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arXiv:1406.5784 · Review of Scientific Instruments

Note: High density pulsed molecular beam for cold ion chemistry

Mark Kokish, Vaishnavi Rajagopal, Joan P. Marler, Brian Odom

Published Aug 1, 2014• 5 citations
arXivHTMLPDF

Abstract

A recent expansion of cold and ultracold molecule applications has led to renewed focus on molecular species preparation under ultrahigh vacuum conditions. Meanwhile, molecular beams have been used to study gas phase chemical reactions for decades. In this paper, we describe an apparatus that uses pulsed molecular beam technology to achieve high local gas densities, leading to faster reaction rates with cold trapped ions. We characterize the beam's spatial profile using the trapped ions themselves. This apparatus could be used for preparation of molecular species by reactions requiring excitation of trapped ion precursors to states with short lifetimes or for obtaining a high reaction rate with minimal increase of background chamber pressure.

DOI
10.1063/1.4894262

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A recent expansion of cold and ultracold molecule applications has led to renewed focus on molecular species preparation under ultrahigh vacuum conditions. Meanwhile, molecular beams have been used to study gas phase chemical reactions for decades. In this paper, we describe an apparatus that uses pulsed molecular beam technology to achieve high local gas densities, leading to faster reaction rates with cold trapped ions. We characterize the beam's spatial profile using the trapped ions themselves. This apparatus could be used for preparation of molecular species by reactions requiring excitation of trapped ion precursors to states with short lifetimes or for obtaining a high reaction rate with minimal increase of background chamber pressure.
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.
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.
This thesis describes the combination of two sources of cold atomic or molecular species which can be used to study a wide range of ion-molecule reactions. The challenges in forming these species and in determining the fate of reactive events are explored throughout. Reactions occur in a volume within a radio-frequency ion trap, in which ions have previously been cooled to sub-Kelvin temperatures. Ions are laser-cooled, with migration of ions slowed sufficiently to form a quasi-crystalline spheroidal structure, deemed a Coulomb crystal. Fluorescence emitted as a consequence of laser-cooling is detected; the subsequent fluorescence profiles are used to determine the number of ions in the crystal and, in combination with complementary simulations, the temperature of these ions. Motion imparted by trapping fields can be substantial and simulations are required to accurately determine collision energies. A beam of decelerated molecules is aimed at this stationary ion target. An ammonia seeded molecular beam enters a Stark decelerator, based on the original design of Meijer and co-workers. The decelerator uses time-varying electric fields to remove kinetic energy from the molecules, which exit at speeds down to 35 m/s. A fast-opening shutter and focussing elements are subsequently used to maximise the decelerated flux in the reaction volume while minimising undecelerated molecule transmission. Substantial fluxes of decelerated ammonia are obtained with narrow velocity distributions to provide a suitable source of reactant molecules. Combination of these two techniques permits studies of reactions between atomic ions and decelerated molecules that can be entirely state-specific. Changes in the Coulomb crystal fluorescence profile denote changes in the ion identities, the rate of these changes can be used to obtain rate constants. Determination of rate constants is even possible despite the fact that neither reactant nor product ions are directly observed. This work has studied reactions between sympathetically cooled Xe+ ions and guided ND3 and has obtained data consistent with prior studies. Determination of reactive events is complicated if ion identities can change without affecting the fluorescence profile, or if multiple reaction channels are possible. A range of spectroscopic techniques are discussed and considered in regards to determining rate constants and product identities. Pulsed axial excitation of trapped ions can follow rapid changes in average ion weights and subtle changes for small crystals. Time-of-flight mass spectrometry is also demonstrated using the trapping electrodes and is suitable for discrimination of ions formed within the trap.
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.
Ultracold and cold molecules have enormous potential for yielding a wealth of new physics and new technologies. Molecules possess several features that greatly increase the possibilities for new interactions, collective quantum states and effects, collisional processes, fundamental symmetry tests and chemical processes. This is due to 1) the strong interactions between the dipole moments of polar molecules, 2) the rotational and vibrational internal structure of all molecules, and 3) the easily orientable internal electric field of many molecules. The promise of cold and ultracold molecules will only be realized when samples can be prepared with at least the same ease in which we now prepare atomic samples. Several approaches towards trapping of molecules, the key first step toward studying ultracold molecules at high density, have already succeeded: direct cooling of molecules via a buffer gas, mechanical slowing of a pulsed molecular beam with electric fields and photoassociation of alkali atoms. In the ICAP 2002 session on molecules I described some of the current theoretical results and predictions about ultracold molecular systems. Profs. Meijer and Heinzen described results on pulsed beam slowing and photoassociation. This contribution describes some of the key results on buffer-gas loading of molecules. A.
The study of chemical reactions between ions and neutral species at very low energies reveals precise informations about the dynamics of collisions and fine details of intermolecular interactions. Here, we report progress towards the development of next- generation experiments for the investigation of cold ion-neutral reactions. First, we present a new ''dynamic" hybrid ion-atom trap which enables the study of collisions with a superior energy resolution accessing a regime in which quantum scattering resonances may become observable. Second, we discuss and numerically characterize the concept and properties of a hybrid trap for cold neutral molecules and molecular ions which paves the way for the study of ion-molecule reactions in the millikelvin regime.