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The possibilities to extract incompletely characterized molecular parameters from experimental thermal rate constants for dissociation and recombination reactions are explored. The reaction system CF4 (+M) ⇄ CF3 + F (+M) is chosen as a representative example. A set of falloff curves is constructed and compared with the available experimental database. Agreement is achieved by minor (unfortunately not separable) adjustments of reaction enthalpy and collisional energy transfer parameters.

The thermal dissociation/recombination reactions of the perfluoromethyl halides CF4↔CF3+F, CF3Cl↔CF3+Cl, CF3Br↔CF3+Br, and CF3I↔CF3+I are analyzed with respect to their transition (for increasing pressures) from second-order to first-order dissociation (or from third-order to second-order recombination). The dependence of this transition on the temperature is documented. Practical aspects of the modelling of falloff curves are discussed.

This article, the eighth in the series, presents kinetic and photochemical data sheets evaluated by the IUPAC Task Group on Atmospheric Chemical Kinetic Data Evaluation. It covers the gas-phase thermal and photochemical reactions of organic species with four, or more, carbon atoms (≥ C4) available on the IUPAC website in 2021, including thermal reactions of closed-shell organic species with HO and NO3 radicals and their photolysis. The present work is a continuation of volume II (Atkinson et al., 2006), with new reactions and updated data sheets for reactions of HO (77 reactions) and NO3 (36 reactions) with ≥ C4 organics, including alkanes, alkenes, dienes, aromatics, oxygenated, organic nitrates and nitro compounds in addition to photochemical processes for nine species. The article consists of a summary table (Table 1), containing the recommended kinetic parameters for the evaluated reactions, and a supplement containing the data sheets, which provide information upon which recommendations are made.

The UV absorption spectrum of C2F5I was studied in shock waves over the temperature range 580–1200 K and, together with room temperature data, was represented in extended Sulzer–Wieland form. As a case study, the work illustrates the properties of high-temperature UV absorption continua for use in kinetics experiments in shock waves. A comparison with experimental data for CF3I and their representation is made. Quantum-chemical model calculations of positions and oscillator strengths are tested and, besides CF3I and C2F5I, applied also to spectra of C2F4I, C2F4, CF2I, C2F2, C2F, CF2, CFI, CF, and IF. Likewise, modelling and experimental results for the high-temperature UV absorption spectra of CF3 and C2F5 radicals are considered.

Simplified representations of branching fractions for thermal unimolecular two-channel reactions are discussed. The dissociation of formaldehyde serves as an illustrative example. Quantum-corrected classical trajectory calculations on an ab initio potential energy surface are combined with master equation calculations for collisional energy transfer. The treatment accounts for roaming atom dynamics. The dependence of the channel branching fractions on the bath gas pressure and temperature, on the collision efficiencies, and on the difference of channel threshold energies, are explored. It is discussed to what extent the derived simplified representations of channel branching fractions can be generalized.

The high-temperature hydrogenation of CF4 in mixtures of CF4 and H2 is assumed to involve the reaction H + CF4 → HF + CF3. The hydrogen atoms here are either formed by the reaction of F and CF3 (i.e., the products of the thermal dissociation of CF4) with H2, or by the thermal dissociation of H2. In the former case, a complicated chain process is started, while the reaction proceeds in a more direct way in the latter. This article determines the rate constant of the reaction H + CF4 → HF + CF3, characterizing its transition state by quantum-chemical methods.Over the temperature range 1000–3000 K, the most accurate results for the rate constant can be represented in the form 1.64 × 1014 (T/1000 K)1.95 exp(−178.8 kJ mol–1/RT) cm3 mol–1 s–1, based on coupled cluster theory extrapolated to the complete basis set limit, and incorporating vibrational anharmonicity, electron correlation through CCSDT(Q), and relativistic and non-Born–Oppenheimer effects., Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas

This article, the eighth in the series, presents kinetic and photochemical datasheets evaluated by the IUPAC Task Group on Atmospheric Chemical Kinetic Data Evaluation. It covers the gas phase thermal and photochemical reactions of organic species with four, or more, carbon atoms (≥ C4) available on the IUPAC website in 2019, including thermal reactions of closed-shell organic species with HO and NO3 radicals, and their photolysis. The article consists of a summary table, containing the recommended kinetic parameters for the evaluated reactions, and a supplement containing the datasheets, which provide information upon which the recommendations are made.

The temperature dependencies of the reactions MnO+ + H2/D2 from 150 to 600 K are measured in a selected- ion flow tube apparatus. The reactions are approximately 15% efficient at room temperature with rate constants (k = 2.7 ± 0.7 × 10−10 cm3 s-1 for H2 and 1.5 ± 0.4 × 10−10 cm3 s-1 for D2 at 300 K) that vary as T-1.1±0.2 and T-1.3±0.2, respectively. Both Mn+ and MnOH+/D+ products are observed, with Mn+ being the minority product in the H2 reaction, but the majority product in the D2 reaction. Reaction coordinates are explored using a variety of density functional methods, with quantitative but not qualitative differences in the results. Unlike the analogous FeO+ + H2 reaction, no evidence for spin inversion prior to the rate-limiting transition state is found. A crossing between septet and quintet potential energy surfaces in the product well exists, and the possibility of spin inversion on the way to products is discussed. The reaction coordinates inform a statistical treatment of the reaction, which well reproduces all kinetic results, including the product branching inversion observed due to isotope substitution.

New flowing afterglow/Langmuir probe investigations of electronic attachment to SF

As we approach the end of the second decade of the 21st century, changes in atmospheric chemical composition due to anthropogenic pollution continue to challenge the well-being of society. The IUPAC effort in atmospheric chemistry data evaluation can be traced back over 40 years. Global concerns over potentially catastrophic stratospheric ozone depletion resulting from emissions of chlorofluorocarbons (CFCs) led to the creation of the CODATA Task Group on Chemical Kinetics in 1977. The task of the CODATA group was to provide the evaluated kinetic data for atmospheric reactions needed to assess the threat to stratospheric ozone. In 1989, sponsorship of the data evaluation effort was transferred to IUPAC, leading to the formation of the IUPAC Task Group on Atmospheric Chemical Kinetic Data Evaluation.

This work suggests a connection between Landau-Zener transition probabilities between two crossing potentials in the classically accessible WKB regime and Landau-Lifshitz transition probabilities in the classically inaccessible WKB regime. It is based on the uniform Airy (UAi) approximation which represents a generalization of quantum transition probabilities for linear crossing potentials with constant coupling. The performance of the UAi approximation is tested by comparison with distorted-wave probabilities for an exponential potential model and illustrated for potentials that determine the intersection of two ab initio vibronic potential surfaces of the NO-Ar system.

The pressure and temperature dependences of the reactions of Fe+ with methyl halides CH3X (X = Cl, Br, I) in He were measured in a selected ion flow tube over the ranges 0.4 to 1.2 Torr and 300-600 K. FeX+ was observed for all three halides and FeCH3+ was observed for the CH3I reaction. FeCH3X+ adducts (for all X) were detected in all reactions. The results were interpreted assuming two-state reactivity with spin-inversions between sextet and quartet potentials. Kinetic modeling allowed for a quantitative representation of the experiments and for extrapolation to conditions outside the experimentally accessible range. The modeling required quantum-chemical calculations of molecular parameters and detailed accounting of angular momentum effects. The results show that the FeX+ products come via an insertion mechanism, while the FeCH3+ can be produced from either insertion or SN2 mechanisms, but the latter we conclude is unlikely at thermal energies. A statistical modeling cannot reproduce the competition between the bimolecular pathways in the CH3I reaction, indicating that some more direct process must be important.

Two-channel and multichannel thermal unimolecular reactions are analyzed by simple models, starting with the calculation of separated-channel rate constants and accounting for intrinsic channel coupling afterward. Reactions with rigid- and with loose-activated complex channels are distinguished. Weak-collision, energy-transfer, effects are suggested to govern the competition between rigid-activated complex channels, while angular-momentum, “rotational channel switching”, effects dominate the competition between rigid- and loose-activated complex channels. The models are tested against master equation treatments of the dissociations of formaldehyde and of glyoxal from the literature. Besides giving insight into the influence of various molecular input parameters, the present approach leads to compact representations of rate constants suitable for inclusion in databases.

We treat the competition between autodetachment of electrons and unimolecular dissociation of excited molecular anions as a rigid-/loose-activated complex multichannel reaction system. To start, the temperature and pressure dependences under thermal excitation conditions are represented in terms of falloff curves of separated single-channel processes within the framework of unimolecular reaction kinetics. Channel couplings, caused by collisional energy transfer and "rotational channel switching" due to angular momentum effects, are introduced afterward. The importance of angular momentum considerations is stressed in addition to the usual energy treatment. Non-thermal excitation conditions, such as typical for chemical activation and complex-forming bimolecular reactions, are considered as well. The dynamics of excited SF6- anions serves as the principal example. Other anions such as CF3- and POCl3- are also discussed. peerReviewed

New flowing afterglow/Langmuir probe investigations of electronic attachment to SF6 are described. Thermal attachment rate constants are found to increase from 1.5 × 10−7 cm3 s−1 at 200 K to 2.3 × 10−7 cm3 s−1 at 300 K. Attachment rate constants over the range of 200–700 K (from the present work and the literature), together with earlier measurements of attachment cross sections, are analyzed with respect to electronic and nuclear contributions. The latter suggest that only a small nuclear barrier (of the order of 20 meV) on the way from SF6 to SF6− has to be overcome. The analysis shows that not only s-waves but also higher partial waves have to be taken into account. Likewise, finite-size effects of the neutral target contribute in a non-negligible manner.

Unravelling elementary steps, reaction pathways, and kinetic mechanisms is key to understanding the behaviour of many real-world chemical systems that span from the troposphere or even interstellar media to engines and process reactors. Recent work in chemical kinetics provides detailed information on the reactive changes occurring in chemical systems, often on the atomic or molecular scale. The optimisation of practical processes, for instance in combustion, catalysis, battery technology, polymerisation, and nanoparticle production, can profit from a sound knowledge of the underlying fundamental chemical kinetics. Reaction mechanisms can combine information gained from theory and experiments to enable the predictive simulation and optimisation of the crucial process variables and influences on the system's behaviour that may be exploited for both monitoring and control. Chemical kinetics, as one of the pillars of Physical Chemistry, thus contributes importantly to understanding and describing natural environments and technical processes and is becoming increasingly relevant for interactions in and with the real world.

To provide a deeper understanding of the kinetics of electron attachment to CF3, the six-dimensional potential energy surfaces of both CF3 and CF3− were developed by fitting ∼3000 ab initio points per surface at the AE-CCSD(T)-F12a/AVTZ level using the permutation invariant polynomial-neural network (PIP-NN) approach. The fitted potential energy surfaces for CF3 and CF3− had root mean square fitting errors relative to the ab initio calculations of 1.2 and 1.8 cm−1, respectively. The main active mode for the crossing between the two potential energy surfaces was identified as the umbrella bending mode of CF3 in C3v symmetry. The lowest energy crossing point is located at RCF = 1.306 A and θFCF = 113.6° with the energy of 0.051 eV above the minimum of the CF3 electronic surface. This value is only slightly larger than the experimental data 0.026 ± 0.01 eV determined by kinetic modeling of electron attachment to CF3. The small discrepancy between the theoretical and experimentally measured values is analyzed.

The rates of the reactions Fe(+) + N2O → FeO(+) + N2 and FeO(+) + CO → Fe(+) + CO2 are modeled by statistical rate theory accounting for energy- and angular momentum-specific rate constants for formation of the primary and secondary cationic adducts and their backward and forward reactions. The reactions are both suggested to proceed on sextet and quartet potential energy surfaces with efficient, but probably not complete, equilibration by spin-inversion of the populations of the sextet and quartet adducts. The influence of spin-inversion on the overall reaction rate is investigated. The differences of the two reaction rates mostly are due to different numbers of entrance states (atom + linear rotor or linear rotor + linear rotor, respectively). The reaction Fe(+) + N2O was studied either with (6)Fe(+) or with (4)Fe(+) reactants. Differences in the rate constants of (6)Fe(+) and (4)Fe(+) reacting with N2O are attributed to different contributions from electronically excited potential energy surfaces, such as they originate from the open-electronic shell reactants.

The pressure and temperature dependences of the reactions of Fe

The relationship between rate constants for dissociation and the reverse association reactions and their potential energy surfaces is illustrated. The reaction systems e(-) + SF6 SF6- -> SF5- + F, H + CH3 CH4, 2 CF2 C2F4, H + O-2 -> HO2, HO + O HO2 H + O-2, and C + HO -> CHO are chosen as representative examples. The necessity to know precise thermochemical data is emphasised. The interplay between attractive and anisotropic components of the potentials influences the rate constants. Spin-orbit and electronic-rotational coupling in reactions between electronic open-shell radicals so far generally has been neglected, but is shown to have a marked influence on low temperature rate constants.

Expressions for representing the pressure dependence of unimolecular dissociation and the reverse recombination reactions are compared. Situations are considered where the broadening of the corresponding falloff curves is particularly pronounced, i.e. where broadening factors at the center of the falloff curves, F cent, are very small and falloff curves correspondingly become very “broad”. Such situations arise when unimolecular reactions of molecules with large numbers of low-frequency modes and high-temperature situations are considered. Recombination reactions of polyatomic species in atmospheric chemistry and dissociation reactions of large molecules in combustion are the fields of application of the present results. While previously proposed expressions for asymmetric broadening factors showed artifacts when F cent decreased to values below about 0.4, alternative functions are described which avoid these problems for situations where F cent decreases to values far below 0.1.

The thermal decomposition of CF4 (+Ar) → CF3 + F (+Ar) was studied in shock waves over the temperature range 2000-3000 K varying the bath gas concentration [Ar] between 4 × 10(-6) and 9 × 10(-5) mol cm(-3). It is shown that the reaction corresponds to the intermediate range of the falloff curve. By combination with room temperature data for the reverse reaction CF3 + F (+He) → CF4 (+He) and applying unimolecular rate theory, falloff curves over the temperature range 300-6000 K are modeled. A comparison with the reaction system CH4 (+M) ⇔ CH3 + H (+M) is made.

The kinetics of the reaction CH3OCH3 + Fe(+) has been studied between 250 and 600 K in the buffer gas He at pressures between 0.4 and 1.6 Torr. Total rate constants and branching ratios for the formation of Fe(+)O(CH3)2 adducts and of Fe(+)OCH2 + CH4 products were determined. Quantum-chemical calculations provided the parameters required for an analysis in terms of statistical unimolecular rate theory. The analysis employed a recently developed simplified representation of the rates of complex-forming bimolecular reactions, separating association and chemical activation contributions. Satisfactory agreement between experimental results and kinetic modeling was obtained that allows for an extrapolation of the data over wide ranges of conditions. Possible reaction pathways with or without spin-inversion are discussed in relation to the kinetic modeling results.

We report for the first time temperature dependences (from 300 to 600 K) of the reactions of Fe(+) and FeO(+) with NO and NO(2). Both ions react quickly with NO(2), and their rate constants have weak negative temperature dependences. The former is consistent with the calculated energy profile along the Fe(+) + NO(2) reaction coordinate. Ground state Fe(+) reacts with NO(2) to produce only FeO(+), while FeO(+) reacts with NO(2) to produce NO(+) exclusively. Certain source conditions produce excited Fe(+), as evidenced by production of primary NO(+), which is endothermic with the ground state by 0.35 eV. The room temperature rate constants are in agreement with previous values. For the reactions of Fe(+) and FeO(+) with NO, we find an upper limit of

Rate coefficients for attachment of electrons to POCl3 have been measured in two FALP (flowing-afterglow Langmuir-probe) apparatuses, extending the temperature range of earlier experiments. The results for equal gas and electron temperatures are compared with data for independently varied temperatures. An analysis of the rate coefficients in terms of electron-polar target capture theory leads to empirical electron–phonon coupling factors which are markedly smaller than unity. These factors depend on the electron energy but do not seem to depend on the gas temperature, which is in contrast to observations made for the electron attachment to SF6. Besides s-wave attachment, contributions from higher partial waves are analyzed and suggested to contribute to some extent. Attachment cross sections and specific rate constants for electron autodetachment are finally constructed in a way which is consistent with the experimental attachment rate coefficients. Autodetachment is shown to be negligible compared to dissociation of POCl3− which is analyzed in a subsequent publication.

The unimolecular thermal decomposition reaction CHF3 (+M)→CF2 + HF (+M) was studied in shock waves by monitoring the UV absorption of the forming CF2 radicals. The results of the present and previous experiments on the temperature and pressure dependence of the rate constants are analyzed in terms of unimolecular rate theory. Falloff curves (for 1500–1900 K) are represented in terms of fitted limiting low pressure rate constants k 0 = [Ar] 1.1 × 1016exp(−53.0 kcal mol−1/RT) cm3mol−1s−1, limiting high pressure rate constants k ∞ = 1.25 × 1015 exp(−75.8 kcal mol−1/RT) s−1 from quantum-chemical calculations, and center broadening factors F cent = 0.170(± 0.04) including strong and weak collision contributions from unimolecular rate theory. With these results, approximate analytical expressions of the falloff curves well represent the measured and calculated rate constants over wide ranges of pressure and temperature.

Experimental rate coefficients for the thermal dissociation H2O2 + (M) → 2OH(+M) and the reverse recombination 2OH(+M) → H2O2(+M) are combined with recent modelling results based on an ab initio potential energy surface. The rate coefficients derived from the complementary experimental and modelling results are represented over wide ranges of temperature and pressure, such as being of interest for practical applications. Limiting low and high pressure rate coefficients as well as detailed and simplified expressions for rate coefficients in the falloff range are given.

The quantum dynamics of the fall of a charged particle (i.e., the capture of a charged particle) onto a stationary dipolar target is considered. Extending previous approaches for the calculation of rate coefficients in the lowest channels, we now determine rate coefficients for all channels until the quantum rate coefficients converge to their classical counterpart. The results bridge the gap between the capture of light particles (electrons) and heavy particles (ions) in the limit of sudden dynamics, when the collision time is short in comparison to the rotational period of the molecular target. The quantum-classical correspondence is discussed in terms of semiclassical numbers of channels which are open for capture in effective potentials formed by charge-dipole attraction and centrifugal repulsion. The quantum capture rate coefficients are presented through classical rate coefficients and correction factors that converge to unity for high temperatures and whose behavior at ultralow temperatures, for not too small values of the dipole moment, is determined by semiclassical numbers of capture channels.

Rate constants for capture-controlled ion-molecule reactions and ion fragmentations are analyzed. It is shown that exothermic ion-molecule reactions governed by long-range potentials (class I reactions) and ion fragmentations and the reverse ion-molecule associations with large contributions from valence potentials (class II reactions) require different treatments. The results are expressed by phase space theory and rigidity factors accounting for the anisotropy of the potentials. Thermal and specific rigidity factors, f rigid(T) and f rigid(E,J)respectively, need to be internally consistent. Simplified two-parameter expressions for the two rigidity factors are designed with the hope that fitting of the parameters to limited experimental data allows one to extrapolate to unexplored ranges of conditions.

Specific rate constants k(E) of the dissociation of the halobenzene ions C6H5X+ --C6H5+ + X* (X* = Cl, Br, and I) were measured over a range of 10(3)-10(7) s-1 by threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy. The experimental data were analyzed by various statistical unimolecular rate theories in order to derive the threshold energies E0. Although rigid activated complex RRKM theory fits the data in the experimentally measured energy range, it significantly underestimates E0 for chloro- and bromobenzene. Phase space theory (PST) does not fit the experimentally measured rates. A parametrized version of the variational transition state theory (VTST) as well as a simplified version of the statistical adiabatic channel model (SSACM) incorporating an energy dependent rigidity factor provide excellent fits to the experimental data and predict the correct dissociation energies. Although both approaches have just two adjustable parameters, one of which is E0, SSACM is effective and particularly simple to apply.

Experimental quantum yields of the photolysis of formaldehyde at lambda310 nm are combined with absolute and relative rate calculations for the molecular elimination H2CO --H2 + CO (1), the bond fission H2CO --H + HCO (2), and the intramolecular hydrogen abstraction H2CO --H ... HCO --H2 + CO (3) taking place in the electronic ground state. Temperature and pressure dependencies of the quantum yields are analyzed with the goal to achieve consistency between experiment and modeling. Two wavelength ranges with considerably different properties are considered: 340-360 nm, where channel 1 competes with collisional deactivation of excited molecules, and 310-340 nm, which is dominated by the competition between the formation of radical and molecular products. The close relation between photolysis and pyrolysis of formaldehyde, such as analyzed for the pyrolysis in the companion paper, is documented and an internally consistent treatment of the two reaction systems is provided. The quantum yields are modeled and represented in analytical form such that values outside the available experimental range can be predicted to some extent.

New experimental results for the thermal dissociation of formaldehyde to radical and molecular products (Proc. Combust. Inst. 2007, 31, in press) form the basis of the present analysis of the respective low-pressure rate coefficients k(Rad,0) and k(Mol,0) of the reaction. The article supersedes an earlier analysis (J. Phys. Chem. A 2005, 109, 8320) which used less accurate and more preliminary input information. In addition, refined rotational factors F(rot) are determined and specific energy and angular momentum dependent branching ratios from a more detailed analysis of photolysis quantum yields (J. Phys. Chem. A 2007, 111, 3868) are employed as well. It is emphasized again that pyrolysis and photolysis are intimately related and should be analyzed in an internally consistent manner. The combination of the new with earlier experimental results for pyrolysis rates allows one to fit the height of the energy barrier for the molecular elimination channel with improved precision. A value of E0,1 = 81.7(+/-0.5) kcal mol(-1) is obtained. In addition, employing anharmonicity factors F(anh) from the earlier work, a total average energy transferred per collision of -DeltaE/hc = 100(+/-20) cm(-1) is fitted from the experiments in the bath gas Ar. This value is consistent with the value -DeltaE/hc = 80(+/-40) cm(-1) for the bath gas N(2) such as fitted from photolysis quenching experiments (using the same molecular parameters as for the pyrolysis). Rate coefficients for the temperature range 1200-3500 K are represented in the form k(Mol,0)/[Ar] = 7.3 x 10(14) T -6.1 exp(-47300 K/T) cm(3) molecule(-1) s(-1) and k(Rad,0)/[Ar] = 2.1 x 10(12) T -5.5 exp(-47300 K/T) cm(3) molecule(-1) s(-1) (accuracy +/-25%) and recommended for use in combustion chemistry.

We investigated spectroscopic and dynamic fluorescence properties of the S1

The rate constants of the recombination reaction of p-fluorobenzyl radicals, p-F-C6H4CH2 + p-F-C6H4CH2 (+M) --C14H12F2 (+M), have been measured over the pressure range 0.2-800 bar and the temperature range 255-420 K. Helium, argon, and CO2 were employed as bath gases (M). At pressures below 0.9 bar in Ar and CO2, and 40 bar in He, the rate constant k1 showed no dependence on the pressure and the nature of the bath gas, clearly indicating that it had reached the limiting high-pressure value of the energy-transfer (ET) mechanism (k(1,infinity)ET). A value of k(1,infinity)ET(T) = (4.3 +/- 0.5) x 10(-11) (T/300 K)(-0.2) cm3 molecule(-1) s(-1) was determined. At pressures above about 5 bar, the k1 values in Ar and CO2 were found to gradually increase in a pressure range where according to energy-transfer mechanism, they should remain at the constant value k(1,infinity)ET. The enhancement of the recombination rate constant beyond the value k(1,infinity)ET increased in the order HeArCO2, and it became more pronounced with decreasing temperature. The dependences of k1 on pressure, temperature, and the bath gas were similar to previous observations in the recombination of benzyl radicals. The effect of fluorine-substitution of the benzyl ring on k1 values is discussed. The present results confirm the significant role of radical complexes in the recombination kinetics of benzyl-type radicals in the gas-liquid transition range. The observations on a rate enhancement beyond the experimental value of k(1,infinity)ET at elevated densities up to the onset of diffusion-control are consistently explained by the kinetic contribution of a "radical-complex" mechanism which is solely based on standard van der Waals interaction between radicals and bath gases.

This review of the role of reaction kinetics in combustion chemistry traces the historical evolution and present state of qualitative and quantitative understanding of a number of reaction systems. Starting from the H2–O2 system, in particular from the reaction between H and O2, mechanisms and key reactions for soot formation, for the appearance of NOx, and for processes of peroxy radicals in hydrocarbon oxidation are illustrated. The struggle for precise rate constants on the experimental and theoretical side is demonstrated for the example of the reaction H + O2 → OH + O. The intrinsic complexity of complex-forming bimolecular reactions, such as observed even in this reaction, also dominates most other key reactions of the systems considered and can be unravelled only with the help of quantum-chemical methods. The multi-channel character of these reactions often also requires the combination with master equation codes. Although kinetics provides an already impressive database for quantitative modelling of simple combustion systems, considerable effort is still required to quantitatively account for the complexities of more complicated fuel oxidation processes.

Laser ablation of polyimide (PI) and polymethyl-methacrylate (PMMA) at 248 nm with pulse lengths τ ranging from 200 fs to 200 ps was investigated. The measured ablation rates show minima for pulse lengths of about 5 ps (PMMA) or 50 ps (PI).

Variational transition state theory (VTST) is widely used for the modelling of barrierless radical-radical recombination reactions. In this application, VTST suffers from a number of limitations some of which are of more technical, others of more fundamental nature. The former are caused by inappropriate averaging over individual adiabatic channel potentials or by the neglect of quantum effects, the latter are due to deviations from adiabatic dynamics. It is shown that most radical-radical recombination reactions are characterized by Massey parameters which are smaller than unity such that the dynamics is nonadiabatic. VTST treatments which generally assume adiabatic dynamics, therefore, have a fundamental problem. Calculations of rate constants by VTST often exceed classical trajectory results by about 10 to 20percent. This is normally attributed to “recrossing trajectories”. In the present work it is shown, however, that deviations of this magnitude also have to be expected for nonadiabatic dynamics in comparison to adiabatic dynamics. It is, therefore, suggested that “recrossing” at least in part has to be attributed to nonadiabatic dynamics. A way out of the dilemma is the use of a combination of statistical adiabatic channel and classical trajectory concepts.

The combination reactions CCl 3 + CCl 3 (+ M) → C 2 Cl 6 (+ M) and CCl 3 + Br (+ M) → CCl 3 Br (+ M) (with rate constants of k 1 and k 2 , respectively) were studied at temperatures of 250 and 300 K over the pressure range of 0.01-1000 bar. Helium, argon, xenon, N 2 , CO 2 , and SF 6 were used as bath gases. CCl 3 radicals were generated via the photolysis of CCl 3 Br at 248 nm, and their absorption was monitored at 223.5 nm. The limiting "high-pressure" rate constants within the energy-transfer mechanism were determined, independent of density and the choice of the bath gas, over the pressure range of 1-10 bar, to be k 1 , ∞ (T) = (1.0 ′ 0.2) × 10 - 1 1 (T/300 K) - 0 . 1 7 cm 3 molecule - 1 s - 1 and k 2 , ∞ (T) = (2.0 ′ 0.2) x 10 - 1 1 (T/300 K) - 0 . 1 3 cm 3 molecule - 1 s - 1 . In the helium, N 2 , and argon bath gases, at pressures above ∼40 bar, the reactions became increasingly faster when the pressure was further raised until they finally started to slow at densities where diffusion-controlled kinetics dominates. This is the first detailed report of such a peculiar density dependence of combination rate constants for larger radicals with five or eight atoms. Possible origins of these pressure effects, such as the influence of the radical-complex mechanism and the density dependence of electronic quenching, are discussed.

The validity of Smoluchowski's theory is demonstrated by its application to the bimolecular triplet–triplet annihilation (TTA) T1 + T1 → S0 + S1 in viscous solution. For that purpose the delayed S1 → S0 fluorescence of anthracene in cis-1,3-dimethylcyclohexane/trans-1,4-dimethylcyclohexane (1∶1) was measured after UV excitation with long laser pulses (∼30 μs) in a temperature range where TTA is completely diffusion controlled. Through the adequate treatment of the kinetics of TTA upon spatially homogeneous and spatially periodic excitation (which is partially based on the results of previous publications) absolute values for the diffusion coefficients D of triplet molecules, for the effective annihilation distance RA and for the Forster radius RST of heterogeneous annihilation S1 + T1 → S0 + Tn were determined. The temperature dependence of all three parameters (D, RA and RST) is discussed. From the decrease of RA with increasing temperature, the parameters A and L can be determined, which define the exponential distance dependence of the annihilation of a triplet pair in the kinetic model of Butler and Pilling (1977). Differences of the two models and applicability criteria are discussed.