Combining Machine Learning and Spectroscopy to Model Reactive Atom + Diatom Collisions

The prediction of product translational, vibrational, and rotational energy distributions for arbitrary initial conditions for reactive atom + diatom collisions is of considerable practical interest in atmospheric re-entry. Because of the large number of accessible states, determination of the necessary information from explicit (quasi-classical or quantum) dynamics studies is impractical. Here, a machine-learned (ML) model based on translational energy and product vibrational states assigned from a spectroscopic, ro-vibrational coupled energy expression based on the Dunham expansion is developed and tested quantitatively. All models considered in this work reproduce final state distributions determined from quasi-classical trajectory (QCT) simulations with R² ∼ 0.98. As a further validation, thermal rates determined from the machine-learned models agree with those from explicit QCT simulations and demonstrate that the atomistic details are retained by the machine learning which makes them suitable for applications in more coarse-grained simulations. More generally, it is found that ML is suitable for designing robust and accurate models from mixed computational/experimental data which may also be of interest in other areas of the physical sciences.

Quantum and quasi-classical dynamics of the C(3P) + O2(3Σ-g) → CO(1Σ+) + O(1D) reaction on its electronic ground state

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 (E_c = 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 E_c > 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.

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.

The C(3P) + O2(3Σg-) → CO2 ↔ CO(1Σ+) + O(1D)/O(3P) reaction: thermal and vibrational relaxation rates from 15 K to 20 000 K

Thermal rates for the C(³P) + O₂(³Σ_g⁻) ↔ 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(³Σ_g⁻, ν = 0) → CO(¹Σ⁺) + O(¹D) lead to CO(¹Σ⁺, ν′ = 17) with an onset at E_c ∼ 0.15 eV, dominated by the ¹A′ surface with contributions from the ³A′ surface. Finally, the barrier for the CO_A(¹Σ⁺) + O_B(³P) → CO_B(¹Σ⁺) + O_A(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).

Reaction and vibrational relaxation rate computed for C(³P) + O₂(³Σ_g⁻) ↔ CO(¹Σ⁺) + O(¹D)/O(³P) for a wide range of temperatures using quasiclassical trajectory calculations on five new potential energy surfaces for different electronic states.

Luminescence measurements of hyperthermal Xe2+ + O2 and O+ + Xe collision systems

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 (E_cm) 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 E_cm = 14 eV and increase in intensity until about E_cm = 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.

The N(4S) + O2(X3Σ−g) ↔ O(3P) + NO(X2Π) reaction: thermal and vibrational relaxation rates for the 2A′, 4A′ and 2A′′ states

Cross sections, rates, equilibrium constants and vibrational relaxation times for the N(⁴S) + O₂(X³Σ−g) ↔ O(³P) + NO(X²Π) reaction from simulations on new, RKHS-based surfaces for the three lowest electronic states.

Accurate reproducing kernel-based potential energy surfaces for the triplet ground states of N2O and dynamics for the N + NO ↔ O + N2 and N2 + O → 2N + O reactions

Reproducing kernel-based potential energy surface based on MRCI+Q/aug-cc-pVTZ energies for the triplet states of N₂O and quasiclassical dynamical study for the reaction, dissociation and vibrational relaxation.

The C(3P) + NO(X2Π) → O(3P) + CN(X2Σ+), N(2D)/N(4S) + CO(X1Σ+) reaction: Rates, branching ratios, and final states from 15 K to 20 000 K

The C + NO collision system is of interest in the area of high-temperature combustion and atmospheric chemistry. In this work, full dimensional potential energy surfaces for the ²A', ²A″, and ⁴A″ electronic states of the [CNO] system have been constructed following a reproducing kernel Hilbert space approach. For this purpose, more than 50 000 ab initio energies are calculated at the MRCI+Q/aug-cc-pVTZ level of theory. The dynamical simulations for the C(³P) + NO(X²Π) → O(³P) + CN(X²Σ⁺), N(²D)/N(⁴S) + CO(X¹Σ⁺) reactive collisions are carried out on the newly generated surfaces using the quasiclassical trajectory (QCT) calculation method to obtain reaction probabilities, rate coefficients, and the distribution of product states. Preliminary quantum calculations are also carried out on the surfaces to obtain the reaction probabilities and compared with QCT results. The effect of nonadiabatic transitions on the dynamics for this title reaction is explored within the Landau-Zener framework. QCT simulations have been performed to simulate molecular beam experiment for the title reaction at 0.06 and 0.23 eV of relative collision energies. Results obtained from theoretical calculations are in good agreement with the available experimental as well as theoretical data reported in the literature. Finally, the reaction is studied at temperatures that are not practically achievable in the laboratory environment to provide insight into the reaction dynamics at temperatures relevant to hypersonic flight.

From in silica to in silico: retention thermodynamics at solid–liquid interfaces

The dynamics of solvated molecules at the solid/liquid interface is essential for a molecular-level understanding for the solution thermodynamics in reversed phase liquid chromatography (RPLC).

Orthogonal time-of-flight mass spectrometry of an ion beam with a broad kinetic energy profile

A combined experimental and modeling effort is undertaken to assess a detection system composed of an orthogonal extraction time-of-flight (TOF) mass spectrometer coupled to a continuous ion source emitting an ion beam with kinetic energy of several hundred eV. The continuous ion source comprises an electrospray capillary system employing an undiluted ionic liquid emitting directly into vacuum. The resulting ion beam consists of ions with kinetic energy distributions of width greater than a hundred of eV and mass-to-charge (m/q) ratios ranging from 111 to 500 000 amu/q. In particular, the investigation aims to demonstrate the kinetic energy resolution along the ion beam axis (axial) of orthogonally extracted ions in measurements of the axial kinetic energy-specific mass spectrum, mass flow rate, and total ion current. The described instrument is capable of simultaneous measurement of a broad m/q range in a single acquisition cycle with approximately 25 eV/q axial kinetic energy resolution. Mass resolutions of ∼340 (M/ΔM, FWHM) were obtained for ions at m/q = 1974. Comparison of the orthogonally extracted TOF mass spectrum to mass flow and ion current measurements obtained with a quartz-crystal microbalance and Faraday cup, respectively, shows reasonable numeric agreement and qualitative agreement in the trend as a function of energy defect.

Reactive collisions for NO(2Π) + N(4S) at temperatures relevant to the hypersonic flight regime

Rate coefficients for the NO(²Π) + N(⁴S) reaction at high temperatures from quasiclassical trajectories using MRCI+Q PESs of the lowest triplet states.

Quantum and quasiclassical trajectory studies of rotational relaxation in Ar–N2+ collisions

Quantum and classical study of the Ar–N₂⁺ collision based on a new potential energy surface.

Integral cross section measurements and product recoil velocity distributions of Xe(2+) + N2 hyperthermal charge-transfer collisions

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 Xe(2+) and N2, in particular, is known to create N2 (+) in both the A and B electronic states. In this work, we present integral charge exchange cross section measurements of the Xe(2+) + N2 reaction as well as axial recoil velocity distributions of the Xe(+) and N2 (+) 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 Å(2) to about 40 Å(2) with increasing collision energy through this range. Analysis of the axial velocity distributions indicates that a Xe(2+) - 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.

Computational study of collisions between O(3P) and NO(2Π) at temperatures relevant to the hypersonic flight regime

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((3)P) and NO((2)Π) 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.

A guided-ion beam study of the collisions and reactions of I(+) and I2 (+) with I2

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 I2 (+)+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.

Electron-catalyzed mutual neutralization of various anions with Ar+: evidence of a new plasma process

The mutual neutralization of anions with Ar+ has been studied by variable electron and neutral density attachment mass spectrometry. Evidence of a previously unobserved plasma loss process, electron-catalyzed mutual neutralization (ECMN), e.g., SF6-+Ar+ + e-→neutrals + e-, is reported. Results for 10 species suggest that ECMN occurs generally and significantly affects the total ion-loss rate in plasmas with electron densities exceeding 10(10) cm-3. ECMN is discussed in the context of other known three-body plasma processes, the mechanisms for which appear insufficient to explain the observed effect. A mechanism for ECMN involving an incident electron facilitating energy transfer to the internal modes of the anion is proposed.

Formation of cold ion-neutral clusters using superfluid helium nanodroplets

A strategy for forming and detecting cold ion-neutral clusters using superfluid helium nanodroplets is described. Sodium cations generated via thermionic emission are directed toward a beam of helium droplets that can also pick up neutral molecules and form a cluster with the captured Na(+). The composition of the clusters is determined by mass spectrometric analysis following a desolvation step. It is shown that the polar molecules H(2)O and HCN are picked up and form ion-neutral clusters with sizes and relative abundances that are in good agreement with those predicted by the statistics used to describe neutral cluster formation in helium droplets. [Na(H(2)O)(n)](+) clusters containing six to 43 water molecules were observed, a size range of sodiated water clusters difficult to access in the gas phase. Clusters containing N(2) were in lower abundance than expected, suggesting that the desolvation process heats the clusters sufficiently to dissociate those containing nonpolar molecules.

Adsorption of Acridine Orange at a C8,18/Water/Acetonitrile Interface

Fully atomistic simulations are used to characterize the molecular dynamics (MD) of acridine orange (3,6-dimethylaminoacridine) at a chromatographic interface. Multiple 1 ns MD simulations were performed for acridine orange at the interface between three different acetonitrile/water mixtures (0/100, 20/80, and 50/50) with C8 and C18 alkyl chains. The diffusion coefficient, D, of acridine orange in pure solvent was found to be 4 times smaller at the water/C18 interface (D = 0.022 x 10(-4) cm2/s) than in bulk water (D = 0.087 x 10(-4) cm2/s), in qualitative agreement with experiment. Rotational reorientation times were 20 and 700 ps, which also agree favorably with the measured time scales of 130 and 740 ps. Contrary to experiment, the simulations found that for increasing surface coverage, the diffusion coefficient for acridine decreased. Detailed analysis of the solvent structure showed that the transport properties of acridine were primarily governed by the solvent distribution above the functionalized surface. The solvent structure, in turn, was largely determined by the surface consisting of the silica layer, the alkyl chains, and their functionalization.

Fragmentation of HCN in optically selected mass spectrometry: Nonthermal ion cooling in helium nanodroplets

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.

Probing charge-transfer processes in helium nanodroplets by optically selected mass spectrometry (OSMS): charge steering by long-range interactions

Electron impact ionization of a helium atom in a helium nanodroplet is followed by rapid charge migration, which can ultimately result in the localization of the charge on an atomic or molecular solute. This process is studied here for the cases of hydrogen cyanide, acetylene, and cyanoacetylene in helium, using a new experimental method we call optically selected mass spectrometry (OSMS). The method combines infrared laser spectroscopy with mass spectrometry to separate the contributions to the overall droplet beam mass spectrum from the various species present under a given set of conditions. This is done by vibrationally exciting a specific species that exists in a subset of the droplets (for example, the droplets containing a single HCN molecule). The resulting helium evaporation leads to a concomitant reduction in the ionization cross sections for these droplets. This method is used to study the charge migration in helium and reveals that the probability of charge transfer to a solvated molecule does not approach unity for small droplets and depends on the identity of the solvated molecule. The experimental results are explained quantitatively by considering the effect of the electrostatic potential (between the charge and the embedded molecule) on the trajectory of the migrating charge.

Electron Impact Ionization in Helium Nanodroplets: Controlling Fragmentation by Active Cooling of Molecular Ions

Reported here is a study of the effects of liquid helium cooling on the fragmentation of ions formed by electron impact mass ionization. The molecules of interest are picked up by the helium nanodroplets as they pass through a low pressure oven. Electron impact ionization of a helium atom in the droplet is followed by resonant charge transfer to neighboring helium atoms. When the charge is transferred to the target molecule, the difference in the ionization potentials between helium and the molecule results in the formation of a vibrationally hot ion. In isolation, the hot parent ion would undergo subsequent fragmentation. On the other hand, if the cooling due to the helium is fast enough, the parent ion will be actively cooled before fragmentation occurs. The target molecule used in the present study is triphenylmethanol (TPM), an important species in synthetic chemistry, used to sterically protect hydroxyl groups. Threshold PhotoElectron PhotoIon COincidence (TPEPICO) experiments are also reported for gas-phase TPM to help quantify the ion energetics resulting from the cooling effects of the helium droplets.