Effects Of Vibrational Relaxation Research Articles

Direct Numerical Simulation of Nonequilibrium Effects on Decaying Isotropic Turbulence

The Direct Numerical Simulation of decaying isotropic turbulence is performed with nonequilibrium effects. Oxygen is used as the test gas in the simulations. The individual and combined effects of vibrational relaxation and chemical reactions are studied by considering four different simulation cases. The time evolution of mean thermodynamic properties and turbulent statistics is analyzed. Budgets for turbulent kinetic energy and mean internal energy are computed to explain the dynamics of energy exchange associated with different nonequilibrium effects. The time evolution of enstrophy and turbulent length scales is also studied. The dissociation reaction is found to significantly affect the evolution of mean thermodynamic quantities as well as turbulence. Vibrational relaxation is found to significantly affect the flow in the presence of chemical reactions. The effects of changing the initial conditions on the resulting flowfield are analyzed.

Thermochemical non-equilibrium flow characteristics of high Mach number inlet in a wide operation range

The high-temperature non-equilibrium effect is a novel and significant issue in the flows over a high Mach number (above Mach 8) air-breathing vehicle. Thus, this study attempts to investigate the high-temperature non-equilibrium flows of a curved compression two-dimensional scramjet inlet at Mach 8 to 12 utilizing the two-dimensional non-equilibrium RANS calculations. Notably, the thermochemical non-equilibrium gas model can predict the actual high-temperature flows, and the numerical results of the other four thermochemical gas models are only used for comparative analysis. Firstly, the thermochemical non-equilibrium flow fields and work performance of the inlet at Mach 8 to 12 are analyzed. Then, the influences of high-temperature non-equilibrium effects on the starting characteristics of the inlet are investigated. The results reveal that a large separation bubble caused by the cowl shock/lower wall boundary layer interaction appears upstream of the shoulder, at Mach 8. The separation zone size is smaller, and its location is closer to the downstream area while the thermal process changes from frozen to non-equilibrium and then to equilibrium. With the increase of inflow Mach number, the thermochemical non-equilibrium effects in the whole inlet flow field gradually strengthen, so their influences on the overall work performance of the high Mach number inlet are more obvious. The vibrational relaxation or thermal non-equilibrium effects can yield more visible influences on the inlet performance than the chemical non-equilibrium reactions. The inlet in the thermochemical non-equilibrium flow can restart more easily than that in the thermochemical frozen flow. This work should provide a basis for the design and starting ability prediction of the high Mach number inlet in the wide operation range.

Vibrational response functions for multidimensional electronic spectroscopy in the adiabatic regime: A coherent-state approach.

Multi-dimensional spectroscopy represents a particularly insightful tool for investigating the interplay of nuclear and electronic dynamics, which plays an important role in a number of photophysical processes and photochemical reactions. Here, we present a coherent state representation of the vibronic dynamics and of the resulting response functions for the widely used linearly displaced harmonic oscillator model. Analytical expressions are initially derived for the case of third-order response functions in an N-level system, with ground state initialization of the oscillator (zero-temperature limit). The results are then generalized to the case of Mth order response functions, with arbitrary M. The formal derivation is translated into a simple recipe, whereby the explicit analytical expressions of the response functions can be derived directly from the Feynman diagrams. We further generalize to the whole set of initial coherent states, which form an overcomplete basis. This allows one, in principle, to derive the dependence of the response functions on arbitrary initial states of the vibrational modes and is here applied to the case of thermal states. Finally, a non-Hermitian Hamiltonian approach is used to include in the above expressions the effect of vibrational relaxation.

Effects of Vibrational Relaxation on Sonic Boom Waveforms in a Stratified Atmosphere

This paper describes the effects of vibrational relaxation on sonic boom waveforms in a stratified atmosphere. Full-field direct simulations with vibrational nonequilibrium are performed over uniform and stratified atmospheres, extending from a supersonic body to the ground level. The governing equations are the three-dimensional Euler equations, combined with the conservation equations for vibrational energies of and . The translational–vibrational energy exchange is evaluated using the Landau–Teller rate model. The full-field simulations reproduce the Drop test for Simplified Evaluation of Non-symmetrically Distributed sonic boom (D-SEND) #1, by the Japan Aerospace Exploration Agency. The computational results show that weak shock waves at a moderate relaxation time are dispersed, and the rise time occurs. A wave structure in one relaxing mode is classified according to the relaxation time. The effects of two relaxing modes are comparable to those obtained from a superposition of each mode. For a stratified atmosphere, the effects of vibrational relaxation at each altitude are similar to those for uniform atmospheres at the same altitudes. However, a wave structure does not monotonically change toward the ground, unlike the case of uniform atmosphere. Such a complex transition mechanism of wave structure in real atmospheric environments is clarified.

Hydrogen Bonding Effects on Vibrational Dynamics and Photochemistry in Selected Binary Molecular Complexes

This brief review presents an overview and analysis of the experimental studies performed recently in the laboratory of the authors demonstrating the role of hydrogen bonding as a promotional factor for intermolecular vibrational energy relaxation, and as a driving force for the occurrences of specific reaction channels in binary molecular complexes. Both vibrational and electronic spectroscopic methods have been used, and measurements have been performed in the supersonic jet expansion, in cold inert gas matrixes and also in suitable liquids at room temperature. Pure and mixed dimers of two types of aromatic chromophores, 7-azaindole and phenol, have been used as the model molecular systems, which involve O–H···O, O–H···N and N–H···N types of hydrogen bonds. The promotional effect of vibrational relaxation has been demonstrated showing large broadening of either of the infrared X–H stretching fundamentals of the donor groups in the hydrogen-bonded X–H···Y network of the binary complexes or broadening of the fluorescence spectra upon excitations to single vibronic levels of the isolated complexes in the gas phase and making comparisons with the corresponding spectra of the monomers. A similar approach has been adopted to demonstrate the promotion of photodissociation and charge transfer reactions in binary complexes by hydrogen bonding.

Stability of axisymmetric supersonic submerged microjet of nonequilibrium sulphur hexafluoride

The influence of vibrational excitation and relaxation of molecules of gaseous sulphur hexafluoride (SF6) on the gas-dynamic structure and stability of high-velocity microjets flowing out of axisymmetric convergent micronozzles is studied numerically. The present numerical simulations are performed with allowance for vibrational relaxation of molecules with the use of a two-temperature model of relaxing flows by means of solving unsteady Navier-Stokes equations in a 3D formulation. The effects of vibrational relaxation of gas molecules on the gas-dynamic structure, intensity of disturbances, and length of the supersonic core of microjets are considered.

Photoinduced anomalous Coulomb blockade and the role of triplet states in electron transport through an irradiated molecular transistor. II. Effects of electron-phonon coupling and vibrational relaxation

We generalize our previous theory [Fu et al., Nano Lett. 18, 5015–5023 (2018)] to investigate the influence of electron-phonon (e-p) coupling and vibrational relaxation on photoinduced anomalous Coulomb blockade, which originates from the triplet states and the energy level alignment. We derive the master equation for an irradiated molecular transistor and obtain the relevant rates via the Redfield theory instead of the phenomenological Fermi golden rule approach. To explore the interplay between e-p coupling and vibrational relaxation, we analyze the charge stability diagrams and the current-voltage characteristics (both gate voltage and source-drain bias voltage) under different e-p coupling strengths in two extreme limits of vibrational relaxation (equilibrated and unequilibrated phonon regimes). From the perspective of energy level alignment, we choose four representative situations and derive the analytical formulas of the photoinduced current in the equilibrated regime. The analytical solution reveals a new type of photocurrent due to e-p coupling that does not require the perfect energy level alignment between charged states and triplet states. In general, our study indicates that photoinduced current and anomalous Coulomb blockade caused by the triplet states are supposed to be experimentally observed.

Assessment of Hypersonic Flow Physics on Aero-Optics

In a hypersonic environment, the high kinetic energy of the oncoming flow causes the molecules in the flow to be thermally excited, leading to dissociation. In such a flow field, an aero-optical analysis that considers thermochemical nonequilibrium may be necessary to assist in sensor design. The present study assesses the optical properties of a nonequilibrium, real gas flow field around a hemisphere cylinder. The simulations are conducted at freestream Mach numbers of 11, 13, and 15. The optical distortions are quantified using optical path length and optical path difference. Optical distortion is also predicted using a perfect gas assumption, and these distortions are provided for comparison. The primary contributions of nonequilibrium parameters on optical distortion are identified. For all Mach numbers, optical path lengths are higher when the effects of dissociation and vibrational relaxation are included. The computational results show that oxygen dissociation is the dominant nonequilibrium flow phenomena affecting the optical distortion. However, as the freestream Mach number increases, atomic nitrogen begins to have an additional influence on optical aberrations.

Electron-Transfer Dynamics in a Zn-Porphyrin-Quinone Cyclophane: Effects of Solvent, Vibrational Relaxations, and Conical Intersections.

Rate constants for photochemical charge separation and recombination in a zinc-porphyrin-benzoquinone cyclophane are calculated by an approach that was developed recently to include effects of vibrational dephasing and relaxation and to reduce the dependence on freely adjustable parameters. The theory is extended to treat the rate of vibrational relaxation individually for each vibrational sublevel of the initial charge-transfer product. Quantum-mechanical/molecular-mechanical simulations of the reactions in iso-octane, toluene, dichloromethane, and acetonitrile suggest that charge separation occurs at conical intersections in the two more polar solvents, but at avoided crossings in the nonpolar solvents. In agreement with experimental measurements, however, the calculated rate constants for charge separation are similar in polar and nonpolar solvents. Charge recombination to the ground state is found to have electronic coupling factors smaller than that of charge separation and to be affected more strongly by interactions with the solvent.

Rayleigh-Brillouin scattering in SF6 in the kinetic regime

Rayleigh-Brillouin spectral profiles are measured with a laser-based scatterometry setup for a 90° scattering angle at a high signal-to-noise ratio (r.m.s. noise below 0.15% w.r.t. peak intensity) in sulfur-hexafluoride gas for pressures in the range 0.2–5bar and for a wavelength of λ=403.0nm. The high quality data are compared to a number of light scattering models in order to address the effects of rotational and vibrational relaxation. While the vibrational relaxation rate is so slow that vibration degrees of freedom remain frozen, rotations relax on time scales comparable to those of the density fluctuations. Therefore, the heat capacity, the thermal conductivity and the bulk viscosity are all frequency-dependent transport coefficients. This is relevant for the Tenti model that depends on the values chosen for these transport coefficients. This is not the case for the other two models considered: a kinetic model based on rough-sphere interactions, and a model based on fluctuating hydrodynamics. The deviations with the experiment are similar between the three different models, except for the hydrodynamic model at pressures p≲2bar. As all models are in line with the ideal gas law, we hypothesize the presence of real gas effects in the measured spectra.

Mutual effect of vibrational relaxation and chemical reactions in viscous multitemperature flows.

We study coupling of vibrational relaxation and chemical reactions in nonequilibrium viscous multitemperature flows. A general theoretical model is proposed on the basis of the Chapman-Enskog method modified for strongly nonequilibrium reacting flows; the model differs from models commonly used in computational fluid dynamics since it is able to capture additional cross-coupling terms arising in viscous flow due to compressibility and mutual influence of all nonequilibrium processes occurring in a mixture. The set of fluid dynamic equations is derived starting from the Boltzmann equation; the relaxation terms in these equations are described using the kinetic transport theory formalism. Reaction and relaxation rates depend on the distribution function and thus differ in the zero-order and first-order approximations of the Chapman-Enskog method. An algorithm for the calculation of multitemperature reaction and relaxation rates in both inviscid and viscous flows is proposed for the harmonic oscillator model. This algorithm is applied to estimate the mutual effect of vibrational relaxation and dissociation in binary mixtures of N(2) and N, and O(2) and O, under various nonequilibrium conditions. It is shown that modification of the Landau-Teller expression for the VT relaxation term works rather well in nitrogen, whereas it fails to predict correctly the relaxation rate in oxygen at high temperatures. In oxygen (in contrast to nitrogen), the first-order cross effects of dissociation and VT relaxation are found to be significant. A method for calculation of vibrational relaxation time based on the kinetic theory definition is suggested. Two-temperature dissociation rate coefficients are calculated in the zero- and first-order approximations and compared to other models.

Direct Dynamics Studies of a Binuclear Metal Complex in Solution: The Interplay Between Vibrational Relaxation, Coherence, and Solvent Effects.

By using a newly implemented QM/MM multiscale MD method to simulate the excited state dynamics of the Ir2(dimen)42+ (dimen = 1,8-diisocyano-p-menthane) complex, we not only report on results that support the two experimentally observed coherent dynamical modes in the molecule but also reveal a third mode, not distinguishable by spectroscopic methods. We directly follow the channels of energy dissipation to the solvent and report that the main cause for coherence decay is the initial wide range of configurations in the excited state population. We observe that the solvent can actually extend the coherence lifetime by blocking channels for intramolecular vibrational energy redistribution (IVR).

A Mixed Quantum-Classical Molecular Dynamics Study of anti-Tetrol and syn-Tetrol Dissolved in Liquid Chloroform II: Infrared Emission Spectra, Vibrational Excited-State Lifetimes, and Nonequilibrium Hydrogen-Bond Dynamics

The effect of vibrational excitation and relaxation of the hydroxyl stretch on the hydrogen-bond structure and dynamics of stereoselectively synthesized syn-tetrol and anti-tetrol dissolved in deuterated chloroform are investigated via a mixed quantum-classical molecular dynamics simulation. Emphasis is placed on the changes in hydrogen-bond structure upon photoexcitation and the nonequilibrium hydrogen-bond dynamics that follows the subsequent relaxation from the excited to the ground vibrational state. The propensity to form hydrogen bonds is shown to increase upon photoexcitation of the hydroxyl stretch, thereby leading to a sizable red-shift of the infrared emission spectra relative to the corresponding absorption spectra. The vibrational excited state lifetimes are calculated within the framework of Fermi's golden rule and the harmonic-Schofield quantum correction factor, and found to be sensitive reporters of the underlying hydrogen-bond structure. The energy released during the relaxation from the excited to the ground state is shown to break hydrogen bonds involving the relaxing hydroxyl. The spectral signature of this nonequilibrium relaxation process is analyzed in detail.

弱い衝撃波の分子振動緩和による減衰過程に関する数値解析(流体工学,流体機械)

Numerical analyses are performed for attenuation of weak shock waves generated by explosion of a unit mass of TNT. Viscous and heat conduction loss and molecular vibrational relaxation effects of oxygen and nitrogen molecules are considered. The effect of each diffusion term on the wave attenuation and the wave form change is investigated in detail. In addition, the characteristics of each attenuation effect are investigated by the technique of frequency analysis.

NeCl2 and ArCl2: Transition from Direct Vibrational Predissociation to Intramolecular Vibrational Relaxation and Electronic Nonadiabatic Effects

Pump-probe results are reported for NeCl(2) excited to the Cl(2) B state, undergoing vibrational predissociation, and then probed via E <-- B transitions. Intensities, lifetimes and product vibrational branching ratios are reported for 16 < or = v' < or = 19 Cl(2) stretching quanta. The intensity of the signal rapidly decreases above v' = 17. Detailed wave packet calculations of the vibrational predissociation dynamics are performed to determine if the experimental results can be explained by the onset of IVR dynamics. The calculations and the experiment are in close accord for low vibrational levels. For higher levels, some, but not all, of the loss of experimental signal can be attributed to IVR. To test whether electronic relaxation dynamics are important for NeCl(2) and ArCl(2), excited state potential surfaces that incorporate spin orbit coupling effects are calculated. These surfaces are then used in a wave packet calculation that includes both vibrational predissociation and electronic predissociation dynamics. The results show that electronic predissociation is important for ArCl(2) levels above v' = 12. For NeCl(2) the calculation suggests that the onset of electronic predissociation should occur for levels as low as v' = 13 but may not contribute markedly to the observed loss of signal above v' = 17. Suggestions are made for further studies of this puzzling problem.

Primary Reaction Dynamics of Proteorhodopsin Mutant D97N Observed by Femtosecond Infrared and Visible Spectroscopy

Femtosecond time-resolved spectroscopy in the visible and IR range was utilized to study the primary reaction dynamics of the proteorhodopsin (PR) D97N mutant in comparison with wild type PR at different pH values. The analysis of the data obtained in the mid-IR closely resembles the results for wild type PR. The observation of the first ground state intermediate K is initially obscured by a complex reaction scheme of vibrational relaxation and heating effects, but its spectral signature clearly emerges at long delay times. In the visible range, a biexponential decay of the excited state within 30 ps and the formation of the K photoproduct is observed. The decay time constants derived for the D97N mutant in D(2)O are slightly larger than in H(2)O due to H/D exchange. This kinetic isotope effect is even less pronounced than for wild type PR at pH 6. These results support the current notion of a pH dependent hydrogen bonding network in the retinal binding pocket of PR and a weaker interaction between the retinal Schiff base and the counter ion complex compared to bacteriorhodopsin.

Vibrational relaxation effects on rise times of weak shocks.

After I finished my Ph.D. on the “Application of Monte Carlo techniques in the analysis of laser induced microfission explosions,” I was unemployed and trying to find a job. Hank had a contract from the Air Force to investigate the generation and propagation of thunder. As part of this work, Hank had assigned an undergraduate student the task of adapting a finite wave propagation program developed by Anderson at the University of Texas to incorporate Hank's newly developed sound attenuation algorithm for the ANSI Standard S1‐26, “Sound absorption in the atmosphere.” The student had graduated without making much progress and Hank offered me a summer job to work on the problem. I managed to find the single error in the program and as a result, Hank and I wrote a paper [J. Acoust. Soc. Am.]. This was the start of my career in acoustics and a lifelong collaboration with Hank. We and several co‐authors published eight papers based on the study of the interaction of vibrational relaxation with finite wave distortion. This paper will present a summary of the findings of this line of research over a 24 year period.

Concerning the Determination of Conformational States in Polymers by Infrared Spectroscopy: A Study of Poly(methyl methacrylate)

Changes in the infrared spectra of poly(methyl methacrylate) as a function of temperature have been examined. The results presented here, together with a consideration of normal coordinate calculations reported in the literature, suggest that previous assignments of bands in the 1300−1050 cm−1 region of the spectrum to specific conformations of the ester pendant group are incorrect. Certain bands in this and other regions of the spectrum are nevertheless very sensitive to temperature. It is proposed that this sensitivity is a function of both conformational effects and vibrational relaxation effects. The latter are usually neglected in vibrational spectroscopic studies of polymers, and it is proposed that more attention be paid to changes in band shapes when considering changes in the spectra of polymers as a function of temperature.

Optimum Performance of Electron Beam Driven Magnetohydrodynamic Generators for Scramjet Inlet Control

This paper is devoted to the analysis and optimization of magnetohydrodynamic control of forebody flow compression and shock incidence in scramjet-powered vehicles that would fly at Mach 5-8. The short (about 30 cm long) magnetohydrodynamic region is created by placing a magnetic coil with B-field strength of several Tesla inside the forebody and ionizing the cold air flow by high-energy electron beams propagating along the magnetic field lines. The Faraday current flowing in the spanwise direction is presumed to be collected with electrodes that are mounted on sidewalls. The purpose of the magnetohydrodynamic device is to restore a shock-on-lip condition at Mach 8 for a vehicle geometry designed for Mach 5, while operating in self-powered mode (the magnetohydrodynamic generated electricity is enough to power the ionizing beams). Location and spatial dimensions of the magnetohydrodynamic region, beam-generated ionization profiles, magnetic field strength, and the magnet size are varied to maximize performance and reduce the magnet size. Two-dimensional inviscid steady-state flow equations are solved jointly with equations describing electron beam-induced ionization profiles, plasma kinetics, vibrational relaxation, and magnetohydrodynamic effects. The model predicts an interesting phenomenon of electric current reversal due to the nonuniformity of velocity, magnetic field, and conductivity in the magnetohydrodynamic region. Vibrational nonequilibrium effects are found to be substantial. Overall, the modeling shows that a considerable reduction in magnet size is possible with the optimum choice of parameters.

Multidimensional infrared spectroscopy of water. I. Vibrational dynamics in two-dimensional IR line shapes

In this and the following paper, we describe the ultrafast structural fluctuations and rearrangements of the hydrogen bonding network of water using two-dimensional (2D) infrared spectroscopy. 2D IR spectra covering all the relevant time scales of molecular dynamics of the hydrogen bonding network of water were studied for the OH stretching absorption of HOD in D2O. Time-dependent evolution of the 2D IR line shape serves as a spectroscopic observable that tracks how different hydrogen bonding environments interconvert while changes in spectral intensity result from vibrational relaxation and molecular reorientation of the OH dipole. For waiting times up to the vibrational lifetime of 700 fs, changes in the 2D line shape reflect the spectral evolution of OH oscillators induced by hydrogen bond dynamics. These dynamics, characterized through a set of 2D line shape analysis metrics, show a rapid 60 fs decay, an underdamped oscillation on a 130 fs time scale induced by hydrogen bond stretching, and a long time decay constant of 1.4 ps. 2D surfaces for waiting times larger than 700 fs are dominated by the effects of vibrational relaxation and the thermalization of this excess energy by the solvent bath. Our modeling based on fluctuations with Gaussian statistics is able to reproduce the changes in dispersed pump-probe and 2D IR spectra induced by these relaxation processes, but misses the asymmetry resulting from frequency-dependent spectral diffusion. The dynamical origin of this asymmetry is discussed in the companion paper.