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Ions in cold electrostatic storage devices

P. Reinhed

Published Jan 1, 2010• 1 citation
OpenAlex

Abstract

We have constructed a compact purely electrostatic ion-beam trap, ConeTrap, which we have mounted inside a double-walled vacuum chamber. In the inner vacuum chamber, we can obtain ultra-high vacuum (UHV) conditions and reach thermal equilibrium at well controlled temperatures down to 10 K. The chamber was constructed partly with the purpose of making high-precision measurements in ConeTrap, but also as a test-chamber for testing components (such as the detector-assembly tested and described in this thesis and paper III) to be used in the DESIREE (Double ElectroStatic Ion Ring ExpEriment) facility. The latter is a double electrostatic ion storage-ring being constructed at Stockholm University, in which the conditions are meant to mimic the environment in the interstellar medium. The interaction between two oppositely charged ions at very low relative velocities (controlled collision energies down to 10 meV) may then be studied in a section of the storage device where the two ion beams merge. The lifetime of loosely bound electronic systems, for example He-, is, at room temperature (and even at much lower temperatures), significantly affected by photons from blackbody radiation from the experimental device and its surroundings. The cryogenic temperature and low pressure obtained in the test chamber have made it possible to use ConeTrap to make the first correction-free lifetime measurement of the long-lived J=5/2 fine-structure level of the metastable 1s2s2p 4Po state of He-. Under the assumption of a statistical population of the fine-structure levels, at the time when the ions are created, we have also deduced the lifetimes of the short-lived J=1/2 and J=3/2 fine-structure levels. Furthermore, we have used ConeTrap to measure the pressure dependent storage lifetimes of He+ and Ar+ ions over wide ranges of temperatures and pressures, and we have thus been able to store positive ions with storage lifetimes of tens of seconds.

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We have constructed a compact purely electrostatic ion-beam trap, ConeTrap, which we have mounted inside a double-walled vacuum chamber. In the inner vacuum chamber, we can obtain ultra-high vacuum (UHV) conditions and reach thermal equilibrium at well controlled temperatures down to 10 K. The chamber was constructed partly with the purpose of making high-precision measurements in ConeTrap, but also as a test-chamber for testing components (such as the detector-assembly tested and described in this thesis and paper III) to be used in the DESIREE (Double ElectroStatic Ion Ring ExpEriment) facility. The latter is a double electrostatic ion storage-ring being constructed at Stockholm University, in which the conditions are meant to mimic the environment in the interstellar medium. The interaction between two oppositely charged ions at very low relative velocities (controlled collision energies down to 10 meV) may then be studied in a section of the storage device where the two ion beams merge. The lifetime of loosely bound electronic systems, for example He-, is, at room temperature (and even at much lower temperatures), significantly affected by photons from blackbody radiation from the experimental device and its surroundings. The cryogenic temperature and low pressure obtained in the test chamber have made it possible to use ConeTrap to make the first correction-free lifetime measurement of the long-lived J=5/2 fine-structure level of the metastable 1s2s2p 4Po state of He-. Under the assumption of a statistical population of the fine-structure levels, at the time when the ions are created, we have also deduced the lifetimes of the short-lived J=1/2 and J=3/2 fine-structure levels. Furthermore, we have used ConeTrap to measure the pressure dependent storage lifetimes of He+ and Ar+ ions over wide ranges of temperatures and pressures, and we have thus been able to store positive ions with storage lifetimes of tens of seconds.
We have developed a small purely electrostatic ion-beam trap which may be operated in thermal equilibrium at precisely controlled temperatures down to 10 K. Thus, we avoid magnetic field induced mixing of quantum states and may effectively eliminate any influence from absorption of photons from blackbody radiation. We report the first correction-free measurement of the lifetime of the 1s2s2p {4}P{5/2}{0} level of 4He(-) yielding the high-precision result 359.0 +/- 0.7 micros. This result is an essential proof-of-principle for cryogenic electrostatic storage rings and traps for atomic and molecular physics.
Ultra-high vacuum conditions are ideal for the study of trapped ions. They offer an almost perturbation-free environment, where ions confined in traps can be studied for extended periods of time-facilitating precision measurements and allowing infrequent events to be observed. However, if one wishes to study processes involving molecular ions, it is important to consider the effect of blackbody radiation (BBR). The vast majority of molecular ions interact with BBR. At 300 K, state selection in trapped molecular ions can be rapidly lost (in a matter of seconds). To address this issue, and to maintain state selectivity in trapped molecular ions, a cryogenic ion trap chamber has been constructed and characterized. At the center of the apparatus is a linear Paul ion trap, where Coulomb crystals can be formed for ion-neutral reaction studies. Optical access is provided, for lasers and for imaging of the crystals, alongside ion optics and a flight tube for recording time-of-flight mass spectra. The ion trap region, encased within two nested temperature stages, reaches temperatures below 9 K. To avoid vibrations from the cryocooler impeding laser cooling or imaging of the ions, vibration-damping elements are explicitly included. These components successfully inhibit the coupling of vibrations from the cold head to the ion trap-confirmed by accelerometer measurements and by the resolution of images recorded at the trap center (at 9 and 295 K). These results confirm that the cryogenic ion trap apparatus meets all requirements for studying ion-neutral reactions under cold, controlled conditions.
This work consists of two major parts, a theoretical and an experimental one. The \nlatter part investigates ion molecule reactions performed in a temperature variable \n22-pole ion trap. The former one analyses different aspects of radio frequency ion \ntraps and the dynamics of charged particles located inside. \nTherefore, fundamental concepts for the description of charged particles in oscil- \nlating electrical fields are discussed on as a first step (Chapter 3). The calculation \nof these fields via different numerical methods is elaborated on subsequently (Chap- \nter 4). The boundary element method (BEM) has proven highly adequate for the calculation of electrostatic potentials. Hence it is depicted in detail. Using analytically solvable electrostatic problems, the previously calculated fields will be tested with respect to their accuracy. To perform the necessary calculations both own numeric routines have been developed and available routines have been employed. \nThis work shows how the numerically, calculated fields can be converted into an analytical description via appropriate multipole expansion. This permits an efficient simulation and investigation of the dynamics of charged particles in ion traps. \nSimulations of different existing multipole ion trap geometries help to analyze the behavior of ions in oscillating fields with respect to, inter alia, the increase of their translational energy, their energy distribution and shifts of the secular frequency caused by higher anharmonicities (Chapter 5). Furthermore, space charge effects and perturbations of the fields due to mechanical misalignments of electrodes are included here. Consequently, this work is meant to contribute to the optimization of existing ion traps and to the development of new and better ones. \nThe experimental part focuses on the reaction of nitrogen ions with hydrogen molecules taking into account the fine structure state of N + and the nuclear-spin state of H 2 . \nThis reaction can be considered as the first process or step in the formation of interstellar ammonia. Up to date this very fundamental reaction – although having been \nstudied in several laboratories for decades – is not fully understood. For example, it remains unclear whether or not the reaction really endothermic or it is just inhibited \nby a barrier. Also, it is not known to what extent the fine structure state energy N + favours/promotes this reaction. All these questions are of great relevance for low temperature astrochemistry. \nApplying a non-adiabatic reaction model and a global fit, fine structure state specific rate coefficients for the reaction mentioned above could be derived (Chapter 7). \nInitially, a large number of test measurements were conducted in order to clarify the impact of different measurement parameters like e.g. temperature or number \ndensity on measured rate coefficients (Chapter 6).On the basis of these test measurements probable conclusions of initial difficulties with reproducing test results could be drawn. Thereupon, experimental processes were optimized to minimize errors in determination the rate coefficients. As a result,+ rate coefficients for the reaction of N + \n2 und CO with hydrogen in a temperature range between 10 to 120 K were determined as well as ternary rate coefficients \nfor the formation of nitrogen-helium-clusters (NHe n ) + (n = 1, 2) in a temperature range between 10 – 13 K (Chapter 9). Also, measurements were performed with respect to fine structure state changing collisions of the nitrogen ion due to helium. Moreover, this work contains the characterization of a piezoelectric valve which was performed using the reaction of Ar + ions with H 2 (Chapter 10). The purpose of \nthe characterization is to develop a new experimental method for the study of ion molecule reactions. This aims at separating translational thermalization of ions as \nwell as their relaxation of the internal excited states from the reaction process. A quantitative model could be developed in order to describe the reaction gas flow as \na function of time, temperature and reaction gas pressure.
The lifetimes of the metastable 1s2s2p4P5/21s2s2p{ }^{4}{P}_{5/2}1s2s2p4P5/2​ level of ${\mathrm{He}}^{\ensuremath{-}},$ as well as the lifetime of the average of the 4P3/2{}^{4}{P}_{3/2}4P3/2​ and 4P1/2{}^{4}{P}_{1/2}4P1/2​ levels, have been measured using a new type of ion trap that stores keV ion beams using electrostatic fields only. The use of a pure electrostatic field avoids the complication of magnetic-field-induced mixing effects, which can interfere with the measurement of the spontaneous decay. The measured lifetime for the 4P5/2{}^{4}{P}_{5/2}4P5/2​ state, after correction for decay induced by blackbody radiation, is $343\ifmmode\pm\else\textpm\fi{}10 \ensuremath{\mu}\mathrm{s}.$ This value is consistent with previous experiments, and in excellent agreement with the most recent theoretical calculations. The average lifetime of the 4P3/2{}^{4}{P}_{3/2}4P3/2​ and 4P1/2{}^{4}{P}_{1/2}4P1/2​ is $8.9\ifmmode\pm\else\textpm\fi{}0.2 \ensuremath{\mu}\mathrm{s},$ which is about 20% lower than the weighted theoretical value.
Charged particles can be confined by electromagnetic fields to a finite volume in space for extended periods of time. The extremely long observation time along with the nearly perturbation-free environment under ultra-high vacuum conditions opens new regions of spectroscopy, which are not accessible by other methods. In microwave spectroscopy resonance lines as narrow as Δv/v = 3 × 10−12 have been obtained, which leads directly to the possible development of a new type of frequency standard. Lifetimes of metastable levels in the region of many seconds can be observed, as well as rare collision processes between ions and molecules, which are important for the formation of larger molecules in interstellar space. Finally, phenomena like crystallisation in a non-neutral plasma may be investigated at sub-Kelvin temperafures. accessible by the new technique of laser cooling of ions.