A variable time step self-consistent mean field DSMC model for three-dimensional environments.

A self-consistent mean field direct simulation Monte Carlo (SCMFD) algorithm was recently proposed for simulating collision environments for a range of one-dimensional model systems. This work extends the one-dimensional SCMFD approach to three dimensions and introduces a variable time step (3D-vt-SCMFD), enabling the modeling of a considerably wider range of different collision environments. We demonstrate the performance of the augmented method by modeling a varied set of test systems: ideal gas mixtures, Poiseuille flow of argon, and expansion of gas into high vacuum. For the gas mixtures, the 3D-vt-SCMFD method reproduces the properties (mean free path, mean free time, collision frequency, and temperature) in excellent agreement with theoretical predictions. From the Poiseuille flow simulations, we extract flow profiles that agree with the solution to the Navier-Stokes equations in the high-density limit and resemble free molecular flow at low densities, as expected. The measured viscosity from 3D-vt-SCMF is ∼15% lower than the theoretical prediction from Chapman-Enskog theory. The expansion of gas into vacuum is examined in the effusive regime and at the hydrodynamic limit. In both cases, 3D-vt-SCMDF simulations produce gas beam density, velocity, and temperature profiles in excellent agreement with analytical models. In summary, our tests show that 3D-vt-SCMFD is robust and computationally efficient, while also illustrating the diversity of systems the SCMFD model can be successfully applied to.

Inverse kinetic isotope effects in the charge transfer reactions of ammonia with rare gas ions.

In the absence of experimental data, models of complex chemical environments rely on predicted reaction properties. Astrochemistry models, for example, typically adopt variants of capture theory to estimate the reactivity of ionic species present in interstellar environments. In this work, we examine astrochemically-relevant charge transfer reactions between two isotopologues of ammonia, NH3 and ND3, and two rare gas ions, Kr+ and Ar+. An inverse kinetic isotope effect is observed; ND3 reacts faster than NH3. Combining these results with findings from an earlier study on Xe+ (Petralia et al., Nat. Commun., 2020, 11, 1), we note that the magnitude of the kinetic isotope effect shows a dependence on the identity of the rare gas ion. Capture theory models consistently overestimate the reaction rate coefficients and cannot account for the observed inverse kinetic isotope effects. In all three cases, the reactant and product potential energy surfaces, constructed from high-level ab initio calculations, do not exhibit any energetically-accessible crossing points. Aided by a one-dimensional quantum-mechanical model, we propose a possible explanation for the presence of inverse kinetic isotope effects in these charge transfer reaction systems.

Strong inverse kinetic isotope effect observed in ammonia charge exchange reactions.

Isotopic substitution has long been used to understand the detailed mechanisms of chemical reactions; normally the substitution of hydrogen by deuterium leads to a slower reaction. Here, we report our findings on the charge transfer collisions of cold [Formula: see text] ions and two isotopologues of ammonia, [Formula: see text] and [Formula: see text]. Deuterated ammonia is found to react more than three times faster than hydrogenated ammonia. Classical capture models are unable to account for this pronounced inverse kinetic isotope effect. Moreover, detailed ab initio calculations cannot identify any (energetically accessible) crossing points between the reactant and product potential energy surfaces, indicating that electron transfer is likely to be slow. The higher reactivity of [Formula: see text] is attributed to the greater density of states (and therefore lifetime) of the deuterated reaction complex compared to the hydrogenated system. Our observations could provide valuable insight into possible mechanisms contributing to deuterium fractionation in the interstellar medium.

Ejection of Coulomb Crystals from a Linear Paul Ion Trap for Ion-Molecule Reaction Studies.

Coulomb crystals are being increasingly employed as a highly localized source of cold ions for the study of ion-molecule chemical reactions. To extend the scope of reactions that can be studied in Coulomb crystals-from simple reactions involving laser-cooled atomic ions, to more complex systems where molecular reactants give rise to multiple product channels-sensitive product detection methodologies are required. The use of a digital ion trap (DIT) and a new damped cosine trap (DCT) are described, which facilitate the ejection of Coulomb-crystallized ions onto an external detector for the recording of time-of-flight (TOF) mass spectra. This enables the examination of reaction dynamics and kinetics between Coulomb-crystallized ions and neutral molecules: ionic products are typically cotrapped, thus ejecting the crystal onto an external detector reveals the masses, identities, and quantities of all ionic species at a selected point in the reaction. Two reaction systems are examined: the reaction of Ca(+) with deuterated isotopologues of water, and the charge exchange between cotrapped Xe(+) with deuterated isotopologues of ammonia. These reactions are examples of two distinct types of experiment, the first involving direct reaction of the laser-cooled ions, and the second involving reaction of sympathetically-cooled heavy ions to form a mixture of light product ions. Extensive simulations are conducted to interpret experimental results and calculate optimal operating parameters, facilitating a comparison between the DIT and DCT approaches. The simulations also demonstrate a correlation between crystal shape and image shape on the detector, suggesting a possible means for determining crystal geometry for nonfluorescing ions.

Product state and speed distributions in photochemical triple fragmentations.

The clearest dynamical signature of a roaming reaction is a very cold distribution of energy into the rotational and translational degrees of freedom of the roaming donor fragment (e.g. CO) and an exceptionally hot vibrational distribution in the roaming acceptor fragment (e.g. H2, CH4). These signatures were initially identified in joint experimental/theoretical investigations of roaming in H2CO and CH3CHO and are now used to infer the presence of roaming mechanisms in other photodissociation reactions. In this paper we construct a phase space theory (PST) model of triple fragmentation (3F) and show that the dynamical signature of 3F is similar to that of the roaming donor fragment. The PST model starts with a calculation of two-body fragmentation (2F) of a generic molecule, ABC into AB + C. Every AB fragment with sufficient energy to undergo subsequence spontaneous dissociation is allowed to dissociate and the PST distribution of energy into A + B products is calculated for every initial AB state. Using CH3CHO --> HCO + CH3 --> H + CO + CH3 as an example, we calculate that the energy disposal into the rotational and translational degrees of freedom of the 3F products is very low, and is similar to the dynamical signature expected for production of CO via a roaming mechanism. We compare the 3F PST model with published experimental data for photodissociation of CH3CHO and CH3OCHO at energies above the 3F threshold.

Laser-induced fluorescence spectrum of 3-vinyl-1H-indene.

The laser-induced fluorescence spectrum of 3-vinyl-1H-indene was recorded between 33,000 and 33,800 cm(-1). An origin band was observed at 33,455 cm(-1) along with several low-frequency modes. With the aid of density functional theory and configuration interaction calculations, the electronic transition was assigned as S1 <-- S0 and the short progression in an 80 cm(-1) mode was identified as a vinyl group torsion. Theoretical, spectroscopic, and thermochemical considerations suggest that the 3-vinyl-1H-indene spectrum results from excitation from both conformational isomers with the vinyl and indene double bonds in trans and cis arrangements. The results are discussed in the context of the identification of species arising from the discharge of benzene in argon.