Abstract I present a novel scheme for producing cold (magnetically trappable) atomic hydrogen, based on threshold photodissociation of the BaH+ molecular ion. BaH+ can be sympathetically cooled using laser cooled Ba+ in an ion trap, before it is photodissociated on the single photon A1Σ+ ← X1Σ+ transition. The small mass ratio between Ba+ and BaH+ ensures a strong overlap within the ion trap for sympathetic cooling, while the large mass ratio between BaH+ and H means that the released hydrogen can be up to 139 times colder than the parent molecular ions. I examine the hydrogen production rate, and describe how the trap dynamics and photodissociation laser detuning influence the achievable energies. The low infrastructure costs and the ion trap nature of the scheme make it suitable for loading hydrogen into an antihydrogen experiment. This would support a direct matter–antimatter comparison, which could provide important clues as to why our Universe contains so little antimatter.
Abstract I present a novel scheme for producing cold (magnetically trappable) atomic hydrogen, based on threshold photodissociation of the BaH+ molecular ion. BaH+ can be sympathetically cooled using laser cooled Ba+ in an ion trap, before it is photodissociated on the single photon A1Σ+ ← X1Σ+ transition. The small mass ratio between Ba+ and BaH+ ensures a strong overlap within the ion trap for sympathetic cooling, while the large mass ratio between BaH+ and H means that the released hydrogen can be up to 139 times colder than the parent molecular ions. I examine the hydrogen production rate, and describe how the trap dynamics and photodissociation laser detuning influence the achievable energies. The low infrastructure costs and the ion trap nature of the scheme make it suitable for loading hydrogen into an antihydrogen experiment. This would support a direct matter–antimatter comparison, which could provide important clues as to why our Universe contains so little antimatter.
Abstract We simulated the motion of BaH + molecular ions which are sympathetically cooled by laser-cooled Ba + ions in linear quadrupole and octupole radiofrequency (RF) ion traps. By varying the trap voltages, we studied the velocity distribution of hydrogen atoms that would be produced upon threshold photodissociation of the BaH + . We found that hydrogen with a kinetic energy below 30 mK × k B can be produced in both traps, making the scheme suitable for loading hybrid ion–atom traps such as those used to synthesize and trap antihydrogen or study ultracold atom–ion collisions. We discuss how the dynamics of RF traps could be exploited to produce hydrogen below this limit by either driving the photodissociation transition with a small laser beam aligned along the axis of the trap, or by pulsing the laser during the micromotion turning points.
Answering the question of why we live in a matter-dominated universe is of great interest to contemporary physicists, as the Standard Model of Particle Physics predicts that matter and an-timatter should only ever be produced in equal parts. Antihydrogen is a good candidate for searches for asymmetries between matter and antimatter as it is the simplest antimatter bound state, and it has an extremely well-understood matter counterpart: the hydrogen atom. The AL-PHA collaboration at CERN can now routinely trap several hundred antihydrogen atoms in a magnetic trap, allowing precise measurements of the fundamental properties of antihydrogen.ALPHA currently traps around 20 antihydrogen atoms every few minutes, so accumulat-ing enough antiatoms to perform precision measurements can take many hours. Increasing this trapping rate would allow for faster or more precise measurements. Simulations and experimen-tal data show that there is a strong correlation between the temperature of the positron plasma used in antihydrogen formation and the trapping rate that can be achieved. This thesis describes work towards using laser-cooled beryllium ions to obtain colder positron plasmas in the ALPHA trapping apparatus. Singly-charged beryllium ions are liberated from the surface of a solid beryl-lium target inside the apparatus via laser ablation, where the ions can be trapped, laser-cooled and mixed with positron plasmas. By mixing the ions and positrons together, they can exchange energy, and the ions can provide cooling to the positrons. Careful tuning of laser and trap pa-rameters, as well as use of other techniques, has allowed for the successful sympathetic cooling of positron plasmas in ALPHA.
A counterintuitive scheme to produce ultracold hydrogen via fragmentation of laser cooled diatomic hydrides is presented where the final atomic H temperature is inversely proportional to the mass of the molecular parent. In addition, the critical density for formation of a Bose-Einstein condensate (BEC) at a fixed temperature is reduced by a factor (mH/mMH)3/2 over directly cooled hydrogen atoms. The narrow Feshbach resonances between a 1S0 atom and hydrogen are well suited to a tiny center of mass energy release necessary during fragmentation. With the support of ab initio quantum chemistry, it is demonstrated that BaH is an ideal diatomic precursor that can be laser cooled to a Doppler temperature of $\ensuremath{\sim}26\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{K}$ with just two rovibronic transitions, the simplest molecular cooling scheme identified to date. Preparation of a hydrogen atom gas below the critical BEC temperature Tc is feasible with present cooling technology, with optical pulse control of the condensation process.
One of the best ways to study antimatter is to investigate antihydrogen, the bound state of an antiproton and a positron. Antihydrogen atoms do not exist naturally and must be synthesized in the lab by merging carefully-prepared plasmas of positrons and antiprotons. If the atoms are created in a magnetic trap like the one used by the ALPHA experiment at CERN, then a fraction of the coldest atoms remain trapped, while the rest escape and annihilate on the trap walls. The trapped atoms may then be probed using microwaves or lasers to make high-precision comparisons with hydrogen. Increasing the trapping rate would allow us to perform precision measurements on antihydrogen in a shorter period of time and with better systematics. Particle simulations indicate that by sympathetically cooling positrons using laser-cooled beryllium ions, we have the ability to improve the antihydrogen trapping rate by up to two orders of magnitude. This thesis describes the effort to design and qualify a beryllium ion source that is compatible with the extreme environment of the ALPHA trapping apparatus. To produce the ions, pulsed laser ablation of a beryllium target is investigated and the ion plume is characterized. By carefully choosing the laser parameters, a plume with a suitable number of ions and kinetic energy distribution can be created for subsequent trapping and laser-cooling in a Penning trap. This thesis reports the successful demonstration of the ion source, in particular its compatibility with the requirements of the ALPHA experiment. The ion source has been installed into the main apparatus at ALPHA and is expected to be commissioned early next year.
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.
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.