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A machine-learned model for predicting product state distributions from specific initial states (state-to-distribution or STD) for reactive atom–diatom collisions is presented and quantitatively tested for the N(4S) + O2( X 3 Σ g − ) → NO(X2Π) + O(3P) reaction. The reference dataset for training the neural network consists of final state distributions determined from quasi-classical trajectory (QCT) simulations for ∼ 2000 initial conditions. Overall, the prediction accuracy as quantified by the root-mean-squared difference (∼0.003) and the R2 (∼0.99) between the reference QCT and predictions of the STD model is high for the test set, for off-grid state-specific initial conditions, and for initial conditions drawn from reactant state distributions characterized by translational, rotational, and vibrational temperatures. Compared with a more coarse grained distribution-to-distribution (DTD) model evaluated on the same initial state distributions, the STD model shows comparable performance with the additional benefit of the state resolution in the reactant preparation. Starting from specific initial states also leads to a more diverse range of final state distributions, which requires a more expressive neural network compared with DTD. A direct comparison between QCT simulations, the STD model, and the widely used Larsen–Borgnakke (LB) model shows that the STD model is quantitative, whereas the LB model is qualitative at best for rotational distributions P(j′) and fails for vibrational distributions P(v′). As such, the STD model can be well-suited for simulating nonequilibrium high-speed flows, e.g., using the direct simulation Monte Carlo method. [ABSTRACT FROM AUTHOR]

High-temperature, reactive gas flow is inherently nonequilibrium in terms of energy and state population distributions. Modeling such conditions is challenging even for the smallest molecular systems due to the extremely large number of accessible states and transitions between them. Here, neural networks (NNs) trained on explicitly simulated data are constructed and shown to provide quantitatively realistic descriptions which can be used in mesoscale simulation approaches such as Direct Simulation Monte Carlo to model gas flow at the hypersonic regime. As an example, the state-to-state cross sections for N(4S) + NO(2Π) → O(3P) + N2( X 1 Σ g + ) are computed from quasiclassical trajectory (QCT) simulations. By training NNs on a sparsely sampled noisy set of state-to-state cross sections, it is demonstrated that independently generated reference data are predicted with high accuracy. State-specific and total reaction rates as a function of temperature from the NN are in quantitative agreement with explicit QCT simulations and confirm earlier simulations, and the final state distributions of the vibrational and rotational energies agree as well. Thus, NNs trained on physical reference data can provide a viable alternative to computationally demanding explicit evaluation of the microscopic information at run time. This will considerably advance the ability to realistically model nonequilibrium ensembles for network-based simulations. [ABSTRACT FROM AUTHOR]

The dynamics of the C(3P) + O2(3Σ −g) → CO(1Σ+) + O(1D) reaction on its electronic ground state is investigated by using time-dependent wave packet propagation (TDWP) and quasi-classical trajectory (QCT) simulations. For the moderate collision energies considered (Ec = 0.001 to 0.4 eV, corresponding to a range from 10 K to 4600 K) the total reaction probabilities from the two different treatments of the nuclear dynamics agree very favourably. The undulations present in P(E) from the quantum mechanical treatment can be related to stabilization of the intermediate CO2 complex with lifetimes on the 0.05 ps time scale. This is also confirmed from direct analysis of the TDWP simulations and QCT trajectories. Product diatom vibrational and rotational level resolved state-to-state reaction probabilities from TDWP and QCT simulations agree well except for the highest product vibrational states (v′ ≥ 15) and for the lowest product rotational states (j′ ≤ 10). Opening of the product vibrational level CO(v′ = 17) requires ∼0.2 eV from QCT and TDWP simulations with O2(j = 0) and decreases to 0.04 eV if all initial rotational states are included in the QCT analysis, compared with Ec > 0.04 eV obtained from experiments. It is thus concluded that QCT simulations are suitable for investigating and realistically describe the C(3P) + O2(3Σ −g) → CO(1Σ+) + O(1D) reaction down to low collision energies when compared with results from a quantum mechanical treatment using TDWPs. [ABSTRACT FROM AUTHOR]

Vibrational energy relaxation (VER) of diatomics following collisions with the surrounding medium is an important elementary process for modeling high-temperature gas flow. VER is characterized by two parameters: the vibrational relaxation time τvib and the state relaxation rates. Here the vibrational relaxation of CO(v = 0 ← v = 1) in Ar is considered for validating a computational approach to determine the vibrational relaxation time parameter (pτvib) using an accurate, fully dimensional potential energy surface. For lower temperatures, comparison with experimental data shows very good agreement whereas at higher temperatures (up to 25 000 K), comparisons with an empirically modified model due to Park confirm its validity for CO in Ar. Additionally, the calculations provide insight into the importance of △v > 1 transitions that are ignored in typical applications of the Landau-Teller framework. Published by AIP Publishing. [ABSTRACT FROM AUTHOR]

Charge exchange from doubly charged rare gas cations to simple diatomics proceeds with a large cross section and results in populations of many vibrational and electronic product states. The charge exchange between Xe2+ and N2, in particular, is known to create N+2 in both the A and B electronic states. In this work, we present integral charge exchange cross section measurements of the Xe2+ + N2 reaction as well as axial recoil velocity distributions of the Xe+ and N+2 product ions for collision energies between 0.3 and 100 eV in the center-of-mass (COM) frame. Total charge-exchange cross sections decrease from 70 Ų to about 40 Ų with increasing collision energy through this range. Analysis of the axial velocity distributions indicates that a Xe2+ - N2 complex exists at low collision energies but is absent by 17.6 eV COM. Analysis of the axial velocity distributions reveals evidence for complexes with lifetimes comparable to the rotational period at low collision energies. The velocity distributions are consistent with quasi-resonant single charge transfer at high collision energies. [ABSTRACT FROM AUTHOR]

The collisional dynamics of N2+(2Σg+) cations with Ar atoms is studied using quasi-classical simulations. N2+-Ar is a proxy to study cooling of molecular ions and interesting in its own right for molecule-to-atom charge transfer reactions. An accurate potential energy surface (PES) is constructed from a reproducing kernel Hilbert space (RKHS) interpolation based on high-level ab initio data. The global PES including the asymptotics is fully treated within the realm of RKHS. From several ten thousand trajectories, the final state distribution of the rotational quantum number of N2+ after collision with Ar is determined. Contrary to the interpretation of previous experiments which indicate that up to 98% of collisions are elastic and conserve the quantum state, the present simulations find a considerably larger number of inelastic collisions which supports more recent findings. [ABSTRACT FROM AUTHOR]

The O(³P) + NO(²∏) → O2(X³∑-g) + N(4S) reaction among the N- and O- involving reactions that dominate the energetics of the reactive air flow around spacecraft during hypersonic atmospheric re-entry. In this regime, the temperature in the bow shock typically ranges from 1000 to 20000 K. The forward and reverse rate coefficients for this reaction derived directly from trajectory calculations over this range of temperature are reported in this letter. Results compare well with the established equilibrium constants for the same reaction from thermodynamic quantities derived from spectroscopy in the gas phase which paves the way for large-scale in silico investigations of equilibrium rates under extreme conditions. [ABSTRACT FROM AUTHOR]

Growing interest in developing and testing iodine Hall effect thrusters requires measurements of the cross sections of reactions that generate low energy plasma following discharge. Limited experimental and theoretical work necessitates a decisive experiment to elucidate the charge exchange and collision-induced dissociation channels. To this end, we have used guided-ion beam techniques to measure cross sections for both I+ + I2 and I+ + I2 collisions. We present total collision cross sections as well as collision-induced dissociation cross sections for center-of-mass collision energies ranging from 0.5 to 200 eV for molecular iodine cations. Similarly, we present total collision cross section and charge-exchange cross sections for atomic iodine cations for center-of-mass collision energies ranging from 0.67 to 167 eV. Time-of-flight measurements of the collision products allow determination of velocity distributions, which show evidence of complex formation of I3+ from the I+ + I2 reaction at collision energies below 6 eV. [ABSTRACT FROM AUTHOR]

Reactions involving N and O atoms dominate the energetics of the reactive air flow around spacecraft when reentering the atmosphere in the hypersonic flight regime. For this reason, the thermal rate coefficients for reactive processes involving O(³P) and NO(²Π) are relevant over a wide range of temperatures. For this purpose, a potential energy surface (PES) for the ground state of the NO2 molecule is constructed based on high-level ab initio calculations. These ab initio energies are represented using the reproducible kernel Hilbert space method and Legendre polynomials. The global PES of NO2 in the ground state is constructed by smoothly connecting the surfaces of the grids of various channels around the equilibrium NO2 geometry by a distance-dependent weighting function. The rate coefficients were calculated using Monte Carlo integration. The results indicate that at high temperatures only the lowest A-symmetry PES is relevant. At the highest temperatures investigated (20 000 K), the rate coefficient for the "O1O2+N" channel becomes comparable (to within a factor of around three) to the rate coefficient of the oxygen exchange reaction. A state resolved analysis shows that the smaller the vibrational quantum number of NO in the reactants, the higher the relative translational energy required to open it and conversely with higher vibrational quantum number, less translational energy is required. This is in accordance with Polanyi's rules. However, the oxygen exchange channel (NO2+O1) is accessible at any collision energy. Finally, this work introduces an efficient computational protocol for the investigation of three-atom collisions in general. [ABSTRACT FROM AUTHOR]

Thermal rates for the C(3P) + O2(3Σg−) ↔ CO(1Σ+)+ O(1D)/O(3P) reaction are investigated over a wide temperature range based on quasi classical trajectory (QCT) simulations on 3-dimensional, reactive potential energy surfaces (PESs) for the 1A′, (2)1A′, 1A′′, 3A′ and 3A′′ states. These five states are the energetically low-lying states of CO2 and their PESs are computed at the MRCISD+Q/aug-cc-pVTZ level of theory using a state-average CASSCF reference wave function. Analysis of the different electronic states for the CO2 → CO + O dissociation channel rationalizes the topography of this region of the PESs. The forward rates from QCT simulations match measurements between 15 K and 295 K whereas the equilibrium constant determined from the forward and reverse rates is consistent with that derived from statistical mechanics at high temperature. Vibrational relaxation, O + CO(ν = 1,2) → O + CO(ν = 0), is found to involve both, non-reactive and reactive processes. The contact time required for vibrational relaxation to take place is τ ≥ 150 fs for non-reacting and τ ≥ 330 fs for reacting (oxygen atom exchange) trajectories and the two processes are shown to probe different parts of the global potential energy surface. In agreement with experiments, low collision energy reactions for the C(3P) + O2(3Σg−, ν = 0) → CO(1Σ+) + O(1D) lead to CO(1Σ+, ν′ = 17) with an onset at Ec ∼ 0.15 eV, dominated by the 1A′ surface with contributions from the 3A′ surface. Finally, the barrier for the COA(1Σ+) + OB(3P) → COB(1Σ+) + OA(3P) atom exchange reaction on the 3A′ PES yields a barrier of ∼7 kcal mol−1 (0.300 eV), consistent with an experimentally reported value of 6.9 kcal mol−1 (0.299 eV). [ABSTRACT FROM AUTHOR]

We have determined absolute apparent excitation cross sections for photoemission in the 300–1050 nm spectral region resulting from collisions of I2 with electrons of 4.8–100 eV translational energy. Experiments were done in single collision conditions and emission is observed from I2, I2+, I and I+. All the emission features appear to turn on at the energy thresholds for formation of the respective excited state species. At higher electron beam energy the spectra are dominated by emissions from the strong I2+ A–X band, as well as the intense emission from several I 5s25p46s states. An attempt to determine the initial I2+ A populations generated from electron impact is discussed. These populations are determined from simulations of the I2+ A–X emission band using both experimentally and theoretically determined potential energy surfaces available in the literature. Direct, vertical formation of I2+ A from I2 X is only supported in the case of the ab initio potential energy surface. [ABSTRACT FROM AUTHOR]

Accurate potential energy surfaces (PESs) have been determined for the 3A′ and 3A′′ states of N2O using electronic structure calculations at the multireference configuration interaction level with Davidson correction (MRCI+Q) and the augmented Dunning-type correlation consistent polarized triple zeta (aug-cc-pVTZ) basis set. More than 20 000 MRCI+Q/aug-cc-pVTZ energies are represented using a reproducing kernel Hilbert space (RKHS) scheme. The RKHS PESs successfully describe all reactant channels with high accuracy and all minima and transition states connecting them are determined. Quasiclassical trajectory (QCT) simulations are then used to determine reaction rates for N + NO and O + N2 collisions. Vibrational relaxation N2(ν = 1) → N2(ν = 0) and dissociation of N2 → 2N for O + N2 collisions are also investigated using QCT. The agreement between results obtained from the QCT simulations and from available experiments is favourable for reaction and vibrational relaxation rates, which provides a test for the accuracy of the PESs. The PESs can be used to calculate more detailed state-to-state observables relevant for applications to hypersonic reentry. [ABSTRACT FROM AUTHOR]

Emission excitation cross sections are recorded for collisions between Xe2+ + O2 and O+ + Xe over a collision energy range of approximately 2 to 900 eV in the center-of-mass (Ecm) frame. Emissive products of the O+ + Xe reaction are examined in the 700–1000 nm optical range and include neutral atomic oxygen emissions and neutral xenon emissions. Atomic emission products of the O+ + Xe collision appear to have measureable cross sections near Ecm = 14 eV and increase in intensity until about Ecm = 60 eV where they remain approximately constant for the remainder of the measured collision energies. For the Xe2+ + O2 collision system, O2+ charge transfer products are measured through fluorescence of the O2+(A–X) and (b–a) manifolds over the 200–850 nm window. Total cross sections for both manifolds do not vary beyond the experimental precision at all measured energies. Vibrational populations are derived from a fitting of the experimental data. The populations are found to deviate from a Franck–Condon distribution at all collision energies and appear to be well-modeled within a multi-channel Landau–Zener framework over the collision energy range measured. [ABSTRACT FROM AUTHOR]

The kinetics and vibrational relaxation of the N(4S) + O2(X3Σ −g) ↔ O(3P) + NO(X2Π) reaction is investigated over a wide temperature range based on quasiclassical trajectory simulations on 3-dimensional potential energy surfaces (PESs) for the lowest three electronic states. Reference energies at the multi reference configuration interaction level are represented as a reproducing kernel and the topology of the PESs is rationalized by analyzing the CASSCF wavefunction of the relevant states. The forward rate matches one measurement at 1575 K and is somewhat lower than the high-temperature measurement at 2880 K whereas for the reverse rate the computations are in good agreement for temperatures between 3000 and 4100 K. The temperature-dependent equilibrium rates are consistent with results from JANAF and CEA results. Vibrational relaxation rates for O + NO(ν = 1) → O + NO(ν = 0) are consistent with a wide range of experiments. This process is dominated by the dynamics on the 2A′ and 4A′ surfaces which both contribute similarly up to temperatures T ∼ 3000 K, and it is found that vibrationally relaxing and non-relaxing trajectories probe different parts of the potential energy surface. The total cross section depending on the final vibrational state monotonically decreases which is consistent with early experiments and previous simulations but at variance with other recent experiments which reported an oscillatory cross section. [ABSTRACT FROM AUTHOR]

A technique that combines infrared laser spectroscopy and helium nanodroplet mass spectrometry, which we refer to as optically selected mass spectrometry, is used to study the efficiency of ion cooling in helium. Electron-impact ionization is used to form He+ ions within the droplets, which go on to transfer their charge to the HCN dopant molecules. Depending upon the droplet size, the newly formed ion either fragments or is cooled by the helium before fragmentation can occur. Comparisons with gas-phase fragmentation data suggest that the cooling provided by the helium is highly nonthermal. An “explosive” model is proposed for the cooling process, given that the initially hot ion is embedded in such a cold solvent. [ABSTRACT FROM AUTHOR]

The characterization of the molecular dynamics (MD) of acridine orange (3,6-dimethylaminoacridine) at a chromatographic interface by using fully atomistic simulations is discussed. Results reveal that the transport properties of acridine were primarily governed by the solvent distribution above the functionalized surface.

A study is conducted to examine the effects of liquid helium cooling on the fragmentation of ions formed by electron impact mass ionization. It is estimated that as much as 90% of the energy released by the charge transfer process is dissipated to the helium on the time scale of fragmentation.