Vibrational Dissociation Research Articles

Multiquantum Transitions in Oxygen and Nitrogen Molecules in Hypersonic Nonequilibrium Flows

A computational fluid dynamics analysis was performed with state-to-state kinetics of oxygen and nitrogen for assessing the influence of vibrational excitation and dissociation on hypersonic flowfields in the range of Mach 7–20. Flows around an axisymmetric forebody geometry were considered by using multiquantum vibrational–translation and dissociation rate coefficients from an ab initio database. The nonequilibrium vibrational energy distributions were modeled by the master kinetic equation. Output quantities of interest included shock standoff distance, translational and vibrational temperatures, and population fractions in the quantum energy states. Both a two-temperature model and a state-to-state kinetics model were used to compare with existing experimental data. The objective of the study was to understand the energy exchange process of multiquantum transitions in the shock layer of a blunt body flow and to assess the maximum number of multiquantum jumps required in describing the nonequilibrium behavior. Multiquantum transitions caused an increase in population in upper energy states due to more efficient energy exchanges. The greater dissociation for the higher multiquantum jumps resulted in reduced shock standoff distance. At the higher Mach numbers, population depletion from the upper energy states demonstrated preferential dissociation. At high temperatures, results indicated that multiquantum transitions had a large influence on the macroscopic parameters of the gas flow behind the shock, especially on the temperature profiles as well as on vibrational population distributions in the upper energy states.

Numerical investigation of oxygen thermochemical nonequilibrium on high-enthalpy double-cone flows

Hypersonic thermochemical nonequilibrium flows over a double-cone configuration are numerically investigated. Simulations with oxygen as the test gas are performed using different coupling models of vibrational excitation and dissociation, including a conventional two-temperature model as the baseline and an improved model established on elementary kinetics and validated against existing shock tube experimental data. For the condition with the highest total enthalpy, the improved model predicts a larger separation region and greater peak heat flux with relative differences of 20.3% and 29.2%, respectively, compared with the baseline two-temperature model. The differences are attributed to inaccurate modeling of the vibration–dissociation coupling effects by the conventional two-temperature model, which overestimates the post-shock degree of dissociation and underestimates the post-shock temperature. The size of the separation bubble is therefore altered due to the change in its density. These findings may help to explain the large discrepancies found between numerical results and experimental data for high-enthalpy double-cone flows in hypersonic studies.

Mechanisms of Coupled Vibrational Relaxation and Dissociation in Carbon Dioxide.

A complete vibrational state-specific kinetic scheme describing dissociating carbon dioxide mixtures is proposed. CO2 symmetric, bending and asymmetric vibrations and dissociation-recombination are strongly coupled through intermode vibrational energy transfers. Comparative study of state-resolved rate coefficients is carried out; the effect of different transitions may vary considerably with temperature. A nonequilibrium 1-D boundary layer flow typical to hypersonic planetary entry is studied in the state-to-state approach. To assess the sensitivity of fluid-dynamic variables and heat transfer to various vibrational transitions and chemical reactions, corresponding processes are successively included to the kinetic scheme. It is shown that vibrational-translational (VT) transitions in the symmetric and asymmetric modes do not alter the flow and can be neglected whereas the VT2 exchange in the bending mode is the main channel of vibrational relaxation. Intermode vibrational exchanges affect the flow implicitly, through energy redistribution enhancing VT relaxation; the dominating role belongs to near-resonant transitions between symmetric and bending modes as well as between CO molecules and CO2 asymmetric mode. Strong coupling between VT2 relaxation and chemical reactions is emphasized. While vibrational distributions and average vibrational energy show strong dependence on the kinetic scheme, the heat flux is more sensitive to chemical reactions.

Vibrational energy transfer and dissociation in O2-N2 collisions at hyperthermal temperatures.

Simulation of vibrational energy transfer and dissociation in O2-N2 collisions is conducted using the quasi-classical trajectory method on an ab initio potential energy surface. Vibrationally resolved rate coefficients are obtained in a high-temperature region between 8000 and 20 000 K by means of the cost-efficient classical trajectory propagation method. A system of master equations is constructed using the new dataset in order to simulate thermal and chemical nonequilibrium observed in shock flows. The O2 relaxation time derived from a solution of the master equations is in good agreement with the Millikan and White correlation at lower temperatures with an increasing discrepancy toward the translational temperature of 20 000 K. At the same time, the N2 master equation relaxation time is similar to that derived under the assumption of a two-state system. The effect of vibrational-vibrational energy transfer appears to be crucial for N2 relaxation and dissociation. Thermal equilibrium and quasi-steady state dissociation rate coefficients in O2-N2 heat bath are reported.

Photodissociation of the Cyclopentadienyl Radical at 248 nm.

The photodissociation of jet-cooled cyclopentadienyl radicals, c-C5H5, at 248 nm was studied using photofragment translational spectroscopy. Two dissociation channels were observed: H + C5H4 and C3H3 + C2H2. The C5H4 fragment was identified as ethynylallene by its ionization energy. The translational energy distribution determined for each channel suggests that both dissociation mechanisms occur via internal conversion to the ground electronic state followed by intramolecular vibrational redistribution and dissociation. The experimental branching ratio and RRKM (Rice-Ramsperger-Kassel-Marcus) calculations favor the formation of C3H3 + C2H2 over the H-atom loss channel. The RRKM calculations also support the observation of ethynylallene as the dominant C5H4 product isomer.

Cross Sections for Electron Collisions with NF3

Cross section data are compiled from the literature for electron collisions with nitrogen trifluoride (NF3) molecules. Cross sections are collected and reviewed for total scattering, elastic scattering, momentum transfer, excitations of rotational and vibrational states, dissociation, ionization, and dissociative attachment. For each of these processes, the recommended values of the cross sections are presented. The literature has been surveyed until end of 2016.

Fully Differential Study of Capture with Vibrational Dissociation in p+H_{2} Collisions.

We have measured fully differential cross sections for electron capture in 75keV p+H_{2} collisions with subsequent dissociation of the intermediate molecular H_{2}^{+} ion by vibrational excitation using different projectile coherence lengths. Data were obtained for two molecular orientations as a function of projectile scattering angle. Two types of interference, single- and molecular two-center interference, were identified. The two-center interference structure is phase shifted by π compared to what we expected. Furthermore, the presence of projectile coherence effects could be reconfirmed.

Stark-induced adiabatic Raman ladder for preparing highly vibrationally excited quantum states of molecular hydrogen

We present a multi-color ladder excitation scheme that exploits Stark-induced adiabatic Raman passage to selectively populate a highly excited vibrational level of a molecule. We suggest that this multi-color coherent ladder excitation provides a practical way of accessing levels near the vibrational dissociation limit as well as the dissociative continuum, which would allow the generation of an entangled pair of fragments with near-zero relative kinetic energy. Specifically, we consider four- and six-photon coherent excitation of molecular hydrogen to high vibrational levels via intermediate vibrational levels, which are pairwise coupled by two-photon resonant interaction. Using a sequence of three partially overlapping, single-mode, nanosecond laser pulses we show that the sixth vibrational level of H2, which is too weakly coupled to be easily accessed by direct two-photon Raman excitation from the ground vibrational level, can be efficiently populated without leaving any population stranded in the intermediate level. Furthermore, we show that the fourteenth vibrational level of H2, which is the highest vibrational level in the ground electronic state with a binding energy of 22 meV, can be efficiently and selectively populated using a sequence of four pulses. The present technique offers the unique possibility of preparing entangled quantum states of H atoms without resorting to an ultracold system.

Thermal modulation in Zeno and anti-Zeno effects on quantum measurement

ABSTRACTWe have proposed the mathematical formulations of quantum Zeno and anti-Zeno effects at non-zero temperature on vibrational population transfer and dissociation of a diatomic system. The proposed formulation has been tested for diatomic systems HBr+ and HI using time-dependent Fourier grid Hamiltonian method. The effect of temperature on the dynamics is incorporated through the population distribution at different vibrational states following Boltzmann distribution formula. It has been found that with increase in temperature Zeno effect increases and anti-Zeno effect decreases in case of survival probability on ground vibrational state. In case of dissociation, with increase in temperature Zeno effect increases while anti-Zeno effect remains almost same.

State-specific simulation of oxygen vibrational excitation and dissociation behind a normal shock

A complete vibration–vibration–translation (V–V–T) bound–bound transitions rates database for O2–O2 collisions is generated using the forced harmonic oscillator (FHO) model with temperature range between 1000 and 20,000K. The postshock process of oxygen vibrational excitation and dissociation is simulated by solving the system of master equations coupled with the one-dimensional compressible flow equations. The results agree well with existing shock tube data. The state-specific calculations show that inclusion of pure vibration–vibration (V–V) processes could reduce the discrepancy with experimental data and the quasi-steady-state (QSS) approximation is valid when dissociation starts to dominate.

Assessment of vibration–dissociation coupling models for hypersonic nonequilibrium simulations

The fidelity of three widely-used two-temperature vibration–dissociation coupling models, including the Park model (1988), the Macheret–Fridman model (1994), and the coupled vibration–dissociation–vibration (CVDV) model (1963), is numerically investigated via a comparison with state-specific results and existing shock tube data for oxygen flows. Under the hypothetical condition where a Boltzmann distribution corresponding to a vibrational temperature is assumed, it is found that the CVDV model with a proper parameter is capable of providing the most accurate results, whereas the Park and Macheret–Fridman models present similar prediction accuracy for which the nonequilibrium dissociation rate coefficients could be as much as three orders of magnitude lower than state-specific values. However, for actual postshock flows with intensive vibrational excitation and dissociation processes, the CVDV model still shows significant discrepancies relative to state-specific results and experimental data, presenting a much lower vibrational temperature and a higher degree of dissociation. The essential cause for the disagreement lies in the existence of non-Boltzmann distributions observed when the incubation period is over and dissociation starts to dominate. Recommendations on further modification of two-temperature vibration–dissociation coupling models are presented.

Cross Sections for Electron Collisions with Acetylene

Cross section data are compiled from the literature for electron collisions with the acetylene (HCCH) molecule. Cross sections are collected and reviewed for total scattering, elastic scattering, momentum transfer, excitations of rotational and vibrational states, dissociation, ionization, and dissociative attachment. The data derived from swarm experiments are also considered. For each of these processes, the recommended values of the cross sections are presented. The literature has been surveyed through early 2016.

Vibrational-Chemical Coupling in mixtures CO2/CO/O and CO2/CO/O2/O/C

The paper deals with modeling of vibration-chemical coupling in mixtures containing CO2 molecules. The temporal vibrational and chemical relaxation in the five-component and three-component space-homogeneous mixtures CO2/CO/O2/C/O and CO2/CO/O is studied taking into account vibrational energy transitions, dissociation, recombination and exchange chemical reactions. The vibrational-chemical coupling in considered mixtures is studied on the basis of three-temperature, two-temperature and one-temperature vibrational CO2 distributions. Governing equations for macroscopic mixture parameters in the three-temperature, two-temperature and one-temperature approximations are solved numerically for different initial conditions. Comparison of the results obtained in three approaches showed the in uence of non-equilibrium vibrational distributions on chemical reaction rates and on the temporal variation of the gas temperature, mixture composition and vibrational temperatures in two considered mixtures. The role of exchange chemical reactions in relaxation processes is also discussed in the paper.

Aerothermochemical Nonequilibrium Modeling for Oxygen Flows

The results from a set of vibrational nonequilibrium models with a range of fidelity are compared to the recent experimental data for several postnormal shock test cases. The present work focuses solely on oxygen flows with an emphasis on implementing a new set of accurate state-specific rate coefficients for collisions. The two-temperature model is presented as the computationally efficient, lower-fidelity approach in this work. The two-temperature model is driven by the relaxation parameters based on the Millikan–White empirical equation as well as on the parameters resulting from a master equation simulation that employs the database of state-resolved rate coefficients. The full state-to-state master equation approach is presented as the higher-fidelity modeling approach. The system uses recently available results of trajectory simulations for state-specific transition rate coefficients The system uses transition rates from the forced harmonic oscillator model. The test case comparison shows that the state-resolved modeling approach is more suitable for describing the vibrational temperature and chemically nonequilibrium zone behind the shock wave. It is shown that the capability of the state-resolved model to capture non-Boltzmann distribution is critical for accurately modeling the vibrational relaxation and dissociation phase.

Study the Addition of Lab-Made Gas Mixing Unit with Varying of Inter-Electrode Distances on the Characterization of DC Magnetron Sputtering System

In this paper, the effect of lab-made gas mixing unit and inter-electrode distance on, current-voltage characteristics of argon and argon-nitrogen plasma, Paschen’s curve and variation of discharge current with working pressure are presented. The results showed the mixing of nitrogen gas with argon at ratio Ar:N2 of 1:1, an increase in the discharge current was observed, while at ratio Ar:N2 of 2.5:1 which increases the discharge current at the distance of 7.5 cm. Paschen’s curves have been showed, it is clear that the breakdown voltage VB increases as nitrogen gas percentage 50%. This effect is due to, the additional energy loss channel nitrogen gas such as vibration, rotational and molecular dissociation. The discharge current was varied by changing the working gas pressure. With increasing of working gas pressure, which is attributed to more molecules which are available for the electrons to collide with and to generate a new free electron and a positive ion and so making the DC voltage more negative. The discharge current is decreasing with increasing gas pressure above 3 mbar. The relation was plotted at certain discharge voltage of 600 V and three certain distances between the electrodes of (2.5, 4.5 and 7.5 cm).

DSMC study of oxygen shockwaves based on high-fidelity vibrational relaxation and dissociation models

This work evaluates high-fidelity vibrational-translational (VT) energy relaxation and dissociation models for pure O2 normal shockwave simulations with the direct simulation Monte Carlo (DSMC) method. The O2–O collisions are described using ab initio state-specific relaxation and dissociation models. The Macheret-Fridman (MF) dissociation model is adapted to the DSMC framework by modifying the standard implementation of the total collision energy (TCE) model. The O2–O2 dissociation is modeled with this TCE+MF approach, which is calibrated with O2–O ab initio data and experimental equilibrium dissociation rates. The O2–O2 vibrational relaxation is modeled via the Larsen-Borgnakke model, calibrated to experimental VT rates. All the present results are compared to experimental data and previous calculations available in the literature. It is found that, in general, the ab initio dissociation model is better than the TCE model at matching the shock experiments. Therefore, when available, efficient ab initio models are preferred over phenomenological models. We also show that the proposed TCE + MF formulation can be used to improve the standard TCE model results when ab initio data are not available or limited.

Theoretical study of Zeno and anti-Zeno effects on photodissociation dynamics: A model approach

Zeno and anti-Zeno effects in the evolution of the multi-photonic dissociation dynamics of the diatomic molecule HBr[Formula: see text] owing to repeated measurements demand if the system in the initial state have been studied. The effects have been calculated numerically for the case of vibrational population transfer and dissociation dynamics of HBr[Formula: see text] taking it as a model. We use time-dependent Fourier grid Hamiltonian (TDFGH) method as a mathematical tool in presence of intense radiation field as perturbation. The effects have been explored through a probable mechanism of population transfer from the ground vibrational state to the different upper vibrational states which ultimately go to the dissociation continuum. The results show significant differences in the mechanism of population transfer and the significant role of time interval of measurement ([Formula: see text] in Zeno and anti-Zeno effects. In case of survival probability of ground vibrational states, there is Zeno effect when the frequency of the laser to which the molecule is submitted is near the vibrational [Formula: see text] to [Formula: see text] resonance, while there is anti-Zeno effect if it is far from this resonance.

Dynamics of nitrogen dissociation from direct molecular simulation

Nitrogen dissociation is studied using direct molecular simulation with a new potential-energy surface. Both ${N}_{2}$ - ${N}_{2}$ and $N$ - ${N}_{2}$ processes are simulated as they concurrently take place in an evolving nonequilibrium gas system. The coupling between vibrational excitation and dissociation is quantified leading to several new insights.

A numerical study of non-equilibrium flows with different vibrational relaxation models

Comparative analysis of a widely used Landau–Teller formula for small deviations from thermal equilibrium and its generalized form, derived from the kinetic theory of gaseous, for an arbitrary deviation from the thermal equilibrium is performed by numerical simulation. Thermally non-equilibrium flows of carbon dioxide near a sharp-edged plate, pure nitrogen flows between two symmetrically located wedges, and the N2/N mixture flow with vibrational relaxation and dissociation over a cone have been considered. A comparison has been performed with the available experimental data.

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.