Ultracold cold molecular ions promise new directions in various studies of fundamental physics, such as precision measurements, ultracold chemistry and quantum information sciences. All these exciting applications require the molecular ion to be prepared at ground state of motional and internal degrees of freedom. It has been proposed that this stringent goal could be achieved through sympa-thetic cooling via collisions with laser-cooled neutral atoms. Three fundamentalissues of this method are addressed in this thesis.First, an analytical model is established to accurately describe collision-induced heating of a single ion in contact with cold neutral atoms. This model reveals that micromotion interruption is the cause of heating, and gives results about steady-state temperature and sympathetic cooling rate verified by Monte-Carlo simulations. It also provides insight into the power-law tails observed in theenergy distribution of the trapped ion.Next, we consider the case of multiple ions, whose inter-particle Coulomb repulsion causes ions in the Coulomb crystal state to spontaneously melt into a gas phase ion cloud, due to the same micromotion interruption mechanism. The analysis of this problem with a plasma model leads to the experimental determination of a quantity central to plasma physics, Coulomb Logarithm, in anion trap.Finally, we demonstrate a molecular ion spectroscopy technique through the example of trap-depletion photodissociation of BaCl + . Although not sensitive to rotational structure, this method already reveals much about the fundamental quantum physics in the photodissociation process. The measured cross-section results paves the road toward state-selective spectroscopy currently going on in our lab.
Ultracold cold molecular ions promise new directions in various studies of fundamental physics, such as precision measurements, ultracold chemistry and quantum information sciences. All these exciting applications require the molecular ion to be prepared at ground state of motional and internal degrees of freedom. It has been proposed that this stringent goal could be achieved through sympa-thetic cooling via collisions with laser-cooled neutral atoms. Three fundamentalissues of this method are addressed in this thesis.First, an analytical model is established to accurately describe collision-induced heating of a single ion in contact with cold neutral atoms. This model reveals that micromotion interruption is the cause of heating, and gives results about steady-state temperature and sympathetic cooling rate verified by Monte-Carlo simulations. It also provides insight into the power-law tails observed in theenergy distribution of the trapped ion.Next, we consider the case of multiple ions, whose inter-particle Coulomb repulsion causes ions in the Coulomb crystal state to spontaneously melt into a gas phase ion cloud, due to the same micromotion interruption mechanism. The analysis of this problem with a plasma model leads to the experimental determination of a quantity central to plasma physics, Coulomb Logarithm, in anion trap.Finally, we demonstrate a molecular ion spectroscopy technique through the example of trap-depletion photodissociation of BaCl + . Although not sensitive to rotational structure, this method already reveals much about the fundamental quantum physics in the photodissociation process. The measured cross-section results paves the road toward state-selective spectroscopy currently going on in our lab.
This dissertation details of the construction of and experiments performed in a hybrid system consisting of a cold neutral atomic magneto-optical trap (MOT) contained within a linear radio-frequency quadrupole ion trap (LQT). The combination of these two workhorses of atomic physics facilitates a variety of developing science such as the controlled investigation of ion-neutral quantum chemistry and the production of ground-state molecular ions. While the primary focus of this work is the production of ultracold molecular ions via sympathetic cooling, there has been investigation of cold (T ~ mK) ion-neutral charge exchange processes in two different ion-neutral pair systems. In addition, a possible method of producing ultracold homonuclear molecular ions via a photo-associative ionization (PAI) pathway is studied in the MOT. The LQT traps ions in spatial overlap with a 40Ca MOT constituting an ultracold buffer gas purposed to sympathetically cool molecular ions.An effective, general method of producing ground-state molecular ions would open a field of physics allowing, for example, precise measurements of molecular transitions which are uniquely sensitive to parity violation, or the possible variation of fundamental constants. Another promising application of cold molecular ions is an implementation of a quantum computing architecture by coupling micro-fabricated strip-line resonators to the microwave rotational transition found in many diatomic molecular ions. The molecular ion BaCl+ is chosen to be used in proof-of-principle sympathetic cooling experiments. Ultimately, a vibrational internal state thermometry experiment shows that the Ca MOT performs very efficiently at quenching the vibrational motion of the BaCl+ molecular ion.As a product of overcoming experimental challenges, this thesis discusses several experiments to characterize trapping and reaction dynamics in the hybrid system. For example, charge transfer measurements are performed with two different laser-cooled atomic ion species, Yb+ and Ba+, which are allowed and disallowed ground-state reaction, respectively. Controlled measurements of these inelastic processes test the cutting edge of quantum theory of ion-neutral interaction in both the ground-state and under optical excitation.
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.
One of the current themes in quantum physics is the creation of cold ( 1 K) molecules to study molecule-molecule, molecule-atom, and molecule-radiation interactions under these novel low-temperature conditions. This book highlights the field of cold trapped molecular ions. In particular, the development of a novel, universal cooling method for charged particles, sympathetic cooling using laser-cooled atoms, together with the experimental and computational techniques required are extensively reviewed. The book also reviews first spectacular applications of the cold ion ensembles produced: the study of the properties of cold non-neutral plasmas, the study of ion-neutral chemical reactions and of photodissociation processes, and high-resolution rotational overtone spectroscopy of cold hydrogen molecular ions with unprecedented spectral resolution. The large potential of this field of research and its possible impact to other fields of physics, chemistry, and biology are highlighted. The book summarizes the main lines of research in cold trapped molecular ion physics, thus, making it useful as an introduction for advanced students and experienced researchers.
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.
We propose a method for the production of ultracold molecular ions. This method utilizes sympathetic cooling due to the strong collisions between appropriately chosen molecular ions and laser-cooled neutral atoms to realize ultracold internal ground-state molecular ions. In contrast to other experiments producing cold molecular ions, our proposed method efficiently cools both the internal and external molecular-ion degrees of freedom. The availability of an ultracold, absolute ground-state sample of molecular ions would have broad impact to fields as diverse as quantum chemistry, astrophysics, and fundamental physics and may lead to the development of a robust scalable quantum computer.
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.