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Physical Review A

Ion-neutral chemical reactions between ultracold localized ions and neutral molecules with single-particle resolution

Bernhard Roth, Peter Blythe, H. Wenz, H. Daerr, Stephan Schiller

Published Apr 19, 2006• 80 citations
Publisher

Abstract

Chemical reactions between ultracold $^{9}\mathrm{Be}^{+}$ ions and room-temperature molecular hydrogen isotopomers and between ultracold ${\mathrm{H}}_{3}^{+}$ ions and room-temperature ${\mathrm{O}}_{2}$ have been studied in a laser-cooling ion trap apparatus. For small Coulomb crystals of beryllium ions, reactions can be followed at the single-ion level. We demonstrate characterization of a chemical reaction in which neither one of the reactants nor the product is directly detectable. In this case molecular dynamics simulations were used for the determination of ion numbers from images of the $^{9}\mathrm{Be}^{+}$ ion ensemble. The observed reaction rates are in agreement with the Langevin ion-neutral reaction theory.

DOI
10.1103/physreva.73.042712

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Chemical reactions between ultracold 9Be+^{9}\mathrm{Be}^{+}9Be+ ions and room-temperature molecular hydrogen isotopomers and between ultracold H3+{\mathrm{H}}_{3}^{+}H3+​ ions and room-temperature O2{\mathrm{O}}_{2}O2​ have been studied in a laser-cooling ion trap apparatus. For small Coulomb crystals of beryllium ions, reactions can be followed at the single-ion level. We demonstrate characterization of a chemical reaction in which neither one of the reactants nor the product is directly detectable. In this case molecular dynamics simulations were used for the determination of ion numbers from images of the 9Be+^{9}\mathrm{Be}^{+}9Be+ ion ensemble. The observed reaction rates are in agreement with the Langevin ion-neutral reaction theory.
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The recent development of a range of techniques for producing cold atoms and molecules at very low translational temperatures T < or = 1 K has provided the opportunity to investigate collisional processes in a new physical regime. We have recently presented a new experimental method to study low-temperature reactive collisions between translationally cold ions and neutral molecules (S. Willitsch et al., Phys. Rev. Lett. 2008, 100, 043203). Our technique relies on the combination of a quadrupole-guide velocity selector for the generation of translationally cold neutral molecules with a facility to produce ordered structures of cold ions (Coulomb crystals) by laser cooling in a linear quadrupole ion trap. The strong localisation of the ions in the trap in combination with the high sensitivity of laser-induced-fluorescence detection enabled us to study chemical reactions on the single-particle level, down to temperatures of T approximately 1 K. In the current paper, we present a detailed characterisation of the scope and limitations of this method based on our study of the reaction between laser-cooled Ca+ ions and velocity-selected CH3F molecules. The properties of our cold-neutrals source and the dependence of the measured rate constant on the shape of the Coulomb crystals, trapping and laser-cooling parameters are discussed. An extension of our technique for the study of low-temperature reactions with sympathetically cooled molecular ions (translational temperature T > 10 mK) is presented and first results on the charge-transfer reaction between OCS+ and ND3 are discussed. Finally, perspectives for further developments of our method are explored.
We have cooled ensembles of the molecular hydrogen ions H2+, H3+, and all their deuterated variants to temperatures of a few mK in a radio frequency trap, by sympathetic cooling with laser-cooled beryllium ions. The molecular ions are embedded in the central regions of Coulomb crystals. Mass spectroscopy and molecular dynamics simulations were used to accurately characterize the properties of the ultracold multispecies crystals. We demonstrate species-selective purification of multispecies ensembles. These molecules are of fundamental importance as the simplest of all molecules, and have the potential to be used for precision tests of molecular structure theory, tests of Lorentz invariance, and measurements of electron to nuclear mass ratios and their time variation.
We measure chemical reactions between a single trapped ^{174}Yb^{+} ion and ultracold Li_{2} dimers. This produces LiYb^{+} molecular ions that we detect via mass spectrometry. We explain the reaction rates by modeling the dimer density as a function of the magnetic field and obtain excellent agreement when we assume the reaction to follow the Langevin rate. Our results present a novel approach towards the creation of cold molecular ions and point to the exploration of ultracold chemistry in ion molecule collisions. What is more, with a detection sensitivity below molecule densities of 10^{14} m^{-3}, we provide a new method to detect low-density molecular gases.
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.
Cold chemical reactions between laser-cooled Ca^+ ions and Rb atoms were studied in an ion-atom hybrid trap. Reaction rate constants were determined in the range of collision energies /k_B = 20 mK-20 K. The lowest energies were achieved in experiments using single localized Ca^+ ions. Product branching ratios were studied using resonant-excitation mass spectrometry. The dynamics of the reactive processes in this system (non-radiative and radiative charge transfer as well as radiative association leading to the formation of CaRb^+ molecular ions) have been analyzed using high-level quantum-chemical calculations of the potential energy curves of CaRb^+ and quantum-scattering calculations for the radiative channels. For the present low-energy scattering experiments, it is shown that the energy dependence of the reaction rate constants is governed by long-range interactions in line with the classical Langevin model, but their magnitude is determined by short-range non-adiabatic and radiative couplings which only weakly depend on the asymptotic energy. The quantum character of the collisions is predicted to manifest itself in the occurrence of narrow shape resonances at well-defined collision energies. The present results highlight both universal and system-specific phenomena in cold ion-neutral reactive collisions.