Vibrational Dissociation Research Articles

Electronic structure, spectroscopy and photophysics of rubidium mono-sulphide

We investigated the nineteen first electronic states of the RbS diatomic using post Hartree-Fock configuration interaction methods at the internally contracted multi-reference configuration interaction (MRCI+(Q)) level. We thus mapped their respective potential energy curves with and without considering spin-orbit effects. After nuclear motion treatments, we derived their equilibrium distances, rotational and vibrational constants, and dissociation and electronic excitation energies. Moreover, an evaluation of the dipole moments of RbS was conducted, which in turn were used to evaluate the radiative lifetimes of the vibrational levels of RbS (X2П and A2Σ+) states. Computations show that spin-orbit interactions are important and cannot be omitted. Afterwards, and utilizing the sum rule approximation and the Franck–Condon factors, we computed both the bound-bound contributions and the bound-free terms. Our findings suggest that RbS exhibits intriguing characteristics suitable for laser cooling experiments. In sum, this work contributes to the understanding in-depth of the spectroscopy of RbS and predicts its use as a promising molecular system for laser cooling applications.

Vibrational spectroscopy and dissociation dynamics of cyclohexyl hydroperoxide.

Organic hydroperoxides (ROOH) are ubiquitous in the atmospheric oxidation of volatile organic compounds (VOCs) as well as in low-temperature oxidation of hydrocarbon fuels. The present work focuses on a prototypical cyclic hydroperoxide, cyclohexyl hydroperoxide (CHHP). The overtone OH stretch (2νOH) spectrum of jet-cooled CHHP is recorded by IR multiphoton excitation with UV laser-induced fluorescence detection of the resulting OH products. A distinctive IR feature is observed at 7012.5 cm-1. Two conformers of CHHP are predicted to have similar stabilities (within 0.2 kcal mol-1) and overtone OH stretch transitions (2νOH), yet are separated by a significant interconversion barrier. The IR power dependence indicates that absorption of three or more IR photons is required for dissociation of CHHP to cyclohexoxy (RO) and OH radical products. Accompanying high-level single- and multi-reference electronic structure calculations quantitatively support the experimental results. Calculations are extended to a range of organic hydroperoxides to examine trends in bond dissociation energies associated with RO + OH formation and compared with prior theoretical results.

Numerical study on the non-equilibrium characteristics of high-speed atmospheric re-entry flow and radiation of aircraft based on fully coupled model

The strong coupling interactions of non-equilibrium flow, microscopic particle collisions and radiative transitions within the shock layer of hypersonic atmospheric re-entry vehicles makes accurate prediction of the aerothermodynamics challenging. Therefore, in this study a self-consistent non-equilibrium flow, collisional–radiative reactions and radiative transfer fully coupled model are established to study the non-equilibrium characteristics of the flow field and radiation of vehicle atmospheric re-entry. The comparison of the present calculation results with flight data of FIRE II and previous results in the literature shows a reasonable agreement. The thermal, chemical and excited energy level non-equilibrium phenomena are obtained and analysed for the different FIRE II trajectory points, which form the critical basis for studying the heat transfer and radiation. The non-equilibrium distribution of excited energy levels significantly exists in the post-shock and near-wall regions due to the rapid vibrational dissociation and electronic under-excitation, as well as the wall catalytic reactions. The analysis of stagnation-point heating of FIRE II illustrates that the translational–rotational convection and the dissociation component diffusion play key roles in the aerodynamic heating of the wall region. The spectrally resolved radiative intensity in the entire flow field indicates that the vacuum ultraviolet radiation caused by the high-energy nitrogen atomic spectral lines makes the main contribution to the radiative transfer. Finally, it is found that the non-equilibrium flow–radiation coupling effect can exacerbate the excited energy level non-equilibrium, and further affect the gas radiative properties and radiative transfer. This fully coupled study provides an effective method for reasonable prediction of atmospheric re-entry flow and radiation fields.

Non-equilibrium dissociation rate coefficient of N2 + N at high temperatures

We study the dissociation rate of N2 + N collision reaction at high temperatures from T = 10000 K to T = 40000 K. We propose a specific vibrational energy dissociation rate model based on the dynamical continuum model recently proposed by Singh et al. We calculate each vibrational energy dissociation rate of the Boltzmann and the QSS distribution. We discuss the non-Boltzmann effect that the overpopulation distribution enhances the dissociation rate. The depletion distribution reduces the dissociation rate, compared to the Boltzmann distribution.

Manipulating hydrogen bond dissociation rates and mechanisms in water dimer through vibrational strong coupling

The vibrational strong coupling (VSC) between molecular vibrations and cavity photon modes has recently emerged as a promising tool for influencing chemical reactivities. Despite numerous experimental and theoretical efforts, the underlying mechanism of VSC effects remains elusive. In this study, we combine state-of-art quantum cavity vibrational self-consistent field/configuration interaction theory (cav-VSCF/VCI), quasi-classical trajectory method, along with the quantum-chemical CCSD(T)-level machine learning potential, to simulate the hydrogen bond dissociation dynamics of water dimer under VSC. We observe that manipulating the light-matter coupling strength and cavity frequencies can either inhibit or accelerate the dissociation rate. Furthermore, we discover that the cavity surprisingly modifies the vibrational dissociation channels, with a pathway involving both water fragments in their ground vibrational states becoming the major channel, which is a minor one when the water dimer is outside the cavity. We elucidate the mechanisms behind these effects by investigating the critical role of the optical cavity in modifying the intramolecular and intermolecular coupling patterns. While our work focuses on single water dimer system, it provides direct and statistically significant evidence of VSC effects on molecular reaction dynamics.

Mode specificity of water dissociating on Ni(100): An approximate full-dimensional quantum dynamics study.

The mode-specific dynamics for the dissociative chemisorption of H2O on rigid Ni(100) is investigated by approximate nine-dimensional (9D) quantum dynamics calculations. The vibrational state-specific 9D dissociation probabilities are obtained by site-averaging the site-specific seven-dimensional results based on an accurate full-dimensional potential energy surface newly developed by neural network fitting to density functional theory energy points with the revised version of the Perdew, Burke, and Ernzerhof functional. The mode specificity of H2O/Ni(100) is very different from that of H2O/Ni(111) or H2O/Cu(111) whose reactivity enhancement by vibrational excitations is quite efficient. For H2O/Ni(100), it is found that the excitation in the symmetric stretching mode is more efficacious than increasing the translational energy in promoting the reaction, while the excitations in the asymmetric stretching mode and bending mode are less efficacious than the translational energy at low collision energies. These interesting observations can be attributed to the near central-barrier reaction for H2O/Ni(100), as well as large discrepancies between the site-specific mode specificities at different impact sites. The mode-specific dynamics obtained in this study is different from that obtained with the reaction path Hamiltonian approach, indicating the importance of full-dimensional quantum dynamics for gas-surface reactions.

Efficient N2 fixation in air enabled by mechanical-energy-driven triboelectric plasma jet

N2 fixation driven by mechanical energy is a promising strategy for production of nitrogen-enriched compounds. However, the activity of mechanical-energy-driven N2 fixation is very low. Herein, a mechanical-energy-driven triboelectric plasma jet was constructed to achieve N2 fixation in air at room temperature and atmospheric pressure. Under optimal conditions, the NOX production rate of 4.82 μmol h−1 is 23-fold better than the previous record using a triboelectric nanogenerator. The electrical to chemical energy conversion efficiency and energy cost for NOX production are 4.92% and 1.76 MJ mol−1 N−1, respectively, and the energy cost is the best result in the reported plasma N2 fixation reaction at room temperature and atmospheric pressure. Because of the lower average energy of electrons in the triboelectric plasma jet, the vibrational excitation dissociation process with low energy barriers is the major mechanism for N2 fixation. This study provides an effective strategy for N2 fixation using mechanical energy.

Two-Temperature Modeling of Nonequilibrium Relaxation and Dissociation in Shock-Heated Oxygen

Two-temperature models for coupled vibrational relaxation and dissociation in shock-heated oxygen are assessed using low-uncertainty measured data from reflected shock tube experiments. A computationally efficient multistep technique is developed to model the unsteady dynamics of shock reflection in a relaxing and dissociating gas. The developed technique is then benchmarked through comparison with unsteady computational fluid dynamic simulations. Results from the benchmarking effort demonstrate that the adopted multistep modeling procedure accurately captures the dominant gas dynamic effects influencing the state of the test gas at the measurement location. A parametric study is then performed to assess several combinations of possible two-temperature modeling approaches for nonequilibrium oxygen dissociation. The current assessment demonstrates that the widely adopted Park model is inconsistent with the measured data, while the recently developed modified Marrone and Treanor (MMT) model demonstrates promising agreement with the data. The results of the present study clearly indicate that the MMT model is more appropriate for two-temperature modeling of nonequilibrium oxygen dissociation than the legacy Park model. Patterns in the parametric comparison also suggest that the approximate treatment of non-Boltzmann vibrational state distributions within the MMT model may require improvement.

Application of Reflected Shock Wave Configuration to Validate Nonequilibrium Models of Reacting Air

The direct simulation Monte Carlo (DSMC) method is used to model transient thermal and chemical relaxation behind reflected shock waves in oxygen–argon and air mixtures under conditions reproducing earlier shock-tube experiments. Two vibration–translation and three popular DSMC chemical reaction models are tested. Where possible, model parameters are adjusted to match equilibrium and nonequilibrium relaxation times and reaction rates. A number of factors that impact relaxation and reaction model validation are examined, including gas–surface interactions, time-varying freestream properties, location of the observation point, electronic excitation, and nonequilibrium populations of vibrational states probed in the experiments. Comparison of numerical and experimental results has demonstrated that the reflected shock configuration is a platform very convenient for validation and analysis of high-temperature chemical reaction models. Computations have shown that the Bias reaction model is superior to the total collision energy and quantum kinetic models, providing reasonable agreement with measured absorbance time histories and vibrational temperatures in oxygen–argon mixtures and pure . There are some modeling-versus-experiment differences observed for air that may warrant additional studies focused on Zeldovich reaction rates and oxygen–nitrogen vibrational excitation and nonequilibrium dissociation rate.

Vibrational relaxation and triggering of the non-equilibrium vibrational decomposition of CO2 in gas discharges

Non-equilibrium vibrational dissociation CO2 → CO + O at translational–rotational temperatures T ⩽ 1200 K is investigated with semi-empiric and computational models. The governing parameter has been introduced, where Q is the specific volumetric power coupled into vibrational states and n 0 is the initial number density of CO2. It has been shown that the non-equilibrium vibrational process can only be triggered when exceeds some critical value determined by the speed of vibrational relaxation. Simple semi-empiric calculations are backed by the state-to-state simulations of the CO2 vibrational kinetics in two-modes approximation performed for conditions of microwave sustained gas discharges. The vibrational kinetics model is benchmarked against the experimental vibrational relaxation times as well as the shock tube data on the rate of the process CO2 + M → CO + O + M for M = Ar and literature data for M = CO2. At T = 300 K the estimated W m3 or 35 W (m− 3 Pa− 2) (p is the gas pressure). is found to always increase with increased T.

Plasmonic Photocatalysis with Nonthermalized Hot Carriers

Hot carriers generated by plasmonic damping have been suggested to promote photocatalysis, yet it remains unclear how the nonthermalized hot carriers dynamically activate and promote the energy transfer processes. Here, we present an Anderson-Newns model to describe the vibrational excitation and bond dissociation induced by plasmonic hot carriers. The nonthermal distribution of the hot carriers generated by plasmon damping is accounted for on equal footing with thermal carriers at a given temperature in the electron-molecule scattering. We found that the nonthermal electrons in the high energy region can, albeit in much smaller populations, provide an efficient and dominant channel for photodissociation especially in the low-temperature and quantum plasmon regime. Our model captures the wavelength dependence and reproduces the enhancement factors observed by experiments for oxygen dissociation on silver nanoparticles. It also paves a way to harvesting nonthermal plasmonic energy for photocatalysis in the quantum regime.

Electron Impact Excitation of Ions from Organics on Singly Protonated Peptides with and without Post-Translational Modifications.

We report on the dissociation of singly protonated peptides by electrons using electron-activated dissociation (EAD), which comprises electron impact excitation of ions from organics (EIEIO), electronic-excitation dissociation (EED), and electron ionization dissociation (EIoD). Various singly protonated peptides were dissociated using a recently reported fast EAD device. The dissociation can be induced through two pathways: (i) vibrational dissociation similar to collision-activated dissociation (CAD, or collision-induced dissociation, CID) by relaxation from a molecular electronic excited state to high vibrational states; and (ii) radical-induced dissociation where molecular electronic excitation is followed by homolytic cleavage. EAD is complementary to CAD as additional molecular information can be obtained; e.g., fragile PTM moieties, such as glycosylation and sulfation, can be localized. Simultaneously, the energetic production of radical z• fragments enables Leu and Ile discrimination, like in a hot ECD process. Using the fast EAD device, LC-EIEIO-time-of-flight mass spectrometry was applied to a tryptic monoclonal antibody digest containing short singly protonated peptides.

Triboelectric Plasma CO2 Reduction Reaching a Mechanical Energy Conversion Efficiency of 2.3.

Mechanical energy‐induced CO2 reduction is a promising strategy for reducing greenhouse gas emissions and simultaneously harvesting mechanical energy. Unfortunately, the low energy conversion efficiency is still an open challenge. Here, multiple‐pulse, flow‐type triboelectric plasma with dual functions of harvesting mechanical energy and driving chemical reactions is introduced to efficiently reduce CO2. CO selectivity of 92.4% is achieved under normal temperature and pressure, and the CO and O2 evolution rates reach 12.4 and 6.7 µmol h−1, respectively. The maximum energy conversion efficiencies of 2.3% from mechanical to chemical energy and 31.9% from electrical to chemical energy are reached. The low average electron energy in triboelectric plasma and vibrational excitation dissociation of CO2 with low barrier is revealed by optical emission spectra and plasma simulations, which enable the high energy conversion efficiency. The approach of triboelectric plasma reduction reported here provides a promising strategy for efficient utilization of renewable and dispersed mechanical energy.

In situ quantification of NO synthesis in a warm air glow discharge by WMS-based Mid-IR QCL absorption spectroscopy

Nitric oxide (NO) is one of the most crucial products in the plasma-based nitrogen fixation process. In this work, in situ measurements were performed for quantifying the NO synthesis spatially in a warm air glow discharge, through the method of Mid-infrared quantum cascade laser absorption spectroscopy (QCL-AS). Two ro-vibrational transitions at 1900.076 cm−1 and 1900.517 cm−1 of the ground-state NO(X) were probed sensitively by the help of the wavelength modulation spectroscopy (WMS) approach to increase the signal/noise (S/N) level. The results show a decline trend of NO synthesis rate along the discharge channel from the cathode to the anode. However, from the point of energy efficiency, the cathode region is of significantly low energy efficiency of NO production. Severe disproportionality was found for the high energy consumption but low NO production in the region of cathode area, compared to that in the positive column zone. Further analysis demonstrates the high energy cost of NO production in the cathode region, is ascribed to the extremely high reduced electric field therein not selectively preferable for the processes of vibrational excitation or dissociation of N2 and O2 molecules. This drags down the overall energy efficiency of NO synthesis by this typical warm air glow discharge, particularly for the ones with short electrode gaps. Limitations of further improving the energy cost of NO synthesis by variations of the discharge operation conditions, such as discharge current or airflow rate, imply other effective manners able to tune the energy delivery selectively to the NO formation process, are sorely needed.

Fully differential investigation of two-center interference in dissociative capture in p+H2 collisions

We have measured and calculated fully differential cross sections for vibrational dissociation following capture in 75-keV $p+{\mathrm{H}}_{2}$ collisions. For a molecular orientation perpendicular to the projectile beam axis and parallel to the transverse momentum transfer we observe a pronounced interference structure. The positions of the interference extrema suggest that the interference term is afflicted with a phase shift which depends on the projectile scattering angle. However, no significant dependence on the kinetic-energy release was observed. Considerable discrepancies between our calculations and experimental data were found.

Influence of Vibration–Dissociation Coupling and Number of Reactions in Hypersonic Nonequilibrium Flows

Abstract During the atmospheric cruising of a hypersonic vehicle, the thermal and chemical nonequilibrium effects characterize the flow field within the shock layer. Therefore an understanding of nonequilibrium flow is essential for the efficient design of a hypersonic vehicle. The present numerical study uses various canonical configurations to study the thermochemical nonequilibrium effects in hypersonic flows. The present study investigates the influence of the vibration–dissociation (V–D) coupling method and the number of reactions on shock standoff distance (SSD), vibrational relaxation process, and surface properties. A finite volume method-based solver using the open-source platform openfoam has been developed to analyze the thermochemical nonequilibrium effects in the hypersonic flow field. The current results show that thermal and chemical nonequilibrium flow assumptions significantly affect SSD, and hence these assumptions are necessary to study the cases with a higher degree of nonequilibrium. The number of reactions influences the vibrational relaxation of diatomic gases in the air. At the same time, the V–D coupling method used to calculate reaction rate constants has a negligible impact on the vibrational relaxation process. Moreover, the V–D coupling method and the number of reactions marginally affect surface pressure. However, in the case of surface heat flux, the 11 reaction model predicts higher peak values than the 17 reaction model.

Vibrational-Specific Model of Simultaneous and Relaxation Under Postshock Conditions

The present paper discusses the development of an aerothermochemistry model that can be efficiently coupled with fluid dynamics simulations and retains the accuracy of the quantum-mechanical and molecular dynamic principles from which it is derived. The present model allows simulations of thermochemical nonequilibrium in nitrogen without relying on the existence of a vibrational temperature. A series of verification tests suggest that the vibrational-specific master equation model is sufficiently accurate and computationally efficient when applied to the modeling of dissociating nitrogen flows under adiabatic conditions. Toward this end, a kinetic dataset describing vibrational energy transfer and dissociation in and collisions is obtained on a high-fidelity potential energy surface, and curve fit coefficients are reported. Validation against shock-tube data indicates that the inclusion of electronically excited and ionic species is potentially required when computing and comparing relaxation parameters with the experimental data derived from the radiative signatures of nitrogen shock flows.

Transfer learned potential energy surfaces: accurate anharmonic vibrational dynamics and dissociation energies for the formic acid monomer and dimer.

The vibrational dynamics of the formic acid monomer (FAM) and dimer (FAD) is investigated from machine-learned potential energy surfaces at the MP2 (PESMP2) and transfer-learned (PESTL) to the CCSD(T) levels of theory. The normal mode (MAEs of 17.6 and 25.1 cm−1) and second order vibrational perturbation theory (VPT2, MAEs of 6.7 and 17.1 cm−1) frequencies from PESTL for all modes below 2000 cm−1 for FAM and FAD agree favourably with experiment. For the OH stretch mode the experimental frequencies are overestimated by more than 150 cm−1 for both FAM and FAD from normal mode calculations. Conversely, VPT2 calculations on PESTL for FAM reproduce the experimental OH frequency to within 22 cm−1. For FAD the VPT2 calculations find the high-frequency OH stretch at 3011 cm−1, compared with an experimentally reported, broad (∼100 cm−1) absorption band with center frequency estimated at ∼3050 cm−1. In agreement with earlier reports, MD simulations at higher temperature shift the position of the OH-stretch in FAM to the red, consistent with improved sampling of the anharmonic regions of the PES. However, for FAD the OH-stretch shifts to the blue and for temperatures higher than 1000 K the dimer partly or fully dissociates using PESTL. Including zero-point energy corrections from diffusion Monte Carlo simulations for FAM and FAD and corrections due to basis set superposition and completeness errors yields a dissociation energy of D0 = −14.23 ± 0.08 kcal mol−1 compared with an experimentally determined value of −14.22 ± 0.12 kcal mol−1.

Kinetic and Continuum Modeling of High-Temperature Oxygen and Nitrogen Binary Mixtures

The present paper provides a comprehensive comparative analysis of thermochemistry models of various fidelity levels developed in leading research groups around the world. Fully kinetic, hybrid kinetic–continuum, and fully continuum approaches are applied to analyze parameters of hypersonic flows starting from the revision of single-temperature rate constants up to the application in 1-D postshock conditions. Comparison of state-specific and two-temperature approaches shows there are very significant and often qualitative differences in the time-dependent nonequilibrium reaction rates and their ratio to the corresponding single-temperature rates. A major impact of the vibration–dissociation coupling on the temporal relaxation of gas properties is shown. For instance, the legacy Park’s model has a strongly nonlinear behavior of nonequilibrium reaction rate with vibrational temperature, while a nearly linear shape exists for all state-specific approaches. Analysis of vibrational level populations in the nonequilibrium region shows a profound impact of the numerical approach and the model on the population ratios, and thus vibrational temperatures inferred from such ratios. The difference in the ultraviolet absorption coefficients, calculated by a temperature-based spectral code using vibrational populations from state-specific and kinetic approaches, is found to exceed an order of magnitude.

Numerical Comparison of Transpiration Cooling Designs Using Real Gas Effects and Approximate Gas Model

In the severe high-temperature environment caused by aerodynamic heating, the vibrational excitation, dissociation and ionization of gas may successively occur, which are known as real gas effects. Under the real gas effects, the thermodynamic properties of gas vary drastically and significantly influence the performances of the active thermal protection system of hypersonic vehicles, especially in the case with coolant outflow, for example transpiration cooling. This paper numerically investigates the transpiration cooling performance with the consideration of the interaction between coolant outflow and hypersonic flow under the real gas effects. The mathematical models and coupled numerical strategy are firstly validated by experimental data, then the influences of real gas effects on the transpiration cooling of a wedged leading edge (WLE) are studied under a flight Mach number range from 8 to 12 and a flight height of 40 km. The analysis and discussions of the numerical results reveal some important phenomena and demonstrate the need to consider real gas effects.