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In-situ Determination of the Population of Nuclear Spin States of H2 Injected to the 22-Pole Trap

Michal Hejduk, Dmytro Mulin, Ricardo Macías Plá

Published Jan 1, 2012• 0 citations
OpenAlex

Abstract

We evaluate a population of ortho nuclear spin states in H2 gas in the 22-pole linear radio-frequency trap using N + ions as a chemical probe. The rate coecient of the reaction N + + H2 ! NH + + H strongly depends on the nuclear spin states of the H2 molecules at the temperature 11 K. To achieve astrophysically relevant conditions in other our experiments, we convert the normal H2 gas (25% of the molecules in para-states) to H2 gas with a distribution of the nuclear spin states corresponding to the temperatures in prestellar cores (> 99 % in para-states at temperatures 20 K) by a catalytic converter (para-hydrogen generator). Tests by the aforementioned probing reaction show that the abundance of ortho-H2 in such H2 gas is (0:5 0:2)% in the trap. Para-to-ortho back-conversion of H2 gas in the generator's outlet tube was studied and its rate was estimated to lie in the interval (10 4 ; 10 2 )s 1 .

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Effect of nuclear spin symmetry in cold and ultracold reactions: D + para/ortho-H2

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We report results for reaction and vibrational quenching of the collision D with para-H 2 ( ) and ortho-H 2 ( ) at cold and ultracold temperatures. We investigate the effect of nuclear spin symmetry for barrier dominated processes ( ) and for one barrierless case ( v = 5). We find resonant structures for energies in the range corresponding to 0.01–10 K, which depend on the nuclear spin of H 2 , arising from contributions of specific partial waves. We discuss the implications on the results in this benchmark system for ultracold chemistry.

arXiv:1505.03172•New Journal of Physics•Jun 1, 2015•17 citations

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

Sven Fanghaenel

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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The spin dynamics of water ice in the presence of external magnetic fields are investigated. The employed model is based on the approach introduced by Buntkowsky et al. [Z. Phys. Chem. 222, 1049 (2008)], which considers two nearest-neighbor water molecules and yields a four-spin system, as the abundant oxygen isotope has zero nuclear spin. The model is extended to include coupling to external magnetic fields, allowing us to analyze the interplay between magnetic dipole-dipole interactions and magnetic field coupling. Two types of configurations are examined: (i) static, homogeneous fields, corresponding to a time-independent interaction, and (ii) spatially varying sinusoidal fields in relative motion with the molecules, leading to a time-dependent interaction. All computations are performed within the density operator formalism. The ortho/para populations and the total spin projections are evaluated during the first tens of milliseconds following the gas-to-solid phase transition. For static homogeneous fields, we show that increasing field strength suppresses dipolar-induced depolarization. Assuming that all molecules are initially in the para state, we show that static homogeneous fields can drive the ortho population up to approximately $50\%$, whereas suitably chosen sinusoidal-field configurations can increase it beyond $90\%$. These results are relevant for schemes aiming to preserve or manipulate nuclear-spin polarization during deposition.

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Complex Formation in Three-Body Reactions of Cl– with H2

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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.

The Journal of Physical Chemistry A•Sep 28, 2021•14 citations

Observation of quantum capture in an ion-molecule reaction

Katharina Höveler, Johannes Deiglmayr, Josef A. Agner +3 more

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 ${\mathrm{H}}_{2}^{+}$ $+$ ${\mathrm{H}}_{2}$ 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 ${\mathrm{H}}_{2}^{+}$ and ground-state ortho ${\mathrm{H}}_{2}$ ($J=1$) was observed. Here we present an experimental study of the reaction of ${\mathrm{H}}_{2}^{+}$ and para ${\mathrm{H}}_{2}$ ($J=0$), 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 ${\mathrm{HD}}^{+}$ with HD support this conclusion.

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Prototype model for nuclear spin conversion in molecules: The case of "hydrogen"

Il'ichov, L.V.

Using the concept of the so-called quantum relaxation, we build a semiphenomenological model for ortho-para conversion of nuclear spin isomers of hydrogen-type molecules.

•Jan 1, 1999•3 citations
We evaluate a population of ortho nuclear spin states in H2 gas in the 22-pole linear radio-frequency trap using N + ions as a chemical probe. The rate coecient of the reaction N + + H2 ! NH + + H strongly depends on the nuclear spin states of the H2 molecules at the temperature 11 K. To achieve astrophysically relevant conditions in other our experiments, we convert the normal H2 gas (25% of the molecules in para-states) to H2 gas with a distribution of the nuclear spin states corresponding to the temperatures in prestellar cores (> 99 % in para-states at temperatures 20 K) by a catalytic converter (para-hydrogen generator). Tests by the aforementioned probing reaction show that the abundance of ortho-H2 in such H2 gas is (0:5 0:2)% in the trap. Para-to-ortho back-conversion of H2 gas in the generator's outlet tube was studied and its rate was estimated to lie in the interval (10 4 ; 10 2 )s 1 .
We report results for reaction and vibrational quenching of the collision D with para-H 2 ( ) and ortho-H 2 ( ) at cold and ultracold temperatures. We investigate the effect of nuclear spin symmetry for barrier dominated processes ( ) and for one barrierless case ( v = 5). We find resonant structures for energies in the range corresponding to 0.01–10 K, which depend on the nuclear spin of H 2 , arising from contributions of specific partial waves. We discuss the implications on the results in this benchmark system for ultracold chemistry.
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.
Spin dynamics and ortho-para conversion in H2_{2}2​O during the phase transition from gas to solid in external magnetic fields
The spin dynamics of water ice in the presence of external magnetic fields are investigated. The employed model is based on the approach introduced by Buntkowsky et al. [Z. Phys. Chem. 222, 1049 (2008)], which considers two nearest-neighbor water molecules and yields a four-spin system, as the abundant oxygen isotope has zero nuclear spin. The model is extended to include coupling to external magnetic fields, allowing us to analyze the interplay between magnetic dipole-dipole interactions and magnetic field coupling. Two types of configurations are examined: (i) static, homogeneous fields, corresponding to a time-independent interaction, and (ii) spatially varying sinusoidal fields in relative motion with the molecules, leading to a time-dependent interaction. All computations are performed within the density operator formalism. The ortho/para populations and the total spin projections are evaluated during the first tens of milliseconds following the gas-to-solid phase transition. For static homogeneous fields, we show that increasing field strength suppresses dipolar-induced depolarization. Assuming that all molecules are initially in the para state, we show that static homogeneous fields can drive the ortho population up to approximately 50%50\%50%, whereas suitably chosen sinusoidal-field configurations can increase it beyond 90%90\%90%. These results are relevant for schemes aiming to preserve or manipulate nuclear-spin polarization during deposition.
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.
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+{\mathrm{H}}_{2}^{+}H2+​ +++ H2{\mathrm{H}}_{2}H2​ reaction is ideally suited to observe this effect because the small reduced mass makes quantum effects related to sss-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+{\mathrm{H}}_{2}^{+}H2+​ and ground-state ortho H2{\mathrm{H}}_{2}H2​ (J=1J=1J=1) was observed. Here we present an experimental study of the reaction of H2+{\mathrm{H}}_{2}^{+}H2+​ and para H2{\mathrm{H}}_{2}H2​ (J=0J=0J=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+{\mathrm{HD}}^{+}HD+ with HD support this conclusion.