High Resolution Image Download MS PowerPoint Slide Three-body reaction rates of Cl – with H 2 to form the weakly bound complex Cl – (H 2 ) are measured between 10 and 26 K in a linear radio-frequency wire trap. Formation of larger clusters of the form Cl – (H 2 ) 2 are also observed. The three-body (or termolecular) rate coefficients follow the form aT –1, with a = 1.12(2) × 10 –29 cm 6 K s –1 . Reverse reactions to repopulate the Cl – parent ion were measured, even though the binding energy of the complex makes bimolecular dissociative collisions energetically inaccessible at low temperatures. The back-reaction was found to be proportional to the cube of the hydrogen density, suggesting that the dissociation mechanism depends on multiple collisions. Comparisons of the rate coefficients measured in a 16-pole wire trap and a 22-pole trap demonstrate significantly lower ion temperatures in the wire trap.
High Resolution Image Download MS PowerPoint Slide Three-body reaction rates of Cl – with H 2 to form the weakly bound complex Cl – (H 2 ) are measured between 10 and 26 K in a linear radio-frequency wire trap. Formation of larger clusters of the form Cl – (H 2 ) 2 are also observed. The three-body (or termolecular) rate coefficients follow the form aT –1, with a = 1.12(2) × 10 –29 cm 6 K s –1 . Reverse reactions to repopulate the Cl – parent ion were measured, even though the binding energy of the complex makes bimolecular dissociative collisions energetically inaccessible at low temperatures. The back-reaction was found to be proportional to the cube of the hydrogen density, suggesting that the dissociation mechanism depends on multiple collisions. Comparisons of the rate coefficients measured in a 16-pole wire trap and a 22-pole trap demonstrate significantly lower ion temperatures in the wire trap.
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 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)
This contribution concentrates on the use of an ion trapping technique which has been developed for studying collision processes and chemical reactions at low temperatures and which utilizes inhomogeneous radio frequency fields. Special emphasis is put onto recent progress achieved with a cryo cooled 22-pole ion trap. Experimental results, measured at a nominal temperature of 10 K, are presented for radiative association of CH 3 + and C 2 H 2 + with H 2 . Another group of results discussed in this paper deals with the dynamics of the growth of hydrogen clusters starting from H 3 + . In all cases, the influence of H 2 rotation was examined by utilizing p-H 2 and n-H 2 . Also the process of isotope fractionation in the presence of HD and D 2 is briefly mentioned.
We theoretically investigate the trap-assisted formation of complexes in atom-ion collisions and their impact on the stability of the trapped ion. The time-dependent potential of the Paul trap facilitates the formation of temporary complexes by reducing the energy of the atom, which gets temporarily stuck in the atom-ion potential. As a result, those complexes significantly impact termolecular reactions leading to molecular ion formation via three-body recombination. We find that complex formation is more pronounced in systems with heavy atoms, but the mass has no influence on the lifetime of the transient state. Instead, the complex formation rate strongly depends on the amplitude of the ion's micromotion. We also show that complex formation persists even in the case of a time-independent harmonic trap. In this case, we find higher formation rates and longer lifetimes than in Paul traps, indicating that the atom-ion complex plays an essential role in atom-ion mixtures in optical traps.
We report on three-body recombination of a single trapped Rb(+) ion and two neutral Rb atoms in an ultracold atom cloud. We observe that the corresponding rate coefficient K(3) depends on collision energy and is about a factor of 1000 larger than for three colliding neutral Rb atoms. In the three-body recombination process large energies up to several 0.1 eV are released leading to an ejection of the ion from the atom cloud. It is sympathetically recooled back into the cloud via elastic binary collisions with cold atoms. Further, we find that the final ionic product of the three-body processes is again an atomic Rb(+) ion suggesting that the ion merely acts as a catalyzer, possibly in the formation of deeply bound Rb(2) molecules.
Heavy-Rydberg ion-pair states, molecular quantum systems that are bound by their long-range pure Coulomb attraction, are formed through Rydberg atom collisions with molecules that attach low energy electrons. Collisions of potassium Rydberg atoms K(np) with the attaching species CCl4 and SF6 lead, respectively, to dissociative and non-dissociative electron attachment with subsequent formation of bound ion-pair states K+..Cl- and K+..SF6-.\nThe experimental apparatus used to produce and detect heavy-Rydberg ion-pair states consists of a reaction region and a separate analysis region. It separates a fraction of the product ion pairs from the parent Rydberg atoms, eliminating spurious background signals associated with blackbody-induced and electric-field ionization of Rydberg atoms. A position sensitive detector in the analysis region is employed to measure their velocity, angular, and binding energy distribution and lifetimes. Measurements of the spatial distribution of ion pairs provide information on their velocity and angular distributions, and their binding energy distribution is measured using electric field-induced dissociation. The lifetime of the ion-pair states is influenced by multiple processes including internal-to-translational energy transfer, autodetachment of the electron, and neutralization through charge transfer.\nThe experimental results are compared with the results of a Monte Carlo collision code that models both the initial Rydberg electron attachment and the subsequent evolution and molecular dynamics of the ion pairs. This model highlights the factors such as the kinematics of the Rydberg atom and the attaching particle and energy released in dissociation (in the case of dissociative attachment) that are important in governing ion pair formation. The model calculations are in good agreement with the experimental data.