) laser. Measurements of the photodestruction rate were performed at different intensities of the UV laser, and rates as low as 0.05 s-1 and less have been determined, which is only possible due to the long ion storage times that can be achieved with this apparatus. For the UV laser wavelength of 266 nm, a destruction cross section of (1.1±0.1)∙10E-17 cm2 for GAH+ has been determined.
In this work, feasibility studies towards internal cooling of trapped complex molecular ions have been performed. Such an additional internal cooling could be realized via collisions with laser-cooled neutral atoms. To test their suitability, neutral atoms of Li, Na, K, Rb, Cs, and Yb have been collided with trapped laser-cooled and not cooled Ba+ ions as well as with complex molecular ions. For all species charge exchange reactions with Ba+ ions have been observed excluding these species for collisional cooling. While K, Rb, and Cs showed charge exchange reactions with ground state and laser-excited Ba+ ions, the charge exchange rates of Yb and Ba+ were found to be state dependent. Collisions of neutral Yb atoms were found to be reactive and lead to the formation of adducts and fragments, in contrast to the alkali metals for which no such reactions have been observed. In addition, the motion of cooled, trapped ions has been studied theoretically via simulations. When the translational energy of a trapped ion ensemble is sufficiently reduced, a first-order phase transition from a liquid to a solid state occurs with the ion ensemble changing from a disordered ion cloud to an ordered ion crystal. As ions in a linear quadrupole trap arrange in shell structures, a free motion in radial direction is disabled, and ions can only move to neighboring shells when their kinetic energy is high enough to overcome a certain potential barrier. The simulations have shown that intershell diffusion rates in the considered ion crystals increase exponentially when the ion temperatures exceed values that well agree with the predictions of other established models. Ion ensembles of different ion numbers (300, 500, and 1000 138Ba+ ions) and symmetries (prolate and spherical) have been investigated showing a clear tendency for the phase transition temperatures: The higher the ion number, the lower is the transition temperature.
This thesis presents the results of my graduate work which has concentrated on applying laser-cooling techniques to molecular ions, specifically CaH⁺ and BH⁺ ions for the study of their rovibronic spectroscopy. The goal of my research has been to control external and internal degrees of freedom of these two ions. Each of these molecules is spectroscopically appealing in its own right. For starters, CaH⁺ has been identified as a potential system for the test for the time variation of fundamental constants, and it's also been speculated to be in space. BH⁺ has a nearly diagonal Franck-Condon structure, and could be directly laser-cooled using only a few lasers. This structure makes it a good candidate for the cooling of internal molecular degrees of freedom. Each of the experiments described in this review involves the trapping and laser-cooling of ions. We cool the molecular ions sympathetically with laser-cooled Ca⁺. The probing techniques used to collect data are mainly: laser induced fluorescence, mass spectrometry, and resonance-enhanced multi-photon dissociation (REMPD). We have demonstrated the ground state sympathetic cooling of CaH⁺ ion to less than 16 muK, and we have used to REMPD to measure for the first time four vibronic transitions in CaH⁺ ions. Together, these results are important for future quantum logic experiments on this molecule. Furthermore, we have also made some progress towards trapping and cooling BH⁺ ions by first trapping B⁺ ions with Ca⁺ ions. We use the motional resonance method to detect the B⁺ ions, which can be reacted with H atoms to produce the molecular ions. Following the measurement of the vibronic spectrum of CaH⁺, we have narrowed the linewidth of the laser source to resolve rotational lines.