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Physical Review A

Quantum-state-controlled reactions between molecular radicals and ions

J. M. Greenberg, Philipp Christoph Schmid, Mikhail I. Miller, John F. Stanton, Heather J. Lewandowski

Published Sep 13, 2018• 23 citations
Publisher

Abstract

Chemical reactions between ions and molecular radicals are important in both terrestrial and extraterrestrial environments. However, due to the challenge of creating dense, pure samples of both species, very few studies have been performed in the laboratory. We present the first measurement of a state-controlled reaction between a radical and an ion, where the ${\mathrm{Ca}}^{+}$ cation and the NO radical molecule combine to form ${\mathrm{CaO}}^{+}$ and a nitrogen atom. The charge transfer between ${\mathrm{Ca}}^{+}$ and NO is simultaneously observed. We utilize a linear Paul ion trap coupled to a time-of-flight mass spectrometer to make direct, simultaneous measurements of multiple ionic product channels. We demonstrate control over the reaction rates by tuning of the excited-state population of the laser cooled $^{40}\mathrm{Ca}^{+}$. This control, coupled with a sensitive detection technique, enables precise measurement of the rate constants and branching ratios for this reaction.

DOI
10.1103/physreva.98.032702

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Chemical reactions between ions and molecular radicals are important in both terrestrial and extraterrestrial environments. However, due to the challenge of creating dense, pure samples of both species, very few studies have been performed in the laboratory. We present the first measurement of a state-controlled reaction between a radical and an ion, where the Ca+{\mathrm{Ca}}^{+}Ca+ cation and the NO radical molecule combine to form CaO+{\mathrm{CaO}}^{+}CaO+ and a nitrogen atom. The charge transfer between Ca+{\mathrm{Ca}}^{+}Ca+ and NO is simultaneously observed. We utilize a linear Paul ion trap coupled to a time-of-flight mass spectrometer to make direct, simultaneous measurements of multiple ionic product channels. We demonstrate control over the reaction rates by tuning of the excited-state population of the laser cooled 40Ca+^{40}\mathrm{Ca}^{+}40Ca+. This control, coupled with a sensitive detection technique, enables precise measurement of the rate constants and branching ratios for this reaction.
We describe a versatile experimental apparatus designed for investigating ion-molecule reaction dynamics at low temperatures under ultra-high vacuum conditions (<8 × 10-11 Torr). The system integrates laser-cooled atomic ions confined in a linear Paul trap with a pulsed molecular beam featuring simultaneous quantum-state and velocity selection. Reaction products are identified using fluorescence imaging and a time-of-flight mass spectrometer. The setup accommodates a broad range of ionic and neutral species-including Be+, C+, H2, N2, O2, H2O, and others-enabling high-sensitivity studies of reaction dynamics. As a benchmark, we present measurements of the Be+ + H2 reaction across a range of collision energies, including reactions with H2 vibrationally excited to the v = 1 level. The apparatus offers fine control over both internal and external degrees of freedom, providing a general platform for cold ion-molecule reaction studies.
This dissertation describes experiments performed in the MOTion trap, a hybrid atom--ion trap comprised of a magneto-optical trap (MOT) and a linear Paul trap with an integrated time-of-flight mass spectrometer (ToF-MS).We aim to sympathetically cool the vibrational and rotational degrees of freedom of a BaCl+^++ molecular ion by overlapping a cold (sim4\\sim4sim4~mK) Ca MOT.\nThis sympathetic cooling can prepare a molecular ion in its rovibrational quantum ground state, providing a method for state preparation for applications in molecular ion quantum logic.\nWhile collisions between the molecular ion and the cold atoms can cool the internal degrees of freedom, chemical reactions are also possible for electronically excited atoms.\nAs such, we study these excited-state atom--ion reactions at low temperature <1<1<1~K and develop tools to control these collisions.\nWe make the first experimental observation of reaction blockading, an effect that suppresses excited-state reactions at low temperature, and develop a method to reverse this suppression if desired with the addition of a catalyst laser, providing a means to optically control such reactions.Beyond its intended purpose of studying and achieving sympathetic cooling, we find that this apparatus is a powerful tool for chemical studies.By imaging the ions, we can observe reactions occurring one atom at a time.\nWe can precisely tune the collision energy or temperature down to the mK regime to as high as $\\sim$10~K using techniques described in this dissertation, allowing controlled interrogation of these low-temperature reactions.\nFurther, using lasers to address electronic transitions, we can precisely control the quantum state of the reactants, and we can measure the reaction products with the ToF-MS.\nA testament to the value of this apparatus is the study of BaOCa$^+$ described in this dissertation.\nUsing the tools of the MOTion trap, we were able to identify the production of BaOCa+^++, the first observation of a mixed hypermetallic oxide, and determine the path of reaction as BaOCH3+_3^+3+​ + Ca(4s4p4s4p4s4p~3PJ^3P_J3PJ​) rightarrow\\rightarrowrightarrow BaOCa+^++ + CH3_33​.In addition to developing control of these atom--ion interactions towards sympathetic cooling of molecular ions, we also work towards further developing new quantum logic schemes for molecular ions, such as dipolar quantum logic, dipole--phonon quantum logic (DPQL), and electric-field gradient gates (EGGs).Specifically, we propose a class of molecules particularly well-suited for DPQL, the alkaline earth monoxide cations, that can also be added to existing atomic ion experiments with minimal additional experimental complexity.\nWe further develop the techniques for DPQL and propose methods for single- and two-qubit gates, outlining a path towards universal quantum computation with molecular ions.
Trapping molecular ions that have been sympathetically cooled with laser-cooled atomic ions is a useful platform for exploring cold ion chemistry. We designed and characterized a new experimental apparatus for probing chemical reaction dynamics between molecular cations and neutral radicals at temperatures below 1 K. The ions are trapped in a linear quadrupole radio-frequency trap and sympathetically cooled by co-trapped, laser-cooled, atomic ions. The ion trap is coupled to a time-of-flight mass spectrometer to readily identify product ion species and to accurately determine trapped ion numbers. We discuss, and present in detail, the design of this ion trap time-of-flight mass spectrometer and the electronics required for driving the trap and mass spectrometer. Furthermore, we measure the performance of this system, which yields mass resolutions of m/Δm ≥ 1100 over a wide mass range, and discuss its relevance for future measurements in chemical reaction kinetics and dynamics.
Metal-containing molecular ions are fundamentally important in both terrestrial and interstellar chemistry, yet their formation mechanisms have been largely unexplored experimentally. To address this lack of fundamental understanding of how these ions are created in space, we conducted an experimental study of a quantum-state-controlled reaction between Ca^{+} and C_{2}H_{2} to investigate a potential unifying mechanism for the formation of metallic molecules. By tuning the quantum-state populations of laser-cooled Ca^{+}, we demonstrate precise control over reactivity. Our results reveal a single ionic product, CaCCH^{+}, providing valuable insights into the mechanisms underlying the formation of metal-containing ions in space.
A range of experimental techniques for application in reaction studies between ionic and neutral atoms/molecules and high resolution spectroscopy experiments with sympathetically cooled molecular ions are presented. A novel ion trap loading scheme using the photo-ionisation of atoms generated by the pulsed laser ablation of a solid calcium target has been characterised. We have identified the range of ablation laser fluences that must be used in order to produce a flux of neutral calcium atoms, which is a prerequisite for isotope selective ion trap loading. Calcium ions are trapped and laser cooled in a linear radio-frequency ion trap. We have developed a spectroscopy scheme that allows the entire fluorescence spectrum of trapped ions to be rapidly collected with high precision while maintaining a low ion temperature and good ion localisation throughout interrogation. The scheme has been demonstrated by measuring the saturation intensity of the calcium ion 4S1/2?4P1/2 transition. We have developed a novel scheme to measure the secular motion of trapped ions and demonstrated the application of the technique to ion-neutral collision reaction experiments. Employing pulsed excitation and Doppler velocimetry, we have measured the centre-of-mass mode frequency of single ions as well as large ion crystals with a frequency precision better than 2x10-3 within an interrogation time on the order of seconds. This method has been used to measure the mass of ions and observe charge exchange collisions between trapped calcium isotopes. In particular, we have measured the 44Ca++40Ca!40Ca++44Ca reaction cross section and demonstrated the single-event resolution of the technique. Finally, we have developed a novel all-optical broadband scheme for exciting, amplifying and measuring the secular motion of ions in the trap. Oscillation induced by optical excitation has been coherently amplified to control and measure the ion's secular motion. Requiring only a single interrogation laser, the ion's oscillation amplitude can be precisely controlled. The application of this technique to non-destructive spectroscopy of trapped molecular ions is discussed.
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.