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 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.
Gas-phase singly protonated organic molecules of mass 410 Da (Alexa Fluor 350) have been cooled from ambient temperature to the hundred millikelvin range by Coulomb interaction with laser-cooled barium ions. The molecules were generated by an electrospray ionization source, transferred to and stored in a radio-frequency trap together with the atomic ions. Observations are well described by molecular dynamics simulations, which are used to determine the spatial distribution and thermal energy of the molecules. In one example, an ensemble of 830 laser-cooled 138Ba+ ions cooled 200 molecular ions to less than 115 mK. The demonstrated technique should allow a large variety of protonated molecules to be sympathetically cooled, including molecules of much higher mass, such as proteins.
We have produced ensembles of cold 16O+2, 40Ar+, 12C16O+2, and various isotopes of barium ions (135Ba+, 136Ba+ and 137Ba+) via sympathetic cooling with laser-cooled 138Ba+ in a linear radiofrequency trap. The sympathetically cooled species were embedded in the centre of large 138Ba+ Coulomb crystals containing up to 2000 ions and were identified by motional resonance excitation. Crystals with molecular fractions exceeding 70% were obtained. The observed multi-species crystal structures agree well with results from molecular dynamics simulations. The simulations were also used to deduce an upper limit for the translational temperature of the molecular ions, as low as 20 mK.
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.
This Dissertation details the development of state-of-the-art hybrid atom-ion trapping architecture and technique towards increasing the quantum control of matter and the detection of chemical processes at cold temperatures. Experimental work discussed herein is performed primarily using the second-generation of the MOTion trap, a hybrid atom-ion trap consisting of a co-located magneto-optical trap (MOT) and a linear quadrupole trap (LQT), with which 40Ca and a variety of atomic and molecular ions (Yb+, Ba+, BaCl+, and others) can be co-trapped and cooled. Such a device enables the immersion of laser-cooled, ionic Coulomb crystals in diffuse gases of laser-cooled, highly-polarizable neutral atoms, allowing for studies in sympathetic cooling, cold atom-ion chemistry, and nonequilbrium collisional dynamics.In the first of these studies, I discuss our method of sympathetic cooling in which we use a laser-cooled gas of Ca atoms to sympathetically cool BaCl+ ions confined in the LQT. Because of the large polarizability of Ca, this method is found to be extremely efficient at cooling the notoriously difficult vibrational degree of freedom. This technique opens the door to research with general, ultracold, highly-localized molecular ions in the quantum ground-state which are not able to be produced using the more traditional methods of Doppler-cooling or photoassociation.In the study of cold chemistry in the hybrid trap, I characterize a fundamental reaction, known as charge exchange, at the single-particle level in which an electron is transferred from a neutral atom to an atomic ion during an inelastic collision. This low-temperature process is thought to be foundational to the chemistry leading to the formation of stars, planets and interstellar clouds in our universe. Herein I discuss how improved experimental architecture has enabled the determination of reaction rate constants for individual electronic states of the reactants.Our theoretical study of the collisional processes in hybrid atom-ion traps has led to the discovery of surprising nonequilibrium physics which exists in the hybrid trap environment. Herein I outline the collisional dynamics between neutral atoms and atomic ions in the presence of a time-dependent confining potential and describe how these dynamics manifest themselves in the emergence of nonequilbrium phenomena as indicated by bifurcations in steady-state energies of trapped ions. I show how the these dynamics can potentially limit the efficiency and overall viability of using neutral buffer gases to sympathetically cool the translational motion of trapped ions to ultracold temperatures.In addition to these experimental studies, the development of architecture is discussed with focus given to the implementation of a novel time-of-flight mass spectrometer (TOF-MS) for use with hybrid atom-ion traps as well as standard LQTs. This simple, modular device couples radially with the LQT and enables the straightforward mass analysis of trapped species without complications derived from typical experimental difficulties, engineering constraints, and financial concerns. With this robust device we precisely characterize chemical processes which can take place in these hybrid atom-ion traps. Additionally, I describe how this new architecture is used to perform the action spectroscopy of several previously unstudied molecular ions in order to further establish the field of cold molecular ion research and the widespread use of hybrid atom-ion traps.
In recent years there has been much progress in the field of cold and ultracold molecular physics and a variety of experimental techniques for producing cold matter now exist. In particular, the generation of trapped molecular ions at mK temperatures has been achieved by sympathetic-cooling with laser-cooled atomic ions. By implementing schemes to selectively prepare and control the internal quantum state of molecular ions, and developing detection techniques, it will be increasingly possible to study cold state-selected chemical collisions in an ion-trap. Most molecular species produced in a selected rovibrational state have a lifetime of a few seconds, before the population is redistributed across numerous rovibrational states by interaction with the ambient blackbody radiation (BBR). Consequently, the investigation of state-selected reaction dynamics at low temperatures in experiments where long time scales (minutes to hours) are required, is hindered. This thesis looks into developing strategies that maintain state selection in molecular ions, allowing one to observe state-selected reactions in cold environments, in particular the state-selected reaction between C2H+2 and ND3. Examining reactive ion molecule collisions under cold conditions provides insight into fundamental reaction dynamics, which are thermally averaged out at higher temperatures. A theoretical model is used to investigate laser-driven, blackbody-mediated, rotational cooling schemes for several 1Σ and 2Π diatomic species. The rotational cooling is particularly effective for DCl+ and HCl+, for which 92% and >99% (respectively) of the population can be driven into the rovibrational ground state. For the other systems a broadband optical pumping source is found to enhance the population that can be accumulated in the rovibrational ground state by up to 29% more than that achieved when exciting a single transition. The influence of the rotational constant, dipole moments and electronic state of the diatomics on the achievable rotational cooling is also studied. This approach is extended to consider the BBR interaction and rotational cooling of a linear polyatomic ion, C2H+2, which has a 2Π electronic ground state. The (1-0) band of the ν5 cis-bending mode is infrared active and strongly overlaps the 300 K blackbody spectrum. Hence the lifetimes of state-selected rotational levels are found to be short compared to the typical timescale of ion trapping experiments. Laser cooling schemes are proposed that could be experimentally viable, which involves simultaneous pumping of a set of closely spaced Q-branch transitions on the 2Δ5/2-2Π3/2 band together with two 2Σ+– 2Π1/2 lines. It is shown that this should lead to >70% of total population in the lowest rotational level at 300 K and over 99% at 77 K. In order to identify states of the acetylene ion that could be trapped sufficiently long enough for state-selected reactions in the ion trap with decelerated ND3, the theoretical work has been complemented by experimental investigations into the production of C2H+2 in selected states, and ion trapping of the same using sinusoidal and digital trapping voltages. Appropriate (2+1) REMPI (Resonance Enhanced Multiphoton Ionization) schemes are used to produce C2H+2 in different quantum states, with (1+1) Resonance Enhanced Multiphoton Dissociation (REMPD) employed to detect the ion thus produced. The concept of digital ion trapping for ejection onto MCPs is introduced. A comprehensive comparison between sinusoidal and digital trapping fields has been performed with respect to trap depth and stability regions. Programs have been developed to calculate the stability regions for different ions under varying experimental conditions. The trap depth has been derived for both digital and sinusoidal trapping fields. It is observed that as τ increases, the trap depth of a digital trap increases. For τ = 0.293, the trap depth and stability diagram for both sinusoidal and digital trapping fields would be equivalent. The trap depth at which the sinusoidal trap operates experimentally in our research group is ˜1.36 eV. In contrast, the experimental parameters at which the digital trap operates generates a trap depth of 1.21 eV. Ca+ Coulomb crystals have been formed, stably trapped and stored for extended periods of time in both sinusoidally and digitally time-varying trapping fields. The sympathetic cooling of a diverse range of ions into Ca+ Coulomb crystals is demonstrated, again using both sinusoidal and digital trapping fields. Mass spectrometric detection of ionic reaction products using a novel ejection scheme has been developed, where ejection is achieved by switching off the trapping voltage and converting the quadrupole trap into an extractor-repeller pair by providing the ion trap electrodes with appropriate ejection pulses. This technique is developed using a digital trapping voltage rather than the sinusoidal trapping voltage, as ejection with sinusoidal trapping voltages is not clean (resonance circuitry used in the electronics induces ringing after switching off the trapping voltage). Coulomb crystals, both pure Ca+ and multi-component crystals, are ejected from the ion trap and the TOF trace obtained is recorded on an oscilloscope. When the integrated, base-line subtracted TOF peak is plotted against the number of ions in a Ca+ crystal and sympathetically-cooled Ca+ – CaF+ crystal, a linear relationship is obtained. This technique is found to be well mass-resolved, with the signal arising from CaOH+ (57 amu) and CaOD+ (58 amu) resolvable on the TOF trace. This technique would enable one to monitor a reaction in a Coulomb crystal where the reactant and product species are both either lighter or heavier than calcium, such as the reaction between C2H+2 and ND3, something which has not been previously possible. It is, also, potentially a very important technique for reactions with many product channels.