Abstract In this article we present the production of a pulsed molecular C2 − beam and the subsequent trapping of C2 − in a digital ion trap (DIT). The anionic molecules were produced in a pulsed discharge valve from acetylene and carbon dioxide gas in a helium carrier. The mass spectrum of the pulsed anion beam is initially recorded using a Wien filter. Subsequently, we measured the mass spectrum using the DIT and its stability diagram. The results are compared to a theoretical description of the trap’s stability conditions. The research is relevant for future laser cooling experiments of trapped C2 − and for sympathetic cooling experiments of other anionic species (antiprotons, electrons, anionic atoms and molecules) and are of interest for precision experiments on antihydrogen as performed at the antiproton decelerator facility at CERN.
Abstract In this article we present the production of a pulsed molecular C2 − beam and the subsequent trapping of C2 − in a digital ion trap (DIT). The anionic molecules were produced in a pulsed discharge valve from acetylene and carbon dioxide gas in a helium carrier. The mass spectrum of the pulsed anion beam is initially recorded using a Wien filter. Subsequently, we measured the mass spectrum using the DIT and its stability diagram. The results are compared to a theoretical description of the trap’s stability conditions. The research is relevant for future laser cooling experiments of trapped C2 − and for sympathetic cooling experiments of other anionic species (antiprotons, electrons, anionic atoms and molecules) and are of interest for precision experiments on antihydrogen as performed at the antiproton decelerator facility at CERN.
The study of ion-molecule reactions plays a vital role in cold chemistry, thus implying the need of well-controlled ion ensembles in a cold environment. The internal and external degrees of freedom of molecular ions, trapped in multipole radio frequency ion traps, can be cooled via collisions with neutral atoms [1]. Usually the coolant undergoes collisions with a thermal shield mounted on a cryostat attaining temperatures of about 4 K. To overcome this lower temperature limit two approaches have been proposed. Firstly, using a laser-cooled buffer-gas localized at the center of the ion cloud [2]. Secondly, sympathetic cooling via a second laser cooled molecular anion like C2− [3]. This, in addition to its potential role in forming carbon chain anions in the interstellar medium, makes C2− an important molecule in anion research. In this contribution we present a newly developed 16-pole cryogenic radio frequency wire trap for spectroscopic and kinetic studies of molecular anions. First results of quenching rates of the first excited vibrational level of C2− due to collisions with H2 are presented. Furthermore, possible reaction pathways with H2 are discussed and recent spectroscopic measurements of the second electronic excited state of C2− are presented. References: [1] R. Wester , J. Phys. B 42, 154001 (2009) [2] B. Höltkemeier et al., Phys. Rev. A 94, 062703, (2016) [3] P. Yzombard et al., Phys. Rev. Lett. 114, 213001 (2015)
The goal of the AEGIS project at CERN is to create ultra-cold antihydrogen in order to perform precise measurements on the direction and magnitude of the gravitational force felt by antimatter. In order to provide the best results it is necessary to create as much antihydrogen as possible. The method of production chosen by AEGIS is a charge exchange process between excited positronium and ultra-cold antiprotons. In order to cool the antiprotons to well below mK temperatures, the AEGIS collaboration is attempting to sympathetically cool the antiprotons using laser cooled C2 anions. Currently we are in the process of conducting experiments on creating and confining a beam of C2 anions. One of these experiments is exciting them with a 2.54micro meter laser and attempting to observe the fluorescence from the C2 anions. This is being done not only to prove that we are creating C2 anions but to also gain a good estimate as to how many we have. The first step to this experiment is to create a model of the fluorescence system and calculate the probability of observing the fluorescence. In this paper I will discuss this model and the results.
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.
The study of cold and controlled molecular ions is pivotal for fundamental research in modern physics and chemistry. Investigations into cooling molecular anions, in particular, have proven to be of key consequence for the production of cold antihydrogen, the creation, and study of anionic Coulomb crystals as well as in atmospheric research and astrochemistry. The commonly used anion cooling technique via collisions with a buffer gas is limited by the temperature of the used cryogenic cooling medium. Here, we demonstrate forced evaporative cooling of anions via a laser beam with photon energies far above the photodetachment threshold of the anion. We reach runaway evaporative cooling of an anionic OH− ensemble from an initial temperature of 370(12) K down to 2.2(8) K. This corresponds to three orders of magnitude increase in the ions' phase space density approaching the near-strong Coulomb coupling regime. A quantitative analysis of the experimental results, via a full thermodynamic model including the role of intrinsic collisional heating, represents the anion cooling dynamics without any fitting parameters. This technique can be used to cool, in principle, any anionic species below liquid helium temperature, providing a novel tool to push the frontiers of anion cooling to much lower than the state-of-the-art temperature regimes.
Trapping molecular ions that have been sympathetically cooled with laser-cooled atomic ions is a useful platform for exploring cold ion chemistry. We designed and characterized a new experimental apparatus for probing chemical reaction dynamics between molecular cations and neutral radicals at temperatures below 1 K. The ions are trapped in a linear quadrupole radio-frequency trap and sympathetically cooled by co-trapped, laser-cooled, atomic ions. The ion trap is coupled to a time-of-flight mass spectrometer to readily identify product ion species and to accurately determine trapped ion numbers. We discuss, and present in detail, the design of this ion trap time-of-flight mass spectrometer and the electronics required for driving the trap and mass spectrometer. Furthermore, we measure the performance of this system, which yields mass resolutions of m/Δm ≥ 1100 over a wide mass range, and discuss its relevance for future measurements in chemical reaction kinetics and dynamics.
In this thesis the dynamics of a trapped ion immersed in \na spatially localized buffer gas is investigated. For a homogeneous buffer gas, the ion’s energy distribution reaches a stable equilibrium only if the mass of the buffer gas atoms is below a critical value. This limitation can be overcome by using multipole traps in combination and/or a spatially confined buffer gas. Using a generalized model \nfor elastic collisions of the ion with the buffer gas atoms, the ion’s energy distribution is numerically determined for arbitrary buffer gas distributions and trap parameters. Three regimes characterized by the respective analytic form of the ion’s equilibrium energy distribution \nare found. One of these is a novel regime at large atom-to-ion mass ratios where the final ion temperature can tuned by adiabatically decreasing the spatial extension of the buffer gas and the effective ion trap depth (forced sympathetic cooling). The second part of the thesis presents a hybrid atom ion trap designed for sympathetic cooling of hydroxide anions. In this hybrid trap the anions are immersed in a cloud of laser cooled rubidium atoms. The translational and rovibrational temperatures of the anions is probed by photodetachment tomography and spectroscopy which shows the first ever indication of sympathetic cooling of anions by laser cooled atoms.