Exhaustive State-to-State Cross Sections and Rate Coefficients for Inelastic N2-N2 Collisions using QCT Combined with Neural Network Models.

Using the quasi-classical trajectory method, we systematically studied the state-to-state vibrational relaxation process of N2(v1) + N2(v2) collisions over a wide temperature range (5000-30,000 K). Different temperature dependencies of the single- and multiquantum VV and VT events in various (v1,v2) collisions are captured, with the dominant channel being related to the initial vibrational energy levels (vmax = 50). At a specified relative translational energy, there is a monotonic relationship of the VT cross sections with the vibrational energy level, particularly in high-energy collisions. Additionally, we constructed well-trained neural network models (R-values reaching 0.99) using limited quasi-classical trajectory (QCT) data sets, which can be used to predict the state-to-state cross sections and rate coefficients of the VV processes N2(v1) + N2(v2) → N2(v1 - Δv) + N2(v2 + Δv) and VT processes N2(v1) + N2(v2) → N2(v1 - Δv) + N2(v2) (Δv = ±1, ±2, ±3) for collisions with arbitrary initial vibrational states. This work not only significantly reduces computational resources but also serves as a reference for the study of the state-to-state dynamics of all four-atom collision systems in hypersonic flows.

Strain and Electric Field Engineering of G-ZnO/SnXY (X, Y = S, Se) S-Scheme Heterostructures for Photocatalyst and Electronic Device Applications: A Hybrid DFT Calculation.

Using hybrid density functional theory calculations, we systematically study the biaxial strain and electric field modulated electronic properties of g-ZnO/SnS2, g-ZnO/SnSe2, and g-ZnO/SnSSe S-scheme van der Waals heterostructures (vdWHs). g-ZnO/SnS2 and g-ZnO/SnSSe are found to be promising photocatalysts for water splitting with high solar-to-hydrogen efficiencies, even under acidic, alkaline, and high-stress conditions. The strain effect on the bandgaps of g-ZnO/SnXY is explained in detail according to the correlation between geometry structure and orbital hybridization of SnXY, which could help understand the strain-induced band structure evolutions in other SnXY (X, Y = S, Se)-based vdWHs. It is surprising that under an external electric field, g-ZnO/SnS2, g-ZnO/SnSe2, and g-ZnO/SnSSe can offer the occupied nearly free-electron (NFE) states. In many materials, NFE states are usually unoccupied and is not conducive to the charge transport. The NFE state in g-ZnO/SnSe2 is the most sensitive to the electric field and might be promising electron transport channel in nanoelectronic devices. g-ZnO/SnSe2 might also have application potential in gas sensors and high-temperature superconductors.

State-to-state dynamics and machine learning predictions of inelastic and reactive O(3P) + CO(1∑+) collisions relevant to hypersonic flows.

The state-to-state (STS) inelastic energy transfer and O-atom exchange reaction between O and CO(v), as two fundamental processes in non-equilibrium air flow around spacecraft entering Mars' atmosphere, yield the same products and both make significant contributions to the O + CO(v) → O + CO(v') collisions. The inelastic energy transfer competes with the O-atom exchange reaction. The detailed reaction mechanisms of these two elementary processes and their specific contributions to the CO relaxation process are still unclear. To address these concerns, we performed systematic investigations on the 3A' and 3A″ potential energy surfaces (PESs) of CO2 using quasi-classical trajectory (QCT) calculations. Analysis of the collision mechanisms reveals that inelastic collisions have an apparent PES preference (i.e., they tend to occur on the 3A' PES), while reactive collisions do not. Reactive rates decrease significantly when the total collision energy approaches dissociation energy, which differs from the inelastic process. Inelastic rates are generally lower than the reactive rates below ∼10 000 K, except for single quantum jumps, whereas the reverse is observed above ∼10 000 K. In addition, by combining QCT with convolutional neural networks, we have established neural network (NN)-STS1 (inelastic) and NN-STS2 (reactive) models to generate all possible STS cross sections. The NN-based models accurately reproduce the results calculated from QCT calculations. In this study, all calculations have been focused on analyzing collisions at the ground rotational level.

Ideal two-dimensional quantum spin Hall insulators MgA2Te4 (A = Ga, In) with Rashba spin splitting and tunable properties.

For decades, topological insulators have played a pivotal role in fundamental condensed-matter physics owing to their distinctive edge states and electronic properties. Here, based on in-depth first-principles calculations, we investigate the MgA2Te4 (A = Ga, In) structures belonging to the MA2Z4 2D material family. Among them, the topological insulator MgGaInTe4 exhibits band inversion and a sizeable bandgap of up to 60.8 meV which satisfies the requirement for room-temperature realization. Under the spin-orbit coupling effect, MgGaInTe4 with inversion asymmetry undergoes Rashba spin splitting. The Rashba-like and Dirac-type edge states emerge from different terminals along (010) for MgGaInTe4. The external vertical electric field is verified to modulate the inverted bandgap and topological state of MgGaInTe4 by converting a nontrivial state to a trivial state and MgIn2Te4 with the original trivial state to a nontrivial one. Accordingly, MgGaInTe4 and MgIn2Te4 have significant potential for application in topological quantum field-effect transistors. Our research identifies that the MgA2Te4 (A = Ga, In) structures have huge potential to be candidate 2D materials for spintronics and topological quantum devices.

Theoretical Study on the Spectroscopic Properties and Line Intensity of Silicon Monosulfide Cation.

The SiS+ cation is considered a potential astromolecule, yet there is limited documentation regarding its spectroscopic properties and spectral line intensities. In this paper, the potential energy curves, dipole moments, and transition dipole moments of the SiS+ cation are calculated using the icMRCI + Q method. By solving the one-dimensional Schrödinger equation for the nucleus, we have obtained spectroscopic constants for both the ground state and 11 low-lying excited states. Utilizing the vibrational transition energy levels of these 12 bound states, we calculated the partition function for the SiS+ cation over the temperature range of 300-10,000 K. Upon deriving the partition function and Einstein coefficients, we have computed the spectral line intensities for the X2Π state, the A2Σ+ state, and the X2Π â†” A2Σ+ transition at 300 and 3000 K for Δν = 0,1,2. At T = 300 K, the intensity of the X2Π state 1-0 band reaches its maximum, while at T = 3000 K, the intensity of the A2Σ+ state 0-0 band reaches its maximum. The spectral line intensities of the X2Π â†” A2Σ+ transition, on the other hand, are relatively small, with the overall intensity being smaller compared to the spectral line intensities of the X2Π and A2Σ+ states by about 10-6.

Dissociation cross sections and rates in O2 + N collisions: molecular dynamics simulations combined with machine learning.

The collision-induced dissociation reaction of O2 (v, j) + N, a fundamental process in nonequilibrium air flows around reentry vehicles, has been studied systematically by applying molecular dynamics simulations on the 2A', 4A' and 6A' potential energy surfaces of NO2 in a wide temperature range. In particular, we have directly investigated the role of the 6A' surface in this process and discussed the applicability of the simplified approximate rate models proposed by Esposito et al. and Andrienko et al. based on the lowest two surfaces. The present work indicates that the state-selected dissociation of O2 + N is dominated by the 6A' surface for all except for the low-lying O2 states. Furthermore, a complete database of rovibrationally detailed cross sections and rate coefficients is a prerequisite for modeling the relevant nonequilibrium air flows in spacecraft reentry. Here, the combination of the quasi-classical trajectory (QCT) and the neural network (NN) has been proposed to predict all state-selected dissociation cross sections and further construct dissociation parameter sets. All NN-based models established in this work accurately reproduce the results calculated from QCT simulations over a wide range of rovibrational quantum numbers with R2 > 0.99. Compared with the explicit QCT simulations, the computational requirement for predicting cross sections and rates based on the NN models significantly reduces. Finally, thermal equilibrium rate coefficients computed from NN models match remarkably well the available theoretical and experimental results in the whole temperature range explored.

Light-responsive organic polaritons from first principles.

Controlling the optical properties of light-responsive organic molecules is essential for their application in photonics. We demonstrate how light-responsive organic polaritons formed inside an optical cavity can be used to modify these properties based on first principles. Specifically, we study the excited state properties of the trans-azobenzene molecule and the free base tetraphenyl porphyrin (H2TPP) molecule under weak to strong light-matter coupling. Our results show that the cavity can modulate the dispersion and absorption properties of organic molecules. Compared to the case outside the cavity, the anomalous dispersion of the trans-azobenzene molecule inside the cavity is suppressed and this suppression decreases with increasing coupling strength, showing the potential of strong light-matter coupling in manipulating the optical dipole trap of organic molecules. Moreover, by adjusting the cavity parameters to tune the strength of the light-matter coupling, we achieve free switching between symmetric Lorentz and asymmetric Fano line shapes for H2TPP polaritonic excitations. During the switching between these spectral features, we also find that the cavity can be used to control the spontaneous radiation of organic molecules via the Purcell effect. These findings provide a new pathway to manipulate the optical properties of light-responsive organic molecules.

Exhaustive state-specific dissociation study of the N2(Σg+1)+N(S4) system using QCT combined with a neural network method.

This work studies the exhaustive rovibrational state-specific collision-induced dissociation properties of the N2+N system by QCT (quasi-classical trajectory) combined with a neural network method based on the ab initio PES recently published by Varga et al. [Phys. Chem. Chem. Phys. 23, 26273 (2021)]. The QCT combined with a neural network for state-specific dissociation (QCT-NN-SSD) model is developed and used to predict the dissociation cross sections and their energy dependence on the thermal range from a sparsely sampled noisy dataset. It is conservatively estimated that using this method can reduce the cost of the calculation by 96.5%. The rate coefficient of thermal non-equilibrium between different energy modes is obtained by combining the QCT-NN-SSD model with the multi-temperature model. The results show that, for the equilibrium state, dissociation mainly occurs at high vibrational and moderately low rotational levels. When the system is in non-equilibrium, there is no obvious vibrational level preference and highly rotationally excited molecules play a major role in promoting the dissociation by compensating for the lack of vibrational energy. The use of neural network training to generate complete datasets based on limited and discrete data provides an economical and reliable way to obtain a complete kinetic database needed to accurately simulate non-equilibrium flows.

Real-time dynamic simulation of laser-induced N2 dissociation on two-dimensional graphene sheets.

Due to its relatively high inertness, nitrogen dissociation at ambient temperature and pressure has always been a challenging task. Plasmon driven photocatalysis has proved to be an effective method. Owing to their unique physical, chemical, and electronic properties, two-dimensional planar materials have become the most promising candidates to replace noble metal catalytic nitrogen reduction. In this study, real-time dynamics of N2 dissociation on graphene sheets under femtosecond laser irradiation was studied by using time-dependent density functional theory. We confirm that electrons generated by plasmon excitation of graphene transfer to the N2 molecular antibonding orbital and activate the N-N bond. The threshold of laser intensity of N2 dissociation can be effectively reduced by mixing CO molecules. This work provides basic insights for understanding the plasmon induced N2 activation process at the atomic scale and proves that graphene can be used as one of the candidate materials for N2 reduction photocatalysts with excellent performance.

The role of the sextet potential energy surface in O2 + N inelastic collision processes.

We have performed molecular dynamics simulations of inelastic collisions between molecular oxygen and atomic nitrogen, employing the quasi-classical trajectory method on the new doublet, quartet, and sextet analytical potential energy surfaces of NO2. A complete database of vibrationally detailed rate coefficients is constructed in a wide temperature range for high vibrational states up to ν = 25. In particular, the present work shows that the sextet potential energy surface plays a crucial role in the rovibrational relaxation process of O2 + N collisions. The state-to-state rate coefficients increase by a factor of 2 to 6 when we consider the contribution of this sextet potential energy surface according to the corresponding weight factor, especially for vibrational energy transfer processes in single quantum jumps and/or high-temperature regimes. Furthermore, we also provide Arrhenius-type accurate fits for the vibrational state-specific rate coefficients of this collision system to achieve the flexible application of rate coefficients in numerical codes concerning air kinetics. Our results have implications for understanding the relaxation mechanism of the collision system with degenerate electronic states.

Elastic and inelastic low-energy electron scattering from pyridine.

A comprehensive investigation of elastic and inelastic electron scattering from molecular pyridine is reported using the ab initio R-matrix method with the static exchange plus polarization and close-coupling approximations for incident energies up to 10 eV. The two well-known low-lying 1 2B1 and 1 2A2 shape resonances as well as a 2 2B1 mixed-character resonance compare well with the theoretical and experimental results. We also detect five core-excited resonances (1 2A1, 1 2B2, 3 2B1, 2 2A2, and 4 2B1), which lie above the first electronic excitation threshold. The total elastic cross sections and momentum transfer cross sections agree reasonably with previous reference data. Comparisons of the differential elastic cross sections of pyridine with those measured for benzene, pyrazine, and pyrimidine show remarkable agreement at scattering angles above 40° but behave differently for forward scattering below 40° below 6 eV, due to the dominant effect of the permanent dipole moment on the differential cross section in the low energy region with narrow scattering angles. Inelastic electronic excitation cross sections are presented, showing the influence of core-excited resonances below the ionization threshold for the first time.

Modeling of laser-pulse induced small water cluster-(H2O)N (N = 1-10) decomposition on suitable metal cluster catalysts.

Understanding the microscopic mechanisms of electronic excitation in water clusters is a very important and challenging problem in a series of solar energy applications, such as solar water evaporation, photolysis, etc. Here we employ real time-time-dependent density functional theory (RT-TDDFT) and Ehrenfest dynamics to investigate the photodissociation dynamic process of (H2O)N=1-10 clusters and photoinduced charge transfer in them. The research presented here confirms that the plane tetramer, (H2O)4, is the most difficult one to be dissociated under laser irradiation in the ten clusters for its high (S4) symmetry; the overall order of the ease of decomposition is as follows: (H2O)6-p > (H2O)8 > (H2O)6-c > (H2O)7 > (H2O)10 > (H2O)1 > (H2O)3 > (H2O)2 > (H2O)9 > (H2O)5 > (H2O)4. Plasmon catalyst-induced water splitting is a promising and feasible way to efficiently convert solar to chemical energy via reducing the laser amplitude threshold significantly; and among the Ag6, Au6, Cu6, Al6 chains and several Cu6 clusters with Oh symmetry, the Cu6 chain seems to be the most cost-effective one. This article aims at unraveling the fundamental mechanisms and providing valuable physical insights into the behavior of water splitting to pave the way for the theoretical and experimental design of the photolysis process.

Out-of-plane dipole-modulated photogenerated carrier separation and recombination at Janus-MoSSe/MoS2 van der Waals heterostructure interfaces: an ab initio time-domain study.

Out-of-plane mirror symmetry-breaking provides a powerful tool for engineering the electronic properties and the exciton behavior of two-dimensional materials. Here, by combining time-domain density functional theory with nonadiabatic dynamics, we investigate the underlying mechanism of how the vertical dipole moment modulates the photoexcited carrier transport and the electron-hole recombination dynamics in polar Janus MoSSe/MoS2 stacked heterostructures. It is shown that the stronger nonadiabatic coupling, interlayer-state delocalization and the built-in electric field caused by charge redistribution facilitate a more rapid photocarrier separation across the interface in the S/S stacked bilayer compared with the S/Se bilayer, explaining the experimentally observed stronger photoluminescence quenching effect in the S/S heterostructure. We also found that the photocarrier recombination of the heterostructure with the S/Se interface has a timescale up to nanoseconds, which is ∼4 times longer than that of the S/S bilayer. Such a prolonged recombination time originates from the dipole-weakened nonadiabatic coupling between occupied and unoccupied states instead of quantum coherence and the band gap effect. Overall, Janus MoSSe/MoS2 heterostructures exhibit superior photocatalytic activity, reflecting the ultrafast photocarrier separation triggered by the built-in electric field, suppressed carrier recombination, high solar-to-hydrogen conversion efficiency and the strong absorption coefficient expanding from visible-light to near-infrared-light. The above atomistic and time-domain findings reveal the intrinsic dipole as an effective freedom to regulate the nonadiabatic photocarrier dynamics in Janus-based 2D heterostructures for efficient energy harvesting and optoelectronic applications.

R-Matrix Calculation of Electron Collisions with Molecular Oxygen in Its Electronically Excited States.

Low-energy electron collisions with the X3Σg- ground state and a1Δg and b1Σg+, the Herzberg states (c1Σu-, A'3Δu, and A3Σu+), and B3Σu- excited states of the O2 molecules are studied using the fixed-nucleus R-matrix method. Integral elastic scattering and electronic excitation cross sections from the X3Σg- ground state overall agree well with the available experimental and theoretical results. The electronic (de-)excitation cross sections for the electron impact with the Herzberg states and the B3Σu- state are reported. The value of elastic cross sections for the six excited states decreases with the increment of electron energy, except for the resonance peaks. As the case of excitation from the X3Σg- ground state, the O2- 2Πu resonance makes a dominant contribution to the (de-)excitation cross sections from the a1Δg, b1Σg+, and the Herzberg states. The magnitude of the de-excitation cross sections at the location of the 2Πu resonance from the Herzberg states to the ground state is about 2 to 8 times those of the excitation cross sections for the corresponding excitation transitions. These results should be significant for models of oxygen plasma and the dynamics of the Herzberg states of molecular oxygen in the earth's atmosphere.

State-to-State Transition Study of the Exchange Reaction for N(4S) and O2(X3Σg-) Collision by Quasi-Classical Trajectory.

Based on the new 2A' and 4A' potential energy surfaces of NO2 fitted by Varga et al., we conducted a quasi-classical trajectory study on the N(4S) +O2(X3Σg- ) → NO(2Π) + O(3P) reaction, focusing on the high vibrational state up to ν = 25. For different rovibrational states of O2, within the relative translational energy (Ec) range of 0.1-30 eV, the total exchange cross section (ECS) is calculated, and it is found that the initial relative translational energy and vibration excitation have a significant effect on ECSs, while rotational excitation has little influence; the rate coefficient of the high rovibrational state of O2 molecules at high temperatures is studied, and it is found that when the vibrational level ν of O2 is in the range of 0-15, the value of log10 k(T, ν, j) with the vibrational level ν is almost linear, while when ν is greater than 15, it becomes gentle with the increase in ν. Finally, the state-to-state rate coefficients are calculated; our results supply the advantageous state-to-state process data in the NO2 system, and they are useful for further studying the related hypersonic gas flow at very high temperature.

Ab initio dynamics simulation of laser-induced photodissociation of phenol.

We theoretically investigated the photodissociation dynamics of phenol molecules steered by a sequence of temporally shaped femtosecond laser pulses with high intensity and ultrashort duration, via the real-time Time-Dependent Density Functional Theory (rt-TDDFT) combined with a Molecular Dynamics (MD) simulation. The principal findings of this research are that the phenol photodissociation can take place in 50 fs; the bonds broke sequentially; the degree of phenol molecular dissociation has a strong linear correlation with the intensity. For an incident laser being 800 nm-40 fs (wavelength-pulse duration), the threshold intensity is 7 × 1014 W cm-2 and the products are hydrogen from OH1 (phenolic hydroxyl group) and C6H5O-fragments. More fragments will be found at stronger intensity, shorter wavelengths, and longer pulse duration. More accurately, we estimated the critical values of bond cleavage of an isolated phenol molecule are 1.779 Å for O-H1 and 2.184 Å for C-Hs via Electron localization function (ELF) analysis. The photodissociation of the phenol molecule was triggered via the excitation of electrons and the dissociation process of phenol here is in good agreement with the characteristics of field-assisted dissociation (FAD) theory. Orthogonal tests with an L9 (34) matrix and threshold intensity decrease tests were conducted to confirm the mechanism. Our research gives an insight into the photodissociation experiment of phenol and provides a simple yet effective way to understand the photochemical experiments of more complex organic pollutants with toxicity.

Separation of C1/C3 binary organic gas mixtures via flexible nanoporous carbon molecular sieves: DFT calculations and MD simulations.

The C1/C3 hydrocarbon gas separation characteristics of nanoporous carbon molecular sieves (CMS) are studied using DFT calculations and MD simulations. To efficiently separate the equimolar CH4/C3H8 and CH4/C3H6 gas mixtures, CNT gas transport channels are embed between the polyphenylene membranes which created structural deformation for both CNT and membrane. The adsorption and permeation of gas molecules via CMS and the effect of nanochannel density and electric field on gas selectivity are analyzed. In addition to the direct permeation, gas molecules that adsorbed on the NPG surface also making a significant contribution to the gas permeability comes from a surface mechanism. Results also uncovered that the gas selectivity is enhanced by the electric field along the + x and +y axes, whereas reduced by the electric field along the + z and -z axes. Plainly, this CMS provides a feasible way for the efficient separation of the C1/C3 organic gas mixtures.

Collision-Induced Rotational Excitation of CO2 by N(4S) Atoms: A New Ab Initio Potential Energy Surface and Scattering Calculations.

Collisional excitations of CO2 molecules are significant to fully understand the physical and chemical processes of astrophysical and atmospheric environments. Rotational excitations of CO2 molecules induced by N(4S) atoms have been studied for the first time. First, we have computed a new highly accurate ab initio potential energy surface (PES) of a CO2-N(4S) van der Waals complex. The PES has been obtained by employing the partially spin-restricted coupled cluster with open-shell single, double, and perturbative triple excitation method with aug-cc-pVQZ basis sets. The full close-coupling calculations have been performed to compute cross sections for kinetic energies up to 800 cm-1. For all of the excitations, rotational cross sections exhibit an overall decrease with the increase of the energy gaps. Rate coefficients are calculated by averaging the cross sections over a Maxwell-Boltzmann distribution for temperatures ranging from 1 to 150 K. The trends in rate coefficients are in good agreement with those of similar collision systems. The decrease in energy gaps and the increase in temperature are the key factors to enhance the rate coefficients of CO2 excitation. Our study will be useful for accurately establishing the atmospheric model of terrestrial planets and determining the abundance of CO2 and N(4S) in space.

New theoretical insights into the photoinduced carrier transfer dynamics in WS2/WSe2 van der Waals heterostructures.

Shedding light on the dynamics of charge transfer is fundamental and important to understand the light-photocurrent power conversion in transition-metal dichalcogenide (TMD) heterostructures. Herein, based on time-dependent ab initio nonadiabatic molecular dynamics simulation, we studied the photoinduced carrier transfer dynamics in the WS2/WSe2 heterostructure and further analyzed the effects of stacking configuration and temperature. Our calculations show that the time scales of ultrafast hole transfer in the C7 and T stacking configurations are 35 fs and 30 fs, respectively, which are mainly caused by the adiabatic charge transfer mechanism. Meanwhile, the time scales of ultrafast electron transfer in the C7 and T stacking configurations are 12 fs and 40 fs, respectively, which are in good agreement with the experimental result. We also investigated in detail the photoinduced carrier transfer pathways of C7 and T stacking configurations, which appear to have some significant differences. In addition, we found that the temperature basically has no effect on the electron transfer dynamics of the WS2/WSe2 heterostructure; this is in excellent agreement with the experimental observation. In short, the reported findings can provide more in-depth insights into the photoinduced carrier transfer dynamics of TMD-based van der Waals heterostructures.

Kohn anomaly and van Hove singularity in IVB and VB group transition metals nitrides and carbides.

The superconducting behavior in IVB-VB group transition metal nitrides and carbides has generally been associated with the phonon anomaly and Fermi surface nesting. However, the origin of phonon anomaly has remained ambiguous (i.e. longitudinal acoustic or transverse acoustic modes). We performed first-principles calculations to investigate the phononic properties of these materials and theoretically confirmed that the Kohn anomaly originates from the lower transverse acoustic mode along the ГX direction, thereby revealing the frequency derivative discontinuity of the mode. In particular, the Kohn anomaly region is found to move from the interior to the boundary X point of the Brillouin zone with increasing number of valence electrons. We deduced that the Kohn anomaly originated from the electrons of the filled energy level near the van Hove singularity. These results suggest that the screening of the ionic electric field decreases, while the coupling of conduction electrons with the highly degenerate modes between the TA∥ and LA via Umklapp scattering process increases. The Fermi surface nesting also plays a role in enhancing the superconductivity. The electronic excitation effect induces a stabilization of the V2 group transition metal nitrides.