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Low Temperature Collisions and Reactions in a 22-Pole Ion Trap

Sven Fanghaenel

Published Jan 1, 2018• 2 citations
OpenAlex

Abstract

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.

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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.
Studying chemical reactions at very low temperatures is of importance for the understanding of fundamental physical and chemical processes. At very low energies, collisions are dominated by only a few partial waves. Thus, studies in this regime allow the characterization of quantum effects which depend on the collisional angular momentum, e.g., reactive scattering resonances and tunneling through centrifugal barriers. Additionally, the dynamics of ultralow-energy collisions is dominated by long-range interactions, i.e., “universal” chemical forces. Thus, experiments in this domain probe the details of intermolecular interactions which is of general relevance for the understanding of chemical processes. Moreover, the increasingly more precise experiments provide valuable data for benchmarking theoretical models and quantum chemical calculations. Studies of cold ion–neutral reactions in the laboratory rely on techniques for the generation of cold ions and cold neutrals. Over the last decades, the technology in this domain has made impressive progress. Many of these techniques allow not only the translational cooling of the molecules, but also a precise preparation of their internal quantum state. Thus, by an accurate preparation of both the energy and state of the reaction partners as well as by the application of external electric, magnetic or optical fields, prospects open up to study and control chemical processes at an unprecedented level of accuracy. In this chapter, we review salient theoretical concepts and recent experimental developments for studies of ion–molecule reactions at low temperatures. We first discuss the theoretical background for the description of low-energy ion–molecule reactions. Next we present key experimental methods for laboratory studies of cold ion–molecule reactions. The chapter concludes with a review of illustrative results and an outlook on future directions.
Studying chemical reactions at very low temperatures is of importance for the understanding of fundamental physical and chemical processes. At very low energies, collisions are dominated by only a few partial waves. Thus, studies in this regime allow the characterisation of quantum effects which depend on the collisional angular momentum, e.g., reactive scattering resonances and tunneling through centrifugal barriers. Additionally, the dynamics of ultralow-energy collisions is dominated by long-range interactions, i.e., "universal" chemical forces. Thus, experiments in this domain probe the details of intermolecular interactions which is of general relevance for the understanding of chemical processes. Moreover, the increasingly more precise experiments provide valuable data for benchmarking theoretical models and quantum-chemical calculations. Studies of cold ion-neutral reactions in the laboratory rely on techniques for the generation of cold ions and cold neutrals. Many of these techniques do not only allow the translational cooling of the molecules, but also a precise reparation of their internal quantum state. Thus, by an accurate preparation of both the energy and state of the reaction partners as well as by the application of external electric, magnetic or optical fields, prospects open up to study and control chemical processes at an unprecedented level of accuracy. In this chapter, we will review salient theoretical concepts and recent experimental developments for studies of ion-molecule reactions at low temperatures. We first discuss the theoretical background for the description of low-energy ion-molecule reactions. Next we present key experimental methods for laboratory studies of cold ion-molecule reactions. The chapter concludes with a review of illustrative results and an outlook on future directions.
This thesis explores the fundamental physical and chemical processes between molecular ions and neutral atoms in the gas phase at very low energies. At temperatures in the mK regime, these are dominated by long-range interactions and only a few partial waves. This study contributes to the understanding of the details of intermolecular interactions and provides valuable data for benchmarking theoretical models and quantum-chemical calculations. Although there is a wide range of previous atomic collision system studies, the investigations of cold collisions with molecular ions are limited. The experimental setup used in this work enabled the measurement of rate coefficients for molecular ion-neutral collisions. The development of a dynamic ion-neutral hybrid trap with improved control over collision energy led to new insights into charge-transfer rate coefficients for molecular ion-neutral atom collisions involving N2+^+_22+​+Rb, O2+^+_22+​+Rb and N2_22​H+^+++Rb. Two different experimental settings enable state-and collision-energy dependent measurements of rate coefficients for all systems revealing an intriguing interplay between long-and short-range effects for the homonuclear diatomics by comparison to theoretical calculations and surprising experimental results for the polyatomic study. A fit of a constant function to the measured CT rate coefficients in the state-dependent experiment of N2_22​H+^+++Rb yielded the similar result as for N2+^+_22+​+Rb(5s5s5s)(2S1/2^2S_{1/2}2S1/2​). In the collision-energy dependent experiment the CT rate coefficient for N2_22​H+^+++Rb(5s5s5s)(2S1/2^2S_{1/2}2S1/2​) and N2_22​H+^+++Rb(5p5p5p)(2P3/2^2P_{3/2}2P3/2​) are both similar to the results for N2+^+_22+​+Rb(5s5s5s)(2S1/2^2S_{1/2}2S1/2​), and also the rate coefficients increase as the collision-energy increases.
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.
This thesis describes the development and demonstration of techniques for the study of ion-neutral reactive collisions at low temperatures conducted using two unique experimental apparatuses comprising radio-frequency Paul traps. One apparatus combines a Stark decelerator with a Paul trap; the other apparatus features a buffer gas cell interfaced with a Paul trap through an electrostatic quadrupole guide. A novel ion trap time-of-flight mass spectrometer has been introduced and characterised. This apparatus is designed to detect the ion products of reactive collisions at low temperatures (< 10 K). This detection approach affords excellent detection efficiency and is broadly applicable to all ion-molecule reactions that one could study within a linear Paul ion trap. A proof-of-principle reaction between xenon ions and neutral ammonia isotopologues is examined to test the performance of the new detection set-up. With the goal of achieving full control over the reaction parameters, the amount of alignment retained by rotationally and translationally cold ammonia molecules exiting the electrostatic guide has been examined using (2 + 1) REMPI detection of the molecules with varying linear polarisation. Simulations suggests how modifications to the current setup will enable full orientation of the cooled molecules reaching the detection region to be achieved. Experiments on cold charge exchange reactions between sympathetically-cooled Xe+ and room temperature ammonia (NH3 and ND3) have been conducted within lasercooled Ca+ Coulomb crystals in a linear Paul ion trap. There is ongoing work to ascertain any difference in reactivity between NH3 and ND3 as the neutral reactant species. Preliminary data are discussed at a qualitative level, with the prospect of observing any isotope effect in this charge transfer process. Charge exchange reaction experiments using cold state-selected ammonia are discussed with the goal of establishing reaction rate constants at different collision energies.
Cooling all degrees of freedom of a molecule, a cluster, or even a nanoparticle which is suspended in a vacuum, is an experimental challenge. Without suitable schemes, cold or ultracold chemical reactions are not feasible. Methods such as laser based preparation of very slow atoms, decelerating molecules to low velocities with electric fields or freezing molecular ions into Coulomb crystals, are generally not suitable to cool the vibrational or rotational motion of molecules. This contribution describes a new method in which a beam of slow atoms or molecules (H, He, H2, or D2) is used for cooling charged particles confined in a multi-electrode rf trap. For reaching sub-K temperatures, the fast part of a cold effusive beam is removed with a shutter before the slow remaining neutrals interact with the ion cloud. The development of a pulsed cold beam source is discussed as well as suitable methods for determining the ion temperature. A challenging application is to prepare internally cold CH5+ for spectroscopy or chemistry. New experimental results for hydrogen abstraction in collisions with slow H atoms are reported at energies of a few meV. For evaluating these measurements and for predicting effective rate coefficients at lower energies, the kinematic conditions of the slow neutral beam-ion trap arrangement have been analyzed in detail. The potential of cooling ions such as protonated methane or H3+ with slow energy selected H atoms is briefly mentioned. An interesting process is the formation of weakly bound ions such as H4+ or CH6+ via radiative or ternary association. Such ions are ideal candidates for preparing the corresponding collision complexes very close (microeV) to the dissociation continuum using infrared transitions.