Volume 215, Issue 8 - 8

A fuel-rich, non-sooting (C/O = 0.773) 1-pentene/oxygen/argon flame is studied experimentally at 50 mbar. The results are compared to computations. The calculations show satisfactory agreement with the data obtained from the measurements. The main channels for 1-pentene decomposition are presented and discussed in view of previous data for a propene flame at similar experimental conditions. Special emphasis is directed towards the formation of the first aromatic ring and the further growth of small aromatic hydrocarbons. By reaction flow analysis, it is found that the major reaction channel for benzene formation results from the recombination of propargyl radicals. Furthermore, the major reaction channels for naphthalene formation are presented.

Ignition delay times have been measured behind reflected shock waves in ethane-oxygen-argon mixtures at temperatures between 1150 and 1500 K and pre-ignition pressures between 10 and 14 atm. Delay times have been measured both by pressure rise and OH absorption at 307 nm. Kinetic modelling of the ignition delays has been made using the GRIMech 3.0 mechanism which with addition of several reactions involving HO 2 radicals and an increase in the barrier for reaction between C 2 H 5 and O 2 can satisfactorily model delays over the studied equivalence ratios of φ = 1.7 to 0.68. Addition of 2H-heptafluoropropane up to 13 mol % (of ethane) had little inhibition effect on stoichiometric ethane-oxygen-argon mixtures but inhibited ignition in a mixture of φ = 1.5. A kinetic mechanism is presented to model the inhibition process.

Variable Pressure Flow Reactor (VPFR) experiments were conducted for ethene/methane mixtures utilizing both FTIR analyses of extracted gas samples and in situ line-of-sight far-UV absorption measurements. In situ observations of ketene prompted the re-evaluation of the presence of ketene in previous acetaldehyde oxidation and pyrolysis experiments in the VPFR. In these experiments only extractive sampling (with FTIR analysis) had been used and ketene had not been observed. In reconsidering the previously obtained FTIR spectra, ketene is shown to be not only present, but also present in levels on the order of other important acetaldehyde reaction intermediates, such as CH 2 O, H 2 , and C 2 H 4 . Through a comparison of the results of the in situ and extractive ketene diagnostics, it has been shown that significant losses of ketene can occur in sample handling and that the principal ketene loss mechanism appears to be surface dominated. It is likely that this difficulty in ketene sampling, common to many experimental systems, has led to the omission of important ketene pathways in many kinetic models.

This work discusses the prospects and feasibility of optical spectroscopy and microscopy of single molecules at nanometer resolution via apertureless, antenna-based near-field scanning optical microscopy. First, different near-field optical methods are compared, which detect the weak scattering or fluorescence from a probe–single molecule interaction at high spatial resolution. Specifically, ultimate sensitivities of coherent (bright-field) and non-coherent (dark-field) apertureless near-field microscopes for resonant (e.g., scattering, absorption) and non-resonant (e.g., n -photon fluorescence) imaging applications are discussed and key differences between absorption and scattering/fluorescence methods are elucidated. Secondly, the enhanced near-field interaction between a probe and a luminescent polystyrene nanosphere (doped with fluorescein-like dye) is investigated as a function of the sample size, which reveals a strong probe-induced enhancement of the fluorescence. Finally, the data are used to predict ultimate sensitivities as well as limitations of current apertureless near-field optical techniques.

The microcanonical variational RRKM formalism combined with high level ab initio and density functional calculations has been applied to the reaction of CH( 2 Π) with molecular nitrogen at low and moderate temperatures. The existing experimental T , P -dependent data for T < 1000 K could be theoretically accounted for. Our simulations reproduced very well the strong negative temperature and strong positive pressure effects observed experimentally. The comparison with less rigorous variational techniques demonstrates the importance of the microscopic rate constant minimization at the E and J resolved level.

The importance of thermal decompositions during the oxidation of methyl tertbutyl ether (MTBE) has motivated a theoretical investigation of reactions: C 4 H 9 OCH 2  → t-C 4 H 9  + CH 2 O Eq. (r.1), C 4 H 8 OCH 3  → i-C 4 H 8  + CH 3 O Eq. (r.2) in comparison with reaction CH 3 OCH 2  → CH 3  + CH 2 O Eq. (r.3). The conventional ab-initio molecular orbital procedures have been used to calculate the potential energy surface of the corresponding reactions and the thermodynamic properties of the reactants and the activated complex. The transition state theory (TST) has allowed to deduce the unimolecular rate constants: k 1  = 4.0 × 10 14 exp(−23800/ RT ) s −1 , k 2  = 1.5 × 10 13 exp(−21800/ RT ) s −1 , k 3  = 2.32 × 10 14 exp(−28190/ RT ) s −1 ( E a : cal mol -1 ). The rate constant obtained for reaction (r.3) from our calculations is in good agreement with the literature data. No experimental data are available for reactions (r.1) and (r.2). Use of Troe´s formalism allows to evaluate the influence of pressure on the rate constants below 1200 K.

The influence of the long range forces between an atom and a diatomic fragment on the density of vibrational bound states is studied theoretically in the vicinity of the dissociation threshold of a model triatomic molecule. In the two-dimensional case (2D), where the non dissociating bond is frozen at the equilibrium geometry, the number of quantum states and their density is shown to increase quickly in a small energy interval close to the dissociation threshold  D 0 , due to the long range forces between the fragments. In addition to the strongly coupled states already known at low energies, a new family of states emerges, whose wavefunctions extend to large internuclear distances and correspond to orbiting motion of the two fragments around each other. The semiclassical number of states in 2D is shown to be in excellent agreement with the exact quantum mechanical result. In contrast, semiclassical calculations of the three-dimensional (3D) density and number of states completely fail in the vicinity of the dissociation limit, because of the quantum character of the high frequency vibration of the diatomic fragment. An adiabatic approach is proposed and discussed, that allows one to estimate the 3D density of states close to the dissociation threshold.