Adiabatic Channel Research Articles

Classical trajectory and statistical adiabatic channel study of the dynamics of capture and unimolecular bond fission. IV. Valence interactions between atoms and linear rotors

The addition of atoms to linear molecules forming linear or nonlinear adducts is treated using standardized valence potentials. The dynamics is analyzed with a combination of classical trajectory (CT) and statistical adiabatic channel (SACM) calculations. For classical adiabatic conditions, the two approaches coincide. The transition from adiabatic to nonadiabatic dynamics is investigated using CT calculations. The low-temperature adiabatic quantum range is studied by SACM. Thermal capture rate constants are represented in analytical form. Thermal rigidity factors are expressed in terms of molecular parameters such as the frequencies of transitional bending modes, the bond dissociation energy, the rotational constant of the linear fragment, and the ratio of the looseness and Morse parameters α/β of the potential-energy surface. The final rate expressions are of simple form suitable for direct practical applications.

Wall shear measurements inside corrugated channels using the electrochemical technique

We report on an investigation of adiabatic single-phase flow inside two-dimensional and three-dimensional channels, with a hydraulic diameter of 0.033 m. This work is a preliminary study to another one that will be carried out with a gas–liquid mixture. The flow structure is studied using the electrochemical technique. Circular single probes and rectangular sandwich probes are inserted flush to the wall. This equipment allows us to give an accurate description of the flow at the wall.

Some New Aspects of Product Distribution Functions for Three-Atom Unimolecular Reactions Performed in Beam Experiments

Energy distribution functions for the products in two classes of three-atom unimolecular reactions performed in beam experiments are derived which are much more realistic than the prior distribution functions (PDF) of Levine and Bernstein without being more complicated. In a first step, we deal with light-atom emission processes governed by short-range forces. Contrary to what is generally believed, we emphasize the applicability of statistical theory of energy distributions to such reactions in which the change in energy partitioning from critical configuration to products is weak if not negligible. In a second step, we treat within the framework of phase space theory (PST) the more usual case of processes governed by long-range forces for which, however, there exists no realistic distribution functions. Comparison with experimental results or with results of simulations and/or results obtained using PDF, PST, or the statistical adiabatic channel model (SACM) is done for the following processes: (i) C2H ...

Prediction of two-phase mixture density using artificial neural networks

In nuclear power plants, the density of boiling mixtures has a significant relevance due to its influence on the neutronic balance, the power distribution and the reactor dynamics. Since the determination of the two-phase mixture density on a purely analytical basis is in fact impractical in many situations of interest, heuristic relationships have been developed based on the parameters describing the two-phase system. However, the best or even a good structure for the correlation cannot be determined in advance, also considering that it is usually desired to represent the experimental data with the most compact equation. A possible alternative to empirical correlations is the use of artificial neural networks, which allow one to model complex systems without requiring the explicit formulation of the relationships existing among the variables. In this work, the neural network methodology was applied to predict the density data of two-phase mixtures up-flowing in adiabatic channels under different experimental conditions. The trained network predicts the density data with a root-mean-square error of 5.33%, being ∼93% of the data points predicted within 10%. When compared with those of two conventional well-proven correlations, i.e. the Zuber-Findlay and the CISE correlations, the neural network performances are significantly better. In spite of the good accuracy of the neural network predictions, the ‘black-box’ characteristic of the neural model does not allow an easy physical interpretation of the knowledge integrated in the network weights. Therefore, the neural network methodology has the advantage of not requiring a formal correlation structure and of giving very accurate results, but at the expense of a loss of model transparency.

Hyperspherical hierarchy of three-electron radial excitations

We investigate hierarchical representation of three-electron radial excitations, relying on the replacement of a sequence of ordered individual electron radial coordinates by a hierarchical sequence of hyperspherical radial variables. In the spirit of the hyperspherical method we argue that the latter variables allow adiabatic quasiseparability of the three-electron wave function. We examine this by approximately evaluating hyperspherical adiabatic potential curves and channel functions for the Li${(s}^{3})$ configuration. The hierarchical representation permits one to display manifold aspects of the triply excited states with a pair of radial quantum numbers.

The influence of local electronic character and nonadiabaticity in the photodissociation of nitric acid at 193 nm

The dissociation of nitric acid upon πnb,O→πNO2* excitation at 193 nm has been studied in a crossed laser-molecular beam apparatus. The primary reaction channels are OH+NO2 and O+HONO. We measure the branching ratio between these two competing processes and determine (OH+NO2)/(O+HONO)=0.50±0.05. Our experiments provide evidence of a minor O+HONO pathway, which we assign to O(3P) and HONO in its lowest triplet state. The dominant pathway correlates to O(1D)+HONO(X 1A′). The translational energy distributions reveal two distinct pathways for the OH+NO2 channel. One pathway produces stable NO2 fragments in the 1 2B2 electronic state. The second pathway produces unstable NO2 fragments which undergo secondary dissociation to NO+O. We examine the influence of nonadiabaticity along the OH+NO2 reaction coordinate in order to explain the significant branching to this other channel. Finally, we introduce a new method for generating correlation diagrams for systems with electronic transitions localized on one moiety, in which we restrict the changes allowed in remote molecular orbitals along the reaction coordinate. Analysis of previously measured X+NO2 photofragment pathways in nitromethane and methyl nitrate provides further support for using a restricted correlation diagram to predict the adiabatic and nonadiabatic product channels.

Warm-up behavior of monolithic reactors under non-reacting conditions

The transient heat transfer characteristics of monolithic reactors are of major importance because the time for the reactor to reach the required reaction temperatures should be minimized. A common application is the automotive catalytic converter. A simple numerical model for the heat transfer in an adiabatic thin wall channel is presented, taking account of the gas-solid convection, axial wall conduction and heat capacity effects in the solid phase. With the aid of the numerical model and the dimensional analysis, the thermal response of the channel after a step change of the inlet feed gas temperature is studied. For the case of negligible axial conduction, the results can be expressed in a single curve using suitably modified coordinates. The critical design and operating parameters are recognized and discussed. The results can be also expressed using common engineering variables to provide reliable assessment of several design and operating parameters of a monolithic reactor, such as void fraction, cell density, space velocity, etc.

Rigidity factors in unimolecular reactions

AbstractUnimolecular reactions with loose activated complexes, i.e. simple bond fission and the reverse bond formation processes, are conveniently characterized by phase space theory (PSD, which accounts for the maximum attraction potential between the fragments, and by additional rigidity factors which are the result of the anisotropy of the potential. This article summarizes recent calculations of specific rigidity factors frigid (E, J) and of thermal rigidity factors frigid (T) for a series of different potentials such as, e.g., ion‐dipole, dipole‐dipole, or standard valence potentials. The calculations have been made by rigorous statistical adiabatic channel (SACM) in combination with classical trajectory (CT) calculations.

Kinetics of the reaction of Cl atoms with FC(O)O and FC(O) OCl

The kinetics of the Cl + FC(O) O + M → FC(O)OCl + M recombination reaction has been studied by using an ArF excimer laser flash photolysis technique over the 3–650 mbar pressure range of the bath gases He and SF 6 at 295 K. The reaction was found to be close to the second-order regime at total pressures greater than about 100 mbar. The derived limiting high-pressure rate coefficient is (3.1 ± 0.3) × 10 −11 cm 3 molecule −1 s −1. For the Cl + FC(O)OCl → FC(O)O + Cl 2 reaction a rate coefficient of (1.5 ± 0.2) × 10 −11 cm 3 molecule −1 s −1 was determined. An analysis of the results in terms of the simplified versions of the statistical adiabatic channel model has carried out.

Specific rigidity factors in simple unimolecular bond fission reactions

Specific rate constants, k(E, J), and thermally averaged rate constants for simple unimolecular bond fission reactions and the corresponding combination reactions to a first approximation are represented by phase space theory (PST) which neglects the anisotropy of the potential-energy surface. A central quantity in this treatment is the number of open reaction channels, W(E, J), which in PST can be expressed in approximate analytical form. Accounting for the anisotropy of the potential introduces additional specific rigidity factors, f rigid (E, J), being between zero and unity, which have to be multiplied by W(E, J) from PST. After thermal averaging, thermal rigidity factors, f rigid (T), are obtained. This article shows examples of specific rigidity factors as derived from the statistical adiabatic channel model (SACM) for selected anisotropies of the potential.

Experimental and theoretical study of the recombination reaction of FC(O)O radicals

Abstract The kinetics of the recombination reaction of FC(O)O radicals has been experimentally and theoretically investigated. The FC(O)O radicals were generated by excimer laser flash photolysis of mixtures of (FCO) 2 and O 2 in excess of CO at 297 K and monitored by absorption spectroscopy at 545 nm. In agreement with the predictions from unimolecular rate theory, no appreciable falloff effects were observed for total pressures ranging from 117 to 727 Torr. The measured limiting high pressure rate coefficient, k rec, ∞ = (7.0 ± 1.1) × 10 −13 cm 3 molecule −1 s −1 , agrees very well with that predicted by a simplified version of the statistical adiabatic channel model. The influence of the dominant potential energy surface features on k rec, ∞ was examined between 200 and 600 K.

Classical trajectory and adiabatic channel study of the transition from adiabatic to sudden capture dynamics. II. Ion–quadrupole capture

Classical trajectory calculations of ion–permanent quadrupole+induced dipole capture processes are performed over wide ranges of conditions. The results are expressed in two-parametric analytical form, and analyzed with respect to the transition from sudden to adiabatic dynamics. The results are compared with the statistical adiabatic channel model (SACM). For this purpose, adiabatic channel potential curves are expressed in approximate analytical form. The agreement between classical trajectory and SACM results is excellent in the range of adiabatic quasiclassical motion of the collision partners. Analytical expressions can well describe the temperature variation of the capture rate constant in the transition range between the quantum and classical limits.

Classical trajectory and adiabatic channel study of the transition from adiabatic to sudden capture dynamics. I. Ion–dipole capture

Classical trajectory calculations of ion–permanent+induced dipole capture processes are performed over very wide ranges of conditions. The results are represented in a simple, two-parametric analytical form of high precision. The transition from adiabatic to nonadiabatic dynamics is expressed in terms of the Massey parameter. In the adiabatic range, perfect agreement (better than 0.4%) of the derived thermal capture rate constants from classical trajectories and results from accurate statistical adiabatic channel (SACM) calculations is obtained.

Classical trajectory and adiabatic channel study of the transition from adiabatic to sudden capture dynamics. III. Dipole–dipole capture

Classical trajectory (CT) calculations of dipole–dipole capture processes are performed over wide ranges of conditions. Besides the anisotropic dipole–dipole potential, an isotropic dispersion potential is accounted for. The Massey parameter of the system characterizes the transition between adiabatic and nonadiabatic (sudden) dynamics. In the adiabatic limit, under classical conditions, CT and statistical adiabatic channel model (SACM) give identical thermal capture rate constants which can be expressed in simple parametrized form. The transition from quantum to classical SACM rate constants is represented analytically as well. Furthermore, harmonic oscillator versions of SACM for dipole–dipole capture are compared with the corresponding models for ion–dipole and ion–quadrupole capture.

Statistical adiabatic channel model for ion–molecule capture processes. II. Analytical treatment of ion–dipole capture

Thermal rate constants for capture of permanent and induced dipoles by ions are calculated by the statistical adiabatic channel model (SACM) using analytical representations of adiabatic channel eigenvalues. Linear, symmetric, and asymmetric top dipoles with closed and open electronic shells are considered. Analytical representations of the capture rate constants over broad temperature ranges are presented for all systems. Numbers of open channels W(E,J) are also investigated. The present analytical SACM treatment provides an economical, transparent, and accurate approach to the considered type of capture processes. At the same time, the implementation to ion fragmentation processes is elucidated.

Adiabatic channel study of the capture of nitrogen and oxygen molecules by an ion: effect of nuclear symmetry and spin-spin interaction

Within the adiabatic channel treatment of ionmolecule capture we have calculated the low temperature capture rate constants of N2(1Σg+) and O2(3Σg−)in collisions with positive and negative ions. In both cases, the charge-quadrupole and the anisotropic charge-induced dipole interactions produce noticeable deviations from the Langevin rate constant. The rate constants calculated with account for the anisotropic interaction, in addition, are substantially affected by nuclear symmetry; in the case of O2(3Σg−), the fine-structure spin-spin interaction strongly manifests itself in the rate constants.

Simulation and measurement of turbulent heat transfer in a channel with a surface-mounted rectangular heated block

Present study numerically and experimentally investigates the turbulent forced convective flow over a heated block mounted on one principal wall of an adiabatic channel. In the computation, thek-ɛ, low-Reynolds-number, two-equation model was adopted for the turbulence closure. In the experiment, the flow measurement was performed by the laser Doppler velocimetry and the mass transfer measurement was carried out via the naphthalene sublimation technique. By virtue of the analogy between heat and mass transfer, the results could then be converted to predict the heat transfer coefficient. The effects of the Reynolds number and the aspect ratio of the block on heat transfer and fluid flow are thoroughly investigated. Distributions of the velocity and the turbulent kinetic energy are presented to gain an insight into the influence of the fluid flow on the heat transfer from the block. The Nusselt number hump is found on every face of the block, which is attributed to the separating bubble there. It is worth noting that the Nusselt number hump is located near the reattachment point of the separating bubble. In the absence of the separating bubble, the Nusselt number decreases or increases monotonously. Comparisons between numerical and experimental results of the local velocity and the heat transfer coefficient show reasonable agreement.

High-pressure range of the addition of HO to HO. III. Saturated laser-induced fluorescence measurements between 200 and 700 K

The addition of HO to HO was studied by saturated laser induced fluorescence at temperatures between 200 and 700 K and at pressures of the bath gas helium up to 100 bar. In combination with earlier measurements at 298 K, a set of falloff curves is constructed for the given temperature range. The limiting high-pressure rate constant for the reaction HO+HO(+He)→H2O2(+He) follows as k1,∞=(2.6±0.8)×10−11 (T/300 K)0±0.5 cm3 molecule−1 s−1, practically independent of the temperature between 200 and 400 K. At higher temperatures, k1,∞ decreases. These results serve as a reference for statistical adiabatic channel model calculations of the recombination rate.

Theoretical study of the unimolecular dissociation HO2→H+O2. II. Calculation of resonant states, dissociation rates, and O2 product state distributions

Three-dimensional quantum mechanical calculations have been carried out, using a modification of the log-derivative version of Kohn’s variational principle, to study the dissociation of HO2 into H and O2. In a previous paper, over 360 bound states were found for each parity, and these are shown to extend into the continuum, forming many resonant states. Analysis of the bound states close to the dissociation threshold have revealed that HO2 is a mainly irregular system and in this paper it is demonstrated how this irregularity persists in the continuum. At low energies above the threshold, these resonances are isolated and have widths that fluctuate strongly over more than two orders of magnitude. At higher energies, the resonances begin to overlap, while the fluctuations in the widths decrease. The fluctuations in the lifetimes and the intensities in an absorption-type spectrum are compared to the predictions of random matrix theory, and are found to be in fair agreement. The Rampsberger–Rice–Kassel–Marcus (RRKM) rates, calculated using variational transition state theory, compare well to the average of the quantum mechanical rates. The vibrational/rotational state distributions of O2 show strong fluctuations in the same way as the dissociation rates. However, their averages do not agree well with the predictions of statistical models, neither phase space theory (PST) nor the statistical adiabatic channel model (SACM), as these are dependent on the dynamical features of the exit channel. The results of classical trajectory calculations agree well on average with those of the quantum calculations.

Statistical adiabatic channel model rate constants for the reaction O+CN→CO+N(2D) at 300–5000 K

The full version of the statistical adiabatic channel model has been employed to calculate rate constants for the title reaction. In very good agreement with available experimental data, the computed values increase by almost one order of magnitude between 300 and 5000 K.