Using a laser-cooled ion trapping apparatus, we have investigated laser-induced chemical reactions between cold trapped ${\mathrm{Ba}}^{+}$ ions and several neutral molecular gases at room temperature, ${\mathrm{O}}_{2}$, $\mathrm{C}{\mathrm{O}}_{2}$, and ${\mathrm{N}}_{2}\mathrm{O}$, leading to the production of cold trapped $(\ensuremath{\approx}20\phantom{\rule{0.3em}{0ex}}\mathrm{mK})$ $\mathrm{Ba}{\mathrm{O}}^{+}$ ions. The $\mathrm{Ba}{\mathrm{O}}^{+}$ ions were converted back to ${\mathrm{Ba}}^{+}$ ions via reaction with room-temperature CO. Reaction rates were determined by employing molecular dynamics simulations. The cold mixed-species ion ensembles produced were used for studying the efficiency of sympathetic cooling, by variation of the ratio of laser-cooled to sympathetically cooled ion numbers. In one extreme case, 20 laser-cooled $^{138}\mathrm{Ba}^{+}$ ions were capable of maintaining the translational temperature of 120 sympathetically cooled barium isotopes $(^{135--137}\mathrm{Ba}^{+})$ and 430 $^{138}\mathrm{Ba}^{16}{\mathrm{O}}^{+}$ molecules at approximately $25\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$.
Using a laser-cooled ion trapping apparatus, we have investigated laser-induced chemical reactions between cold trapped Ba+ ions and several neutral molecular gases at room temperature, O2, CO2, and N2O, leading to the production of cold trapped (\ensuremath≈20mK)BaO+ ions. The BaO+ ions were converted back to Ba+ ions via reaction with room-temperature CO. Reaction rates were determined by employing molecular dynamics simulations. The cold mixed-species ion ensembles produced were used for studying the efficiency of sympathetic cooling, by variation of the ratio of laser-cooled to sympathetically cooled ion numbers. In one extreme case, 20 laser-cooled 138Ba+ ions were capable of maintaining the translational temperature of 120 sympathetically cooled barium isotopes (135−−137Ba+) and 430 138Ba16O+ molecules at approximately 25mK.
We have produced sympathetically cooled barium monohalide ions BaX+ (X=F, Cl, Br) by reacting trapped, laser-cooled Ba+ ions with room-temperature gas-phase neutral halogen-containing molecules. Reaction rates for two of these (SF6 and CH3Cl) have been measured and were found to be in agreement with classical models. BaX+ ions are promising candidates for cooling to the rovibrational ground state, and our method presents a straightforward way to produce these polar molecular ions.
This thesis reports on experimental and theoretical studies of the sympathetic cooling of complex molecular ions demonstrating that this general method for cooling atomic and molecular ions is reliable and efficient.
For this purpose, complex molecular ions and barium ions have been confined simultaneously in a linear Paul trap. The complex molecular ions are generated in an electrospray ionization system and transferred to the trap via a 2 m long octopole ion guide. These molecular ions are pre-cooled by room temperature helium buffer gas so that they can be captured by the trap. The atomic barium ions are loaded from a barium evaporator oven and are laser-cooled by a 493 nm cooling laser and a 650 nm repumping
laser. Due to the mutual Coulomb interaction among these charged particles, the kinetic energy of the complex molecular ions can be reduced significantly.
In our experiments we have demonstrated the sympathetic cooling of various molecules (CO2, Alexa Fluor 350, glycyrrhetinic acid, cytochrome c) covering a wide mass range from a few tens to 13000 amu. In every case the molecular ions could be cooled down to millikelvin temperatures.
Photo-chemical reactions of the 138Ba+ ions in the (2P1/2) excited state with gases such as O2, CO2, or N2O, could be observed. If the initial 138Ba+ ion ensemble is cold, the produced 138BaO+ ions are cold as well, with a similar temperature as the laser-cooled barium ions (a few tens of millikelvin). The back-reaction of 138BaO+ ions with neutral CO to 138Ba+ is possible and was observed in our experiments as well.
A powerful molecular dynamics (MD) simulation program has been developed. With this program dynamic properties of ion ensembles, such as sympathetic interactions or heating effects, have been investigated and experimental results have been analyzed to obtain, for example, ion numbers and temperatures. Additionally, the feasibility of nondestructive spectroscopy via an optical dipole excitation and the lineshape of rovibrational
transitions of 1.4 and 5.1 μm have been studied.
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.
Chemical reactions between ultracold 9Be+ ions and room-temperature molecular hydrogen isotopomers and between ultracold H3+ ions and room-temperature O2 have been studied in a laser-cooling ion trap apparatus. For small Coulomb crystals of beryllium ions, reactions can be followed at the single-ion level. We demonstrate characterization of a chemical reaction in which neither one of the reactants nor the product is directly detectable. In this case molecular dynamics simulations were used for the determination of ion numbers from images of the 9Be+ ion ensemble. The observed reaction rates are in agreement with the Langevin ion-neutral reaction theory.
Ion traps are an incredibly versatile tool which have many applications throughout the physical sciences, including such diverse topics as mass spectrometry, precision frequency metrology, tests of fundamental physics, and quantum computing. In this thesis, experiments are presented which involve trapping and measuring properties of Th³⁺. Th³⁺ ions are of unique interest in part because they are a promising platform for studying an unusually low-lying nuclear transition in the 229Th nucleus which could eventually be used as an exceptional optical clock. Here, experiments to measure electronic lifetimes of Th³⁺ are described. A second experimental topic explores the production of sympathetically cooled molecular ions. The study of cold molecular ions has a number of applications, some of which include spectroscopy to aid the study of astrophysical objects, precision tests of quantum electrodynamics predictions, and the study of chemical reactions in the quantum regime. The experiments presented here involve the production of barium monohalide ions, BaX⁺ (X = F, Cl, Br). This type of molecular ion proves to be particularly promising for cooling to the rovibrational ground state. The method used for producing BaX⁺ ions involves reactions between cold, trapped Ba⁺ ions and neutral gas phase reactants at room temperature. The Ba⁺ ion reaction experiments presented in this thesis characterize these reactions for producing Coulomb crystals composed of laser cooled Ba⁺ ions and sympathetically cooled BaX⁺ ions.
The recent development of a range of techniques for producing cold atoms and molecules at very low translational temperatures T < or = 1 K has provided the opportunity to investigate collisional processes in a new physical regime. We have recently presented a new experimental method to study low-temperature reactive collisions between translationally cold ions and neutral molecules (S. Willitsch et al., Phys. Rev. Lett. 2008, 100, 043203). Our technique relies on the combination of a quadrupole-guide velocity selector for the generation of translationally cold neutral molecules with a facility to produce ordered structures of cold ions (Coulomb crystals) by laser cooling in a linear quadrupole ion trap. The strong localisation of the ions in the trap in combination with the high sensitivity of laser-induced-fluorescence detection enabled us to study chemical reactions on the single-particle level, down to temperatures of T approximately 1 K. In the current paper, we present a detailed characterisation of the scope and limitations of this method based on our study of the reaction between laser-cooled Ca+ ions and velocity-selected CH3F molecules. The properties of our cold-neutrals source and the dependence of the measured rate constant on the shape of the Coulomb crystals, trapping and laser-cooling parameters are discussed. An extension of our technique for the study of low-temperature reactions with sympathetically cooled molecular ions (translational temperature T > 10 mK) is presented and first results on the charge-transfer reaction between OCS+ and ND3 are discussed. Finally, perspectives for further developments of our method are explored.