Dissociation Rate Coefficients Research Articles

Monomer–excimer kinetics in solution. I. Stochastic many-particle approach

A stochastic many-particle approach is applied to study the kinetics of reversible excimer formation in solution. Coupled dynamic equations for the macroscopic concentrations and for the radial distribution function are derived, and applied to analyze (i) time resolved kinetics after a short pulse, and (ii) steady-state kinetics. Renormalization of the phenomenological excimer dissociation rate coefficient due to nonequilibrium effects is discussed. A relation is demonstrated between steady-state, reversible monomer–excimer kinetics and irreversible fluorescence kinetics. Explicit results are given for the excimer fluorescence yield, assuming the Smoluchowski–Collins–Kimball reactivity model.

Isotope effect and electron-temperature dependence in volume H- and D- ion sources.

We present the results of a numerical study of isotope effects in volume negative-ion sources and compare them with experimental results. We have studied both hydrogen and deuterium discharges over a range of discharge current. For basic plasma parameters, the model reproduces the behavior measured in the discharge region of a tandem multicusp source. The theory predicts a local maximum in negative-ion production at an optimal electron temperature ${\mathit{T}}_{\mathit{e}}^{\mathrm{opt}}$\ensuremath{\approxeq}0.6--0.8 eV. This prediction is supported by our analysis of ${\mathrm{D}}^{\mathrm{\ensuremath{-}}}$ and ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ densities measured in a hybrid multicusp source and in the extraction region of a tandem source. We show how the local maximum is related to the temperature dependence of the dissociative attachment rate coefficients. The most significant isotope effect suggested by this modeling effort is a stronger cooling of the vibrational distribution in deuterium due to vibration-translation reactions between molecules and atoms. This vibrational cooling reduces negative-ion production in high-power deuterium discharges below that in equivalent hydrogen discharges.

A simple model for state-specific diatomic dissociation

Accurate quasiclassical trajectory (QCT) calculations have been performed recently to compute thermal state-specific dissociative cross sections for atom-diatom collisions at a specific temperature. However, these calculations require the generation of a realistic potential energy surface on which to propagate thousands of trajectories, making the QCT method an expensive alternative for parametric studies. In this paper we develop approximate expressions from which an entire set of state-specific dissociative rate coefficients (or thermal cross sections) for atom-diatom collisions may be computed efficiently. These expressions, which depend on the single parameter λ D , are based on a vibrational-rotational bias model for dissociation and are an extension of the vibrational bias models of previous workers

Photodissociation in the atmosphere of Mars: Impact of high resolution, temperature‐dependent CO2 cross‐section measurements

We have investigated the impact of high resolution, temperature‐dependent CO2 cross‐section measurements (Lewis and Carver, 1983) on calculations of photo dissociation rate coefficients in the Martian atmosphere. Since the cross‐section measurements exhibit large variations on a 10–20 Å scale, the calculations are sensitive to the wavelength resolution. We find that the adoption of 50 Å intervals for the purpose of computational efficiency result in errors in the calculated values for photodissociation of CO2, H2O and O2 which are generally ≤10% but as large as 20% in some instances. These are acceptably small errors, especially considering the uncertainties introduced by the large temperature dependence of the CO2 cross‐section. The inclusion of temperature‐dependent CO2 cross‐sections is shown to lead to a decrease in the diurnally averaged rate of CO2 photodissociation as large as 33% at some altitudes, and increases of as much as 950% and 80% in the photodissociation rate coefficients of H2O and O2, respectively. The actual magnitude of the changes depends on the assumptions used to model the CO2 absorption spectrum at temperatures lower than the available measurements, and at wavelengths longward of 1970 Å. Given our current understanding, these results minimize or even reverse the sense of the CO2 chemical stability problem.

Atomic resonance absorption spectroscopy measurements on high-temperature CO dissociation kinetics

The dissociation rate coefficient of CO was measured behind reflected shock waves in the temperature range 5500 < J<9000 K. By the use of the atomic resonance absorption spectroscopy (ARAS), the time-dependent formation of C and O atoms could be directly observed in the postshock reaction zone. The experiments were performed in mixtures of CO highly diluted in argon at pressures between 0.5 and 1.3 bar. The initial CO concentrations varied from 100 to 10,000 ppm. Assuming vibrational equilibrium, the data for the thermal decomposition of CO, (CO + Ar ^ C + O + Ar), can be represented by the following modified Arrhenius expression: k = 4.3 X1027 J-3-1 exp(-129,000 K/J) cn^mol-^-1 ±50%. Computer simulations based on a two-temperature model were not able to explain observed reaction induction times. Vibrational relaxation alone seems to have a negligible influence under the present conditions. The effect of impurities was minimized by keeping background pressure in the shock tube below 5xlO~ 7 mbar.

Electron and vibrational kinetics in an N2-H2 glow discharge with application to surface processes

The electron and vibrational kinetics in an N2-H2 glow discharge have been analysed using a self-consistent theoretical approach valid in the conditions of a low-pressure moderate current, positive column. This model is based on the solutions to the homogeneous Boltzmann equation and the two systems of rate balance equations for the vibrational levels N2(X1 Sigma g+, v) and H2(X1 Sigma g+, v'), which take into account e-V, V-V, and V-T processes for the N2-N2 and H2-H2 systems as well as those involving N2-H2 collisions. The paper presents a large amount of calculated data on the vibrational distribution functions of N2(X,v) and H2(X,v') molecules, electron energy distribution function, rate of dissociation and electron rate coefficients for excitation. This formulation provides a relationship between the characteristic vibrational temperatures Tv(N2) and Tv(H2) and a dependence of Tv(N2) on the fractional H2 concentration, which are in satisfactory agreement with measurements by emission spectroscopy and by CARS. The model also predicts the strong decrease of the rate of dissociation of N2 as it is observed in flowing discharges when a few per cent of H2 is added to N2.

Further measurements of the H+3(v=0,1,2) dissociative recombination rate coefficient

A new flowing afterglow apparatus that utilizes a Langmuir probe/mass spectrometer to monitor both electron and ion decay in a hydrogen plasma has been used to measure the dissociative recombination rate coefficient of H+3 at two different electron temperatures. At 300 K a rate coefficient of 1.5×10−7 cm3 s−1 was found for H+3 ions with a low degree of vibrational excitation (v≤2). The rate coefficient for ground state ions H+3(v=0) was measured as 1.1×10−7 cm3 s−1 at 650 K. A discussion is given of the excitation states of H+3 ions in the afterglow in the light of slow deexcitation rates for low vibrational states. A new model for the recombination of H+3 is presented.

Infrared multiphoton excitation dynamics of CF3I. I. Populations and dissociation rates of highly excited rovibrational states

Transient detection of I(2P3/2) by resonantly enhanced multiphoton ionization (REMPI) was used to monitor in-pulse and after-pulse dissociation of CF3I excited by infrared (IR) multiphoton absorption. After-pulse reaction is characterized by time-dependent dissociation rate coefficients. The apparent reaction rates increase with increasing laser intensity. These observations are attributed to strong rotational dependencies of the specific dissociation rate constants k(E,J) and to CO2 laser-intensity-dependent rotational distributions of the excited CF3I. The corresponding rotational distributions are reconstructed from the observed experimental time profiles of the reaction rates. In addition to the detection of the I(2P3/2) dissociation product from infrared multiphoton excitation, populations of vibrationally highly excited CF3I* were identified via electronic excitation in the visible, subsequent fast dissociation and REMPI detection of the resulting I(2P1/2). At weak IR laser intensities these vibrational distributions were found to be strongly bimodal. However, the bimodal character weakens with increasing laser intensity, tending towards a single broad distribution at very high laser intensities.

A mathematical model for the computation of carboxyhaemoglobin in human blood as a function of exposure time

A mathematical model is developed for the carbon monoxide (CO) uptake by the blood by taking into account the molecular diffusion, convection, facilitated diffusion and the non-equilibrium kinetics of CO with haemoglobin. The overall rate for the combination of CO with haemoglobin is derived by including the dissociation of CO from carboxyhaemoglobin (COHb). The resulting coupled system of nonlinear partial differential equation with physiologically relevant initial, entrance and boundary conditions is solved numerically. A fixed point iterative technique is used to deal with nonlinearities. The concentration of COHb in the blood is computed as a function of exposure time and ambient CO concentration. The COHb levels computed from our model are in good agreement with those measured experimentally. Also, results computed from our model give better approximation to the experimental values compared with the results from other models. The time taken by the blood COHb to attain 95% of its equilibrium value is computed. The COHb concentration in the blood increases with the increase in ventilation rate, association rate coefficient of CO with haemoglobin and total haemoglobin content in the blood, and with the decrease in dissociation rate coefficient of CO with haemoglobin and mean capillary blood PO2. It is found that the COHb level in the blood is not affected significantly because of endogenous production of CO in the body under normal condition. However, the effect may be significant in the patients with haemolytic anaemia.

Diagnostics and modelling of ECRH microwave discharges

Stationary plasmas in various gases were generated at pressures of some 10 Pa in weak magnetic fields by microwave heating. Quantitative spectroscopy of atomic lines and molecular bands was applied for diagnostics of plasmas in hydrogen, nitrogen, methane and argon-helium mixtures. Simple model calculations were based on the well-known balance equations of glow discharges including chemical reactions especially for methane. Some tests were also performed in nitrogen glow discharges. Excitation, ionization and dissociation rate coefficients for diagnostic or modelling purposes were taken from nuclear fusion research or calculated from experimental cross-sections assuming a Maxwellian energy distribution. The electron temperatures of the discharges, predicted from electron confinement times and ionization rate coefficients ( approximately=1-4 eV), were well confirmed by the experiment. The electron density in molecular plasmas is of the order of the cut-off density, i.e. approximately=1011 cm-3, almost independent of the pressure.

Electron-ion recombination in high pressure Ar/Xe mixtures

Electron-ion recombination processes in high pressure (at 5 and 10 atm) Ar/Xe mixtures were investigated experimentally. Measurements of electron drift currents in the gas mixtures initiated by an ionizing short pulse were made. Taking into account the cathode potential drop caused by the space charge, calculations of the signal time histories yielded good fits with measurements for a dissociative recombination rate coefficient kr=6*10-6 (300/Te(K))0.6 cm3 s-1. Thus, kr at room temperature (300 K) is more than twice that previously measured for the recombination of electron and Xe2+. Based on kinetic considerations, the authors conclude that the dominant recombination process in high pressure Ar/Xe plasmas is that between electron and triatomic xenon ion Xe3+ (i.e. e+Xe3+ to Xe*+2 Xe).

Determination of the electron-ion dissociative recombination coefficients for several molecular ions at 300 K

The electron-ion dissociative recombination rate coefficients, alpha e, have been determined for ten molecular ions, seven of which have not previously been measured. The data were obtained under truly thermal conditions at 300 K using a flowing afterglow/Langmuir probe (FALP) apparatus. Recombination coefficients were obtained for the following ions: CO2+ ( alpha e=3.1*10-7 cm3 s-1), N2+ (2.0), CO+ (1.6), KrH+ (<0.2), XeH+ (<0.4), HSO2+ (2.7), CH3CNH+ (3.3), CH3CHOH+ (3.9), C2H5CNH+ (4.7) and ((CH3)2CO)2H+ (14). The dissociative recombination of many of the ions studied may be important in planetary, cometary and terrestrial atmospheres and be involved in the synthesis of interstellar molecules.

Diagnostics and modeling of RF discharge dissociation in N/sub 2/O

Measurements were made of the RF discharge dissociation of N/sub 2/O in a parallel-plate reactor by downstream mass spectrometry using a wide range of gas flows and powers at 10 kHz and 13.56 MHz. The results show that the mass 44 signal (N/sub 2/O/sup +/), which is a measure of the amount of undissociated N/sub 2/O, is a function of the discharge input energy per N/sub 2/O molecule (eV/N/sub 2/O). A plug flow, rate equation model of the discharge was used to predict the experimental dissociation rates. A DC Monte Carlo simulation was used to calculate rate coefficients for electron-impact neutral dissociation, ionization, and dissociative ionization. The rate equation model also includes reactions among the dissociation products of N/sub 2/O and species which are synthesized in the discharge, as well as neural and electron-ion recombination at the electrodes. The model predictions identify the major reaction pathways and the sensitivities of the results to the rate coefficient values used. >

Salt effects on steady-state quantum yields of ultrafast, diffusion-influenced, reversible photoacid dissociation reactions

The steady-state fluorescence quantum yields of the protonated and deprotonated froms of 8-hydroxypyrene-1,3,6-trisulfonate are determined in three solvents as a function of NaNO 3 concentration. The measured quantum yilds, and therefore also the overall dissociation rate coefficients, decrease with increasing salt concentration, in contrast to Bronsted's classical model for primary salt effects. This is a consequence of the reversibility and the nearly diffusion-controlled recombination rate of the proton in the reaction studied. A quantitative analysis is based on newly derived results for reversible diffusion-influenced reactions

Electron Energy Distribution Functions in CO2 Laser Mixture: The Effects of Second Kind Collisions from Metastable Electronic States

AbstractElectron energy distribution functions (eedf) in CO2 laser discharge (He—CO2—N2—CO mixture) have been calculated by solving the Boltzmann equation in the presence of given concentrations of excited (vibrational and electronic) states. The results show a well structured eedf as a result of second kind collisions coming from metastable electronic states of N2 and He as well as a strong dependence of rate coefficients for CO2 dissociation and for the ionization of the different species.

The dissociative recombination rate coefficients of H+3, HN+2, and HCO+

The dissociative recombination rate coefficients for H+3, HN+2, and HCO+ are determined at 110, 210, and 273 K by monitoring the decay of the infrared absorption signals as a function of time. The rate coefficients are 1.8, 7.0, and 3.1 in units of 10−7 cm3 s−1 for H+3, HN+2, and HCO+, respectively, at 273 K. These values agree very well with those obtained using the stationary afterglow or the merged beam techniques, but the values for H+3 disagree with that obtained by Smith and co-workers (≤2×10−8 cm3 s−1) using the flowing afterglow/Langmuir probe method. The rate coefficients for H+3 and HCO+ disagree with theory which has predicted very slow dissociative recombinations in the lower vibrational states. The temperature dependences obtained here, although the temperature range is rather limited, are consistent with those obtained previously using the stationary afterglow (for H+3 and HCO+) and the merged beam (for HN+2) techniques. The measurements are extended to several vibration–rotation levels and no significant rotation dependence of the rate coefficients is observed. It has also been found that the ions investigated here can be equally abundant at ice temperature as at liquid nitrogen temperature.

ARAS measurements on the thermal decomposition of CO2 behind shock waves

The dissociation rate coefficient of COi has been measured behind reflected shock waves in the temperature range 2400<T<4400 K at pressures 1.2<p <2.0 bar. By the use of the atomic resonance absorption spectroscopy, the formation of O atoms could directly be observed in the postshock reaction zone. The experiments have been performed in mixtures of CCh highly diluted in argon. The initial COi concentrations varied from 10 to 5000 ppm. The Arrhenius expression for the thermal decomposition of COz, CCh + M_^iCO + O + M was found to be k = 3.65 x 10 14exp(-52,525 K/T) cm3mol~1 s1. The effect of impurities was minimized by keeping background pressures in the shock tube below 5x 10 ~7 mbar.

ELectron Collision Rates and Transport Coefficients of a Weakly Ionized dc Plasma in Ar/Sih4‐Mixtures

AbstractThe paper deals with electron kinetics of a Ar/SiH4 dc plasma, using the stationary and spatially uniform Boltzmann equation. The solution of this kinetic equation has been obtained by applying higher order Legendre polynomial expansion of the electron velocity distribution. For varying mixture composition the energy distribution and relevant macroscopic quantities as mean energy, drift velocity, rate coefficients for excitation, dissociation and ionization and relevant energy transfer rates for these processes have been calculated. In particular, it has been found that a most effective activation of the feed gas occurs for SiH4 admixtures in the range from 5 to 10 per cents.

On the stability of CF +·4 ions

Experimental results on the identification of CF +· 4 ions by measurement of the mass number and the mean natural lifetime [τ = (1.5 ± 0.3) × 10 −5 s] of these ions by changing the flight time in a double-focusing mass spectrometer (MCH 1310) are presented. The rate coefficient for collision-induced dissociation of CF +· 4 ions in collisions with CF 4 molecules ( k 4 = 2 × 10 −7 cm 3 s −1) is determined. The decay of CF +· 4 ions in the regions of the mass spectrometer is discussed.

The relationship between recombination, chemical activation and unimolecular dissociation rate coefficients

A new solution to the master equation relating the rate coefficients for unimolecular, recombination (association) and chemical activation reactions, incorporating weak collision effects, is presented. The solution establishes conditions for the validity of the commonly used procedure of relating the recombination rate coefficient, throughout the falloff regime, to the reverse single-channel unimolecular rate coefficient via the equilibrium constant. In addition, a relationship between the rate coefficient for stabilization in a chemical activation reaction and the reverse multichannel unimolecular dissociation rate coefficient is derived. This result, in conjunction with recently developed methods for fully incorporating angular momentum conservation into the solution of the master equation for unimolecular dissociation, enables both angular momentum and weak collision effects to be accurately incorporated into the solution of the master equation for chemical activation reactions in the falloff regime. Application of this method to a typical ion/molecule chemical activation reaction, that of CH+3 with NH3, illustrates the importance of weak collision and angular momentum effects in this system.