To form or not to form a reaction complex: exploring ion-molecule reactions between C3H4 isomers and Xe+ and O2.

Ion-molecule reactions are an essential contributor to the chemistry of a diverse range of environments. While a great deal of work has been done to understand the fundamental mechanisms driving these reactions, there is still much more to discover. Here, we expand upon prior studies on ion-molecule reactions involving two isomers of C3H4, allene (H2C3H2) and propyne (H3C3H). Specifically, we probe the previously observed isomeric dependent reactivity of these molecules by reacting them with two ions with nearly identical ionization potentials, Xe+ and O2+. Our goal is to determine if the isomer-dependent reaction mechanisms previously observed are universal for C3H4 or if they depend on the ion character as well. Through the combination of experimental measurements and theoretical calculations, we found that both isomeric structure and identity of the ion contribute to the propensity of a reaction complex forming or for only long-range charge transfer to occur.

Cold Ion-Molecule Reactions in the Extreme Environment of a Coulomb Crystal.

Coulomb crystals provide a unique environment in which to study ion-neutral gas-phase reactions. In these cold, trapped ensembles, we are able to study the kinetics and dynamics of small molecular systems. These measurements have connections to chemistry in the Interstellar Medium (ISM) and planetary atmospheres. This Feature Article will describe recent work in our laboratory that uses Coulomb crystals to study translationally cold, ion-neutral reactions. We provide a description of how the various affordances of our experimental system allow for detailed studies of the reaction mechanisms and the corresponding products. In particular, we will describe quantum-state resolved reactions, isomer-dependent reactions, and reactions with a rarely studied, astrophysically relevant ion, CCl+.

Reactions of Acetonitrile with Trapped, Translationally Cold Acetylene Cations.

The reaction of the acetylene cation (C2H2+) with acetonitrile (CH3CN) is measured in a linear Paul ion trap coupled to a time-of-flight mass spectrometer. C2H2+ and CH3CN are both noted for their astrochemical abundance and predicted relevance for understanding prebiotic chemistry. The observed primary products are c-C3H3+, C3H4+, and C2NH3+. The latter two products react with excess CH3CN to form the secondary product C2NH4+, protonated acetonitrile. The molecular formula of these ionic products can be verified with the aid of isotope substitution via deuteration of the reactants. Primary product reaction pathways and thermodynamics are investigated with quantum chemical calculations and demonstrate exothermic pathways to two isomers of C2NH3+, two isomers of C3H4+, and the cyclopropenyl cation c-C3H3+. This study deepens our understanding of the dynamics and products of a pertinent ion-molecule reaction between two astrochemically abundant molecules in conditions that mimic those of the interstellar medium.

Characterization of a vacuum ultraviolet light source at 118 nm.

Vacuum ultraviolet (VUV) light at 118 nm has been shown to be a powerful tool to ionize molecules for various gas-phase chemical studies. A convenient table top source of 118 nm light can be produced by frequency tripling 355 nm light from a Nd:YAG laser in xenon gas. This process has a low efficiency, typically producing only nJ/pulse of VUV light. Simple models of the tripling process predict that the power of 118 nm light produced should increase quadratically with increasing xenon pressure. However, experimental 118 nm production has been observed to reach a maximum and then decrease to zero with increasing xenon pressure. Here, we describe the basic theory and experimental setup for producing 118 nm light and a new proposed model for the mechanism limiting the production based on pressure broadened absorption.

Isotope-specific reactions of acetonitrile (CH3CN) with trapped, translationally cold CCl.

The gas-phase reaction of CCl+ with acetonitrile (CH3CN) is studied using a linear Paul ion trap coupled to a time-of-flight mass spectrometer. This work builds on a previous study of the reaction of CCl+ with acetylene [K. J. Catani et al., J. Chem. Phys. 152, 234310 (2020)] and further explores the reactivity of CCl+ with organic neutral molecules. Both of the reactant species are relevant in observations and models of chemistry in the interstellar medium. Nitriles, in particular, are noted for their relevance in prebiotic chemistry and are found in the atmosphere of Titan, one of Saturn's moons. This work represents one of the first studied reactions of a halogenated carbocation with a nitrile and the first exploration of CCl+ with a nitrile. Reactant isotopologues are used to unambiguously assign ionic primary products from this reaction: HNCCl+ and C2H3 +. Branching ratios are measured, and both primary products are determined to be equally probable. Quantum chemical and statistical reaction rate theory calculations illuminate pertinent information for interpreting the reaction data, including reaction thermodynamics and a potential energy surface for the reaction, as well as rate constants and branching ratios for the observed products. In particular, the reaction products and potential energy surface stimulate questions regarding the strength and role of the nitrile functional group, which can be further explored with more reactions of this class.

Using isotopologues to probe the potential energy surface of reactions of C2H2 ++C3H4.

Investigations into bimolecular reaction kinetics probe the details of the underlying potential energy surface (PES), which can help to validate high-level quantum chemical calculations. We utilize a combined linear Paul ion trap with a time-of-flight mass spectrometer to study isotopologue reactions between acetylene cations (C2H2 +) and two isomers of C3H4: propyne (HC3H3) and allene (H2C3H2). In a previous study [Schmid et al., Phys. Chem. Chem. Phys. 22, 20303 (2020)],1 we showed that the two isomers of C3H4 have fundamentally different reaction mechanisms. Here, we further explore the calculated PES by isotope substitution. While isotopic substitution of reactants is a standard experimental tool in the investigation of molecular reaction kinetics, the controlled environment of co-trapped, laser-cooled Ca+ ions allows the different isotopic reaction pathways to be followed in greater detail. We report branching ratios for all of the primary products of the different isotopic species. The results validate the previously proposed mechanism: propyne forms a bound reaction complex with C2H2 +, while allene and C2H2 + perform long-range charge exchange only.

Cold and controlled chemical reaction dynamics.

The prospect of studying state-to-state chemical reaction dynamics, with full control over all of the reaction parameters, is becoming a reality for a small number of systems. Thanks to the rapid development of new experimental techniques (alongside novel combinations of existing methods), an increasingly diverse range of reactants can be prepared under cold conditions and manipulated with external fields. These tools are enabling the study of reactions at previously inaccessible collision energies; the role of long-range forces and quantum effects are beginning to be experimentally probed-challenging the accuracy of theoretical predictions and fundamental models of reactivity. In this perspective article, we outline the key methodologies that are adopted for the study of cold and controlled reaction dynamics. We discuss the motivation for these studies, detail the progress made to date, and highlight the future prospects for the field.

Isomer-selected ion-molecule reactions of acetylene cations with propyne and allene.

One of the fundamental goals of chemistry is to determine how molecular structure influences interactions and leads to different reaction products. Studies of isomer-selected and resolved chemical reactions can shed light directly on how form leads to function. In the following, we present the results of gas-phase reactions between acetylene cations (C2D2+) with two different isomers of C3H4: propyne (DC3D3) and allene (H2C3H2). Our highly controlled, trapped-ion environment allows for precise determination of reaction products and kinetics. From these results, we can infer details of the underlying reaction dynamics of C2H2+ + C3H4. Through the synergy of experimental results and high-level quantum chemical potential energy surface calculations, we are able to identify distinct reaction mechanisms for the two isomers. We find long-range charge exchange with no complex formation is favored for allene, whereas charge exchange leads to an intermediate reaction complex for propyne and thus, different products. Therefore, this reaction displays a pronounced isomer-selective bi-molecular reactive process.

Reactions of translationally cold trapped CCl+ with acetylene (C2H2).

Ion-neutral chemical reactions are important in several areas of chemistry, including in some regions of the interstellar medium, planetary atmospheres, and comets. Reactions of CCl+ with C2H2 are measured, and the main products include C3H2 + and C3H+, both relevant in extraterrestrial environments. Accurate branching ratios are obtained, which favor the formation of C3H2 + over C3H+ by a factor of four. The measured rate constants are on the order of Langevin, and complementary electronic structure calculations are used to aid in the interpretation of experimental results.

Production of carbon clusters C3 to C12 with a cryogenic buffer-gas beam source.

Cryogenic buffer-gas beam sources are capable of producing intense beams of a wide variety of molecules and have a number of advantages over traditional supersonic expansion sources. In this work, we report on a neon matrix isolation study of carbon clusters produced with a cryogenic buffer-gas beam source. Carbon clusters created by laser ablation of graphite are trapped in a neon matrix and detected with a Fourier-transform infrared spectrometer in the spectral range 4000-1000 cm-1. Through a study of carbon cluster production as a function of various system parameters, we characterize the behavior of the buffer-gas beam source and find that approximately 1011-1012 of each cluster is produced with each pulse of the ablation laser. These measurements demonstrate the usefulness of cryogenic buffer-gas beam sources for producing molecular beams of clusters.

Large effects of electric fields on atom-molecule collisions at millikelvin temperatures.

Controlling interactions between cold molecules using external fields can elucidate the role of quantum mechanics in molecular collisions. We create a new experimental platform in which ultracold rubidium atoms and cold ammonia molecules are separately trapped by magnetic and electric fields and then combined to study collisions. We observe inelastic processes that are faster than expected from earlier field-free calculations. We use quantum scattering calculations to show that electric fields can have a major effect on collision outcomes, even in the absence of dipole-dipole interactions.

Velocity-tunable beam of continuously decelerated polar molecules for cold ion-molecule reaction studies.

Producing high densities of molecules is a fundamental challenge for low-temperature, ion-molecule reaction studies. Traveling-wave Stark decelerators promise to deliver high density beams of cold, polar molecules but require non-trivial control of high-voltage potentials. We have overcome this experimental challenge and demonstrate continuous deceleration of ND3 from 385 to 10 m/s, while driving the decelerator electrodes with a 10 kV amplitude sinewave. In addition, we test an alternative slowing scheme, which increases the time delay between decelerated packets of ND3 and non-decelerated molecules, allowing for better energy resolution of subsequent reaction studies. We characterize this source of neutral, polar molecules suitable for energy-resolved reaction studies with trapped ions at cold translational temperatures. We also propose a combined apparatus consisting of the traveling-wave decelerator and a linear ion trap with a time-of-flight mass spectrometer and discuss to what extent it may achieve cold, energy-resolved, ion-neutral reactions.

A high-voltage amplifier for traveling-wave Stark deceleration.

Controlling high voltage is a critical aspect of Stark deceleration, a technique that uses electric fields to produce slow molecules. Traditionally, Stark deceleration required only commercial high-voltage switches to operate. However, a new continuous version of Stark deceleration, which promises significantly improved performance, requires chirped sinusoidal voltages. The complexity of the electronics needed to run this new decelerator has restricted the implementation for all but a few groups. The challenge is to create high-voltage amplifiers that have a frequency range of tens of kilohertz down to DC and that can source and sink enough current (∼1 A) to drive the capacitive load of the decelerator. We present a new high-voltage linear amplifier for driving in-vacuum electrodes for traveling-wave Stark deceleration. The amplifier has a gain of 12 000, output voltages up to ±10 kV, instantaneous currents up to 1.5 A, and a frequency range from 30 kHz down to DC. This makes the amplifier suitable for traveling-wave Stark deceleration of a supersonic molecular beam down to rest.

An ion trap time-of-flight mass spectrometer with high mass resolution for cold trapped ion experiments.

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.

A search for varying fundamental constants using hertz-level frequency measurements of cold CH molecules.

Many modern theories predict that the fundamental constants depend on time, position or the local density of matter. Here we develop a spectroscopic method for pulsed beams of cold molecules, and use it to measure the frequencies of microwave transitions in CH with accuracy down to 3 Hz. By comparing these frequencies with those measured from sources of CH in the Milky Way, we test the hypothesis that fundamental constants may differ between the high- and low-density environments of the Earth and the interstellar medium. For the fine structure constant we find Δα/α=(0.3 ± 1.1) × 10⁻7, the strongest limit to date on such a variation of α. For the electron-to-proton mass ratio we find Δµ/µ=(-0.7 ± 2.2) × 10⁻7. We suggest how dedicated astrophysical measurements can improve these constraints further and can also constrain temporal variation of the constants.

Experimental demonstration of classical Hamiltonian monodromy in the 1:1:2 resonant elastic pendulum.

The 1:1:2 resonant elastic pendulum is a simple classical system that displays the phenomenon known as Hamiltonian monodromy. With suitable initial conditions, the system oscillates between nearly pure springing and nearly pure elliptical-swinging motions, with sequential major axes displaying a stepwise precession. The physical consequence of monodromy is that this stepwise precession is given by a smooth but multivalued function of the constants of motion. We experimentally explore this multivalued behavior. To our knowledge, this is the first experimental demonstration of classical monodromy.

Cold molecule spectroscopy for constraining the evolution of the fine structure constant.

We report precise measurements of ground-state, Lambda-doublet microwave transitions in the hydroxyl radical molecule (OH). Utilizing slow, cold molecules produced by a Stark decelerator we have improved over the precision of the previous best measurement 25-fold for the F'=2-->F=2 transition, yielding (1 667 358 996 +/- 4)Hz, and by tenfold for the F'=1-->F=1 transition, yielding (1 665 401 803 +/-12)Hz. Comparing these laboratory frequencies to those from OH megamasers in interstellar space will allow a sensitivity of 1 ppm for Delta(alpha/alpha) over approximately 10(10) yr.

Decoherence-driven cooling of a degenerate spinor Bose gas.

We investigate the relationship between the coherence of a partially Bose-condensed spinor gas and its temperature. We observe cooling of the normal component driven by decoherence as well as the effect of temperature on decoherence rates.

Phase space manipulation of cold free radical OH molecules.

We report bunching, slowing, and acceleration of a supersonically cooled beam of diatomic hydroxyl radicals (OH). In situ observation of laser-induced fluorescence along the beam propagation path allows for detailed characterization of longitudinal phase-space manipulation of OH molecules through the Stark effect by precisely sequenced inhomogeneous electric fields.