Set Of Rate Coefficients Research Articles

Fine and hyperfine excitation of nitric oxide by collision with para-H2 at low temperature

ABSTRACT Nitric oxide is an open-shell molecule abundantly detected in the interstellar medium. A precise modelling of its radiative and collisional processes opens the path to a precise estimate of its abundance. We present here the first rate coefficients for fine and hyperfine (de-)excitation of NO by collisions with the most ubiquitous collision partner in the interstellar medium, para-H2 hydrogen molecules, using a recently developed accurate interaction potential. We report quantum scattering calculations for transitions involving the first 74 fine levels and the corresponding 442 hyperfine levels belonging to both F1 and F2 spin–orbit manifolds. To do so, we have calculated cross-sections by means of the quantum mechanical close-coupling approach up to 1000 cm−1 of total energy and rate coefficients from 5 to 100 K. Propensity rules are discussed and the new NO–H2 rates are compared to those available in the literature, based on scaled NO–He rates. Large differences are observed between the two sets of rate coefficients, and this comparison shows that the new collision rates must be used in interpreting NO emission lines. We also examined the effect of these new rates on the NO excitation in cold clouds by performing radiative transfer calculations of the excitation and brightness temperatures for the two NO lines at 150.176 and 250.4368 GHz. This shows that the local thermodynamic equilibrium is not fulfilled for this species for typical conditions. We expect the use of the rates presented in this study to improve the constraints on the abundance of NO.

Rate coefficients for the reactions of CO[formula omitted] with O: Lessons from MAVEN at Mars

In the lower ionosphere of Mars, the relative density distributions of the major ions CO2+, O2+, and O+ are largely determined by three reactions, including the two channels of the reaction of CO2+ with O, which yield either O2+ (R1) or O+ (R2), and the reaction of O+ with CO2, which yields solely O2+ (R3). There have been only two measurements of the rate coefficients for reactions (R1) and (R2) in the last 50 years, and they are very different (Fehsenfeld et al., 1970; Tenewitz et al., 2018). Although we have carried out a fairly thorough exploration of parameter space in our quest to fit the density profiles of the major ions and O atoms as measured by instruments on the MAVEN spacecraft, we report here only a small fraction of that exploration. In this investigation, we have used the atmosphere of Mars as a laboratory to distinguish between the two sets of rate coefficients. We find that the best fits to the O2+, CO2+, and O+ densities are for the three models in which the Fehsenfeld et al. rate coefficients are adopted, each of which has its advantages. We conclude that the favored O density profile is 1.5±0.5 times the measured profile, and that the rate coefficients measured 50 years ago are much better than those recently measured at explaining the density profiles of O2+, CO2+, and O+ in the Martian ionosphere as we know it.

Determining rate coefficients for ion adsorption at the solid/water interface: better from desorption rate than from adsorption rate.

One of the most common approaches in the adsorption kinetics literature is to compare the fitting performance of several empirical or non-empirical equations (pseudo-first order, pseudo-second order, Elovich, parabolic diffusion, etc.) with the aim of selecting the equation that best describes the experimental data. This is normally a futile fitting exercise that leads to the determination of ambiguous rate parameters, without providing insights into the behaviour of the studied system. A more realistic approach is to treat it as a combination of mass transport and chemical reaction under controlled conditions, and thus actual adsorption-desorption rate parameters are readily estimated. This article applies a simple and realistic physicochemical model to describe and understand the adsorption-desorption kinetics of ions at the solid/water interface. The model is applied to an ATR-FTIR study of phosphate adsorption-desorption on goethite, which is a very well-known and reference system, ideal for testing the performance of a physicochemical treatment that combines transport and reaction. Always the same phosphate species (monodentate mononuclear protonated) was present at the goethite surface during adsorption-desorption. There was an excellent agreement between theory and experiments at a variety of phosphate concentrations and surface coverages for adsorption kinetics, desorption kinetics and equilibrium situations, employing just one set of rate coefficients. The use of rate vs. adsorption curves permitted easy detection of conditions of transport- and reaction-controlled kinetics. The phosphate-goethite system is a fast-adsorbing/slow-desorbing system, with an adsorption rate constant k = 1.26 × 103 s-1 and a desorption rate constant kd = 1.66 × 10-5 s-1. Therefore, adsorption was transport-controlled and desorption was reaction-controlled. The half-life of the desorption reaction is 41 700 s (11.6 h) but for adsorption it would take only a few seconds in the absence of transport control. For this kind of system, which is ubiquitous in nature and technological processes, it is easier to determine rate constants from desorption than from adsorption experiments.

Quantifying Error Correction through a Rule-Based Model of Strand Escape from an [n]-Rung Ladder.

The rational design of 3D structures (MOFs, COFs, etc.) is presently limited by our understanding of how the molecular constituents assemble. The common approach of using reversible interactions (covalent or noncovalent) becomes challenging, especially when the target is made from multivalent building blocks and/or under conditions of slow exchange, as kinetic traps and nonequilibrium product distributions are possible. Modeling the time course of the assembly process is difficult because the reaction networks include many possible pathways and intermediates. Here we show that rule-based kinetic simulations efficiently model dynamic reactions involving multivalent building blocks. We studied "strand escape from an [n]-rung ladder" as an example of a dynamic process characterized by a complex reaction network. The strand escape problem is important in that it predicts the time a dynamic system needs to backtrack from errors involving [n]-misconnections. We quantify the time needed for error correction as a function of the dissociation rate coefficient, strand valency, and seed species. We discuss the simulation results in relation to a simple probabilistic framework that captures the power law dependence on the strand's valency, and the inverse relationship to the rung-opening rate coefficient. The model also tests the synthetic utility of a one-rung (i.e., hairpin) seed species, which, at intermediate times, bifurcates to a long-lived, fully formed [n]-rung ladder and a pair of separated strands. Rule-based models thus give guidance to the planning of a dynamic covalent synthesis by predicting time to maximum yield of persistent intermediates for a particular set of rate coefficients and valency.

Calculated electron impact dissociation cross sections for molecular chlorine (Cl2)

Electron impact dissociation of Cl2 is a key process for the formation of Cl atoms in low-temperature plasmas used for industrial etching processes. Despite this, relatively little cross section data exist for this process. In this work, electron impact dissociation cross sections were calculated for Cl2 molecules using the UK molecular R-matrix code in the low electron energy range and extended to high energies using a scaling depending on the specific nature of each transition. Our results are compared with both previous calculations and with experimental measurements, and the similarities and differences are discussed. In addition, the rate coefficients for electron impact dissociation of Cl2 are calculated by integrating the cross sections derived in this (and previous) work, with electron energy distribution functions representative of those normally found in low-temperature plasmas used in industry. Depending on the shape and effective temperature of the distribution function, significant differences arise between the rate coefficients calculated from our cross sections and those calculated using previous data. Deviations between the two sets of rate coefficients are particularly pronounced at the low electron temperatures typical of electron beam and remote plasma sources of interest for atomic layer etching and deposition. These differences are principally caused by the higher energy resolution in the near-threshold region in this work, emphasising the importance of accurate, high-resolution cross sections in this energy range.

Multitemperature Dissociation Rate of Calculated Using Selective Sampling Quasi-Classical Trajectory Analysis

The dissociation rate of nitrogen for the reaction was calculated as a function of a translational, vibrational, and rotational temperature, each ranging from 6000 to 60,000 K. The rate coefficients were calculated using quasi-classical trajectory analysis, in which approximately 5.35 billion trajectories were directly simulated. Furthermore, a new selective sampling procedure was implemented so that only trajectories with sufficient energy to cause dissociation were sampled. At 6000 K, this method reduced the computational cost by nearly two orders of magnitude. Finally, the set of rate coefficients was used to extend the two-temperature model conventionally used in computational-fluid-dynamics simulations. For temperatures between 13,000 and 40,000 K, where rotational energy of should be independently modeled, the fit shows good agreement with the quasi-classical trajectory-calculated rate coefficients.

State-specific dissociation in O2–O2 collisions by quasiclassical trajectory method

State-specific dissociation rate coefficients of oxygen in collisions with another oxygen molecule are obtained using the quasiclassical trajectory method on a six-dimensional potential energy surface at high temperatures characteristic of hypersonic flows. The present results indicate that O2–O2 rate coefficients can not be obtained by a simple scaling of a simpler O2–O dataset. A difference is observed between the present rate coefficients and those obtained via the preferential dissociation and forced harmonic oscillator models. Depletion via an exchange mechanism significantly contributes to the total dissociation rate at low temperatures and at low vibrational energies. The new set of rate coefficients is used in a one-dimensional shock flow simulation. The computed vibrational temperature is then compared to data obtained by absorption spectroscopy in the Schumann-Runge bands. A better agreement with the experimental data is achieved at moderate levels of nonequilibrium when the new dataset is implemented.

LABORATORY STUDY OF RATE COEFFICIENTS FOR H 2 O:He INELASTIC COLLISIONS BETWEEN 20 AND 120 K

State-to-state rate coefficients for ortho-H2O:He and para-H2O:He inelastic collisions in the 20–120 K thermal range are investigated by means of an improved experimental procedure. This procedure is based on the use of a kinetic master equation (MEQ) which describes the evolution of populations of H2O rotational levels along a supersonic jet of H2O highly diluted in helium. The MEQ is expressed in terms of experimental observables and rate coefficients for H2O:He inelastic collisions. The primary experimental observables are the local number density and the populations of the rotational energy levels of H2O, quantities which are determined along the jet with unprecedented accuracy by means of Raman spectroscopy with high space resolution. Sets of rate coefficients from the literature and from present close-coupling calculations using two different potential energy surfaces (PESs) have been tested against the experiment. The Green et al. rate coefficients are up to 50% too low compared to the experiment, while most rates calculated here from the Hodges et al. PES and the Patkowski et al. PES are much closer to the experimental values. Experimental rates with an estimated accuracy on the order of 10% have been obtained for ortho-H2O:He and para-H2O:He inelastic collisions between 20 and 120 K by scaling and averaging the theoretical rates to the experiment.

Hyperfine excitation of CN by para- and ortho-H2

Abstract Among the interstellar molecules, the CN radical is of particular interest since it is a good probe of cold dark molecular clouds, and especially prestellar cores. Modelling of CN emission spectra from these dense molecular clouds requires the calculation of rate coefficients for excitation by collisions with the most abundant species. We calculate fine- and hyperfine-structure-resolved excitation rate coefficients of CN(X2Σ+) by para- and ortho-H2. The calculations are based on a new potential energy surface obtained recently from highly correlated ab initio calculations. State-to-state rate coefficients between fine and hyperfine levels of CN were calculated for low temperatures ranging from 5 to 100 K. The new results are compared to available CN rate coefficients. Significant differences are found between the different sets of rate coefficients. This comparison shows that the new CN–H2 rate coefficients have to be used for observations interpretations. We expect that their use will help significantly to have a new insight into the physical conditions of prestellar cores.

Hyperfine collisional rate coefficients of CN with H2(j= 0)

Modelling of molecular emission spectra from interstellar clouds requires the calculation of rate coefficients for excitation by collisions with the most abundant species. Among the interstellar molecules, the CN radical is of particular interest since it is a good probe of dense region and can be used as a tracer of magnetic fields. We calculate fine- and hyperfine-structureresolved excitation rate coefficients of CN(X 2 � + )b y H 2(j = 0), the most abundant collisional partner in the interstellar medium. The calculations are based on a new potential energy surface obtained from highly correlated ab initio calculations. State-to-state rate coefficients between the first fine and hyperfine levels of CN were calculated for low temperatures ranging from 5 to 100 K. The new results are compared to the CN–He rate coefficients. Significant differences are found between the two sets of rate coefficients. This comparison shows that the CN–H2 rate coefficients have to be used for observation interpretations and we expect that their use will significantly help the astronomers in the interpretation of the CN emission lines observed with current and future telescopes.

A mechanistic model of fast pyrolysis of glucose-based carbohydrates to predict bio-oil composition

Bio-oil obtained from fast pyrolysis of biomass has the potential to be upgraded to renewable, drop-in fuel intermediates. The quality of bio-oil produced is critical to downstream process development and is determined by a number of factors like reaction timescale, extent of primary and secondary reactions, and the presence of minerals. In this work, we provide a mechanistic understanding of the various competing reactions in fast pyrolysis of cellulose and other glucose-based carbohydrates through a unified microkinetic model. The model incorporates the reactions of the cellulose chain and of the glucose intermediate to form a variety of bio-oil components, which are confirmed by either experiments or theoretical calculations reported in the literature. The majority of the Arrhenius rate parameters were based on existing studies, while a small number were fitted to match the experimental product distribution over a wide range of temperatures. The model yields of all the major primary fast pyrolysis products, viz., levoglucosan, formic acid, glycolaldehyde, 5-hydroxymethyl furfural, furfural and char, match reasonably well with the experimental data over the temperature range of 400–550 °C. The model predicted that the time required for complete conversion corresponds to 2.5–3 s. Net rate analysis identified key pathways to levoglucosan via either mid-chain initiation, or depropagation and end-chain initiation that were dominant over the timescales of 0–100 ms and 100–1000 ms, respectively, at 500 °C. Unlike the existing lumped kinetic models, our model provides a detailed picture of levoglucosan formation that involves mid-chain glycosidic bond cleavage followed by unzipping of levoglucosan from the chain ends. The model, utilizing the same set of rate coefficients, was able to predict the dominant products of fast pyrolysis of maltohexaose, cellobiose and glucose that also are in good agreement with experimental data. The initial chain length of cellulose was found to have a profound effect on the product yields for chain lengths shorter than 50, while it had no effect on the total reaction time.

First rate coefficients for an interstellar anion: application to the CN−-H2 collisional system

Recently, several molecular anions have been detected in the interstellar medium. Accurate modelling of their microwave emission spectra requires the calculation of rates for rotational excitations by collisions with the most abundant species. We report the first collisional data for an interstellar anion, the rate coefficients for the rotational excitation of CN− by H2. State-to-state rate coefficients between the 11 lowest rotational levels of CN− were calculated for temperatures ranging from 5 to 100 K. Collisions with para- and ortho-H2 are taken into account. The CN− rate coefficients are compared with those of the neutral CO molecule, and, as expected, it is found that the rate coefficients for the anion are significantly larger than those of the neutral species. The new set of rate coefficients will help significantly in the interpretation of the CN− emission lines observed with current and future telescopes, and enable this molecule to become a powerful astrophysical tool.

Comparison between experiment and simulation for argon inductively coupled plasma

In order to include the nonlocal characteristics of electrons and investigate the inductively coupled plasma (ICP) resources more completely, we have developed a hybrid Monte Carlo (MC)/fluid hybrid model and calculated the axial and radial distributions of electron density, electron temperature, plasma potential, and electron energy distribution functions (EEDFs) of Ar discharge in a planar ICP. Furthermore, to make the model more practical, we still incorporate the effects of metastable atoms, whose sets of rate coefficients and density are, respectively, calculated through the electron MC part and fluid module. Besides, the corresponding Langmuir probe measurements are used to compare these data to validate the simulated results. Under all the selected discharge powers and pressures, the theoretically simulated and experimentally measured quantity profiles agree reasonably with each other, embodied in the generally identical magnitude ranges and spatial distributions. Furthermore, the interpretations about their detailed differences are given, which are based on the designs of both experimental schematic and model configuration. The analysis implements that the inclusions of electron-electron collision and a neutral density distribution into the hybrid model are likely to improve the comparison between the model predictions and experiment diagnostics. Furthermore, the evolution of plasma parameters and EEDFs with discharge conditions is discussed.

Application of Genetic Algorithm to Chemical Kinetics: Systematic Determination of Reaction Mechanism and Rate Coefficients for a Complex Reaction Network

We develop a systematic computational approach, which includes the genetic algorithm (GA) search method, to the minimal bromate reaction system, to automate the determination of its reaction mechanism and rate coefficients. We take the 10 species of the system as given, and then determine all possible bimolecular elementary reaction steps and all possible combinations of these steps that fulfill the given overall reaction. The optimization criteria used in the GA are chosen to be the following: The reaction mechanism must have an unstable focus, or must have oscillations; has specified ranges of periods of oscillations; and has specified ranges of inflow conditions. With rate coefficients given by Field, Koros, and Noyes1, we find reaction mechanisms with four and five elementary reaction steps that show oscillations but do not fit well the experimental [Br-]0 - [BrO3-]0 oscillation domain. We then determine, by GA, a set of rate coefficients in one particular five-step reaction mechanism that gives very...

Inelastic collisions of n=3 helium atoms with normal helium atoms: associative ionisation

Helium atoms are excited to an n=3 level by pulsed laser light in a glow discharge plasma, and the subsequent temporal development of the upper-level populations and the electron density are determined from the emission line intensities and by the microwave cavity resonance method, respectively. Associative ionisation rate coefficients are obtained for 31P-, 31D- and 33D-state atoms. When combined with the previous determination of the excitation transfer and total depopulation collision rate coefficients a consistent set of rate coefficients is obtained. A qualitative discussion on the detailed mechanism of these transitions is given on the basis of the diabetic molecular potential curve crossings.

Luminescence from excited mercury vapour. Quantum yield and lifetime of the ultraviolet reservoir states and mechanisms for the termolecular formation of excited dimers

Mixture of Hg vapour with low pressures of N2 and an excess of added inert gas have been excited with resonance radiation, both pulsed and c.w., to determine the quantum yield and mechanism of the ultraviolet emission which arises in the combination of metastable and ground-state mercury atoms.The time-resolved measurements demonstrate that the u.v. band is supplied from a higher energy reservoir than the A 0±g first excited states of the dimer. By estimating the ratio of the partition function of the reservoir to that of the first excited states from the effective emission rates, it is adduced that the C 0u– dimer is the sole reservoir state and is in equilibrium with the D 1u carrier with pressures of Ar > 300 Torr; there is little deactivation of the coupled ungerade states to the lower gerade states by Ar at up to one atmosphere pressure. Addition of He, however, does quench the ultraviolet emission. A set of rate coefficients for deactivation of the C 0–u state is reported.The quantum yield for formation of the C 0–u state, relative to the total formation of excited dimers, has been measured by comparing the u.v. intensity with that of the green trimer emission, after allowance for non-radiative decay. Only one combination occurs into the vibrationally relaxed C 0–u state per six or seven combinations into A 0±g. It is proposed that the low quantum yield of the C 0–u dimers is due to collisional coupling of pairs of states in high vibrational levels, where the potential energy functions merge, which preferentially drains the excitation into the state of lower energy.

Atomic oxygen densities in the lower thermosphere as derived from in situ 5577-A night airglow and mass spectrometer measurements

Atomic oxygen number densities measured by a mass spectrometer with a cryoion source and a 5577-A night airglow photometer flown in the same rocket payload are discussed. The airglow data are further compared with the number densities obtained from a number of previous photometer experiments and from another cryomass spectrometer. The volume emission rates of the previous airglow measurements were taken from the literature and converted to O number densities by a new set of rate coefficients. The comparison with the mass spectrometer data indicates that these coefficients yield usable oxygen number densities. The oxygen densities are correlated with several geophysical parameters. A seasonal variation of the peak density of the oxygen layer is found showing a flat summer maximum. The altitude at which the peak of the layer occurs shows a seasonal variation with a minimum in summer. Combination of these two counteracting effects yields two results: (1) it indicates a quasisemiannual variation of the 5577-A light emission of the oxygen layer and (2) by an extrapolation it yields a seasonal variation of the atomic oxygen number density at higher altitudes with a pronounced summer minimum. (auth)