Following our general approach to Lambda-doubling specificity in the capture of dipolar molecules by ions [M. Auzinsh et al., J. Chem. Phys. 128, 184304 (2008)], we calculate the rate coefficients for the title process in the temperature range 10(-4)<T<10(2) K. Three regimes considered are as follows: (i) nonadiabatic capture in the regime of high-field Stark effect with respect to the Lambda-doubling components, (10(-1)<T<10(2) K), (ii) adiabatic capture in the regime of intermediate Stark effect (10(-3)<T<10(-1) K), and (iii) adiabatic capture in the limit of very low temperatures (T<<10(-3) K) in the regime of quadratic Stark effect with respect to the Lambda-doubling and hyperfine components. The results predict a high specificity of the capture rates with respect to the Lambda-doublet states even under conditions when the collision energy of the partners strongly exceeds the Lambda-doubling splitting.
Following our general approach to Lambda-doubling specificity in the capture of dipolar molecules by ions [M. Auzinsh et al., J. Chem. Phys. 128, 184304 (2008)], we calculate the rate coefficients for the title process in the temperature range 10(-4)<T<10(2) K. Three regimes considered are as follows: (i) nonadiabatic capture in the regime of high-field Stark effect with respect to the Lambda-doubling components, (10(-1)<T<10(2) K), (ii) adiabatic capture in the regime of intermediate Stark effect (10(-3)<T<10(-1) K), and (iii) adiabatic capture in the limit of very low temperatures (T<<10(-3) K) in the regime of quadratic Stark effect with respect to the Lambda-doubling and hyperfine components. The results predict a high specificity of the capture rates with respect to the Lambda-doublet states even under conditions when the collision energy of the partners strongly exceeds the Lambda-doubling splitting.
Vogt and Wannier [Phys. Rev. 95, 1190 (1954)] predicted that the capture rate of a polarizable neutral atom or molecule by an ion should increase by a factor of 2 compared to the classical Langevin rate as the collision energy approaches zero. This prediction has not been verified experimentally. The H2++H2 reaction is ideally suited to observe this effect because the small reduced mass makes quantum effects related to s-wave scattering observable at higher collision energies than in other systems. Moreover, the reaction rate for this barrierless, strongly exothermic reaction follows the classical Langevin capture model down to cold-collision conditions (about ${k}_{\mathrm{B}}\ifmmode\times\else\texttimes\fi{}1$ K) and is not affected by short-range interactions. Below this temperature, a strong enhancement of the reaction rate resulting from charge--quadrupole interaction between H2+ and ground-state ortho H2 (J=1) was observed. Here we present an experimental study of the reaction of H2+ and para H2 (J=0), which has no dipole and no quadrupole moments, at collision energies below ${k}_{\mathrm{B}}\ifmmode\times\else\texttimes\fi{}1$ K. We observe an enhancement at the lowest collision energies which is attributed to the quantum enhancement predicted by Vogt and Wannier. Measurements of the reaction of HD+ with HD support this conclusion.
Inelastic collisions between dipolar molecules, assumed to be trapped in a static electric field at cold (> 10−3K) temperatures, are investigated and compared with elastic collisions. For molecules with a Λ-doublet energy-level structure, a dipole moment arises because of the existence of two nearly degenerate states of opposite parity, and the collision of two such dipoles can be solved entirely analytically in the energy range of interest. Cross sections and rate constants are found to satisfy simple, universal formulas. In contrast, for molecules in a Σ electronic ground state, the static electric field induces a dipole moment in one of three rotational sublevels. Collisions between two rotor dipoles are calculated numerically; the results scale simply with molecule mass, rotational constant, dipole moment, and field strength. It might be expected that any particles interacting only under the influence of the dipole-dipole interaction would show similar behavior; however, the most important and general result of this research is that at cold temperatures inelastic rate constants and cross sections for dipoles depend strongly upon the internal structure of the molecules. The most prominent difference between the Λ-doublet and rotor molecules is variation of the inelastic cross section with applied field strength. For Λ-doublet dipoles, cross sections decrease with increasing field strength. For rotor dipoles, cross sections increase proportionally with the square of field strength. Furthermore, the rate constants of the two types of molecules depend very differently on the angular orientations of the dipoles in the electric field.
Two elementary models of molecular structure are used to investigate inelastic collisions in cold trapped dipolar gases. A two-state model of a polar molecule (such as one with a Λ-doublet) permits analytic description of the inelastic collision process at energies considerably higher than the gap energy, predicting rate constants and cross sections given by elementary formulae. Alternatively, a three-state model of a rotor molecule in an electric field is investigated, with resultant rate constants and cross sections scaling simply with molecular mass, rotational constant, dipole moment and polarizing field strength. Semiclassical inelastic collision cross sections and rate constants are demonstrated to depend strongly on the model used, especially as regards their dependence on the polarizing field. Inelastic cross sections among doublet dipoles decrease with increasing field strength, while they increase in proportion to the square of the field strength for collisions between rotor dipoles.
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.
Rate constants for the charge transfer reaction between N2H+ and Rb in the mK regime are measured in an ion-atom hybrid trap and are found to be lower than the Langevin capture limit. Multireference ab initio computation of the potential energy surfaces involved in the reaction reveals that the low-temperature charge transfer is hindered by short-range features highly dependent on the collision angle and is promoted by a deformation of the molecular frame. The present study highlights the importance of polyatomic effects and of stereodynamics in cold molecular ion-neutral collisions.
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++Rb, O2++Rb and N2H++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 N2H++Rb yielded the similar result as for N2++Rb(5s)(2S1/2). In the collision-energy dependent experiment the CT rate coefficient for N2H++Rb(5s)(2S1/2) and N2H++Rb(5p)(2P3/2) are both similar to the results for N2++Rb(5s)(2S1/2), and also the rate coefficients increase as the collision-energy increases.