Band 219, Heft 5 - 5

Laser-based imaging diagnostics allow the quantitative determination of scalar fields (concentrations, temperature) in combustion environments. Beyond instantaneous two-dimensional distributions, multi-dimensional imaging can provide simultaneous information about concentration of more than one species and temperature, about three-dimensional concentration and temperature distributions as well as the temporal evolution of reactions, flows and evaporation processes. Various examples in combustion environments are presented.

Incubation prior to the thermal decomposition of CO 2 is observed for the first time behind shock waves, confirming the expected bottleneck in collisional activation of this triatomic molecule. The thermal decomposition of carbon dioxide has been investigated behind reflected shock waves at temperatures of 3200–4600 K and pressures of 45–100 kPa. Ultraviolet laser absorption was used to monitor the CO 2 concentration with microsecond time resolution, allowing observation of a pronounced incubation period prior to steady CO 2 dissociation. Master equation simulations, with a simple model for collisional energy transfer, were carried out to describe the measured incubation times and dissociation rate coefficient. The second order rate coefficient for CO 2 dissociation was found to be 3.14 × 10 14 exp(−51300 K/ T ) cm 3 mol −1 s −1 . The number of incubation collisions was found to range from 7 × 10 3 at 4600 K to 3.5 × 10 4 at 3200 K. The master equation simulations suggest that the energy transferred per collision must have a greater than linear dependence on energy.

H and D photofragments, produced via vibrationally mediated photodissociation of jet-cooled C 2 HD allowed measurement of vibrational and vibronic excitation action spectra of the parent as well as H and D Doppler profiles. The initial vibrational preparation at energies around 9700 cm −1 enhanced C—H and C—D bond cleavage for transitions from an initially excited bent state to the upper A ~ state, rather than from a close lying state containing three C—H stretch quanta, due to favorable Franck—Condon factors for the former. The bound region of the A ~ state was accessed, resulting in non-adiabatic dissociation and slight preferential C—H over C—D bond cleavage.

Soot is one of the most important pollutants originating from combustion. Despite recent advances in the measurement of size and composition of soot particles, their actual formation mechanism is still under debate. It depends on fuel, stoichiometry, temperature, flow conditions and the concentration of a large number of intermediate species. An adequate characterization of this complex reaction system generally requires the use of several complementary techniques. In this article, we present measurements aiming to study reactions in fuel-rich flames using several complementary techniques. Only with a combination of optical and mass-spectrometric measurements, important features of the early polyaromatic hydrocarbon (PAH) and soot formation chemistry are accessible in detail. Three different techniques are combined to investigate one-dimensional laboratory flames on the same low-pressure burner and their respective merits are discussed: (i) cavity ring-down spectroscopy (CRDS) for the detection of small radicals and measurement of the temperature, (ii) mass spectrometry with electron-impact (EI) ionization in order to measure species with molecular weights up to m / e = 90, and (iii) mass spectrometry with resonantly-enhanced multi-photon ionization (REMPI) to distinguish isomers with masses up to m / e = 178. Measurements of this type may prove a valuable input to improve kinetic and combustion models.

Time-resolved infrared-ultraviolet double resonance (IR-UV DR) spectroscopy is used to study the kinetics of collision-induced state-to-state molecular energy transfer between rovibrational states in the 12700-cm −1 4ν CH manifold of the electronic ground state of acetylene (C 2 H 2 ). Particular initial and final rovibrational J -states are prepared and monitored by a pair of tunable laser pulses (IR PUMP and UV PROBE) and the kinetic results recorded by continuously varying the time delay between those pulses at a set sample pressure. After allowing for collision-induced quenching of fluorescence and mass transfer from the IR-UV optical excitation zone (by beam flyout and diffusion), an array of kinetic data for J -resolved energy-transfer channels can be interpreted by means of a mechanistically structured master-equation model. This paper focuses on kinetics derived by probing C 2 H 2 in its 4ν CH J = 12 state (which is affected by intramolecular perturbations and implicated in unusual collision-induced quasi-continuous background effects) and J -resolved collision-induced rovibrational energy transfer with both even Δ J and (supposedly forbidden) odd Δ J .

By applying joint velocity/scalar measurements, conditional mean axial and radial velocity components are determined in the vicinity of the turbulent flame front of a premixed swirling natural gas/air flame. Two velocity components are measured by laser Doppler velocimetry (LDV) simultaneously with planar laser-induced fluorescence (PLIF) monitoring the hydroxyl radical (OH). The spatial OH distribution is used to tag the instantaneous position of the flame front and can be thereby used to derive conditional velocities. From these measurements the turbulent flux of the mean progress variable is determined which is an important quantity to model premixed flames. One important finding is that for this rather complex but technically realistic geometry, at some locations in the flame brush conditional axial and radial velocities show different behaviour. Thus gas expansion effects can dominate axially whereas in radial direction turbulent fluctuations determine the direction of turbulent transport.

The chemical vapor deposition (CVD) process of silicon from silane has been analyzed by a combined experimental and modeling approach with the aim to optimize growth rate, yield and deposition quality of polycystalline silicon while minimizing gas phase nucleation. The effects of precursor concentration, flow rate and buffer gas have been investigated. Modeling of the gas phase and of the deposition process has been used in a parameter screening to select appropriate deposition conditions in a stagnation-point cold-wall CVD reactor. Deposition experiments have been performed for several silane concentrations with helium, nitrogen and hydrogen as the buffer gases. Polycrystalline films of good quality were obtained with growth rates up to 8 μm/min. The observed trends in the experiments are in good qualitative agreement with model predictions. To achieve high film growth rates and minimal gas phase nucleation, hydrogen is suggested as the diluent gas, potential reasons for this being its involvement in gas phase and surface reactions.

We demonstrate the potential of modern confocal fluorescence microscopy in combination with quenched peptide-based fluorescence probes for sensitive detection of p53 antibodies directly in homogeneous solution. Single tryptophan (Trp) residues in the sequences of short, synthetic peptide epitopes of the human p53 protein efficiently quench the fluorescence of an oxazine fluorophore attached to the amino terminal ends of the peptides. The fluorescence quenching mechanism is thought to be a photoinduced electron transfer reaction from Trp to the dye enabled by the formation of intramolecular complexes between dye and Trp. Specific recognition of the epitope by the antibody confines the conformational flexibility of the peptide. Consequently, complex formation between dye and Trp is abolished and fluorescence is recovered. Using fluorescence correlation spectroscopy (FCS), antibody binding can be monitored observing simultaneously two parameters: the diffusional mobility of the peptide as well as the quenching amplitude induced by the conformational flexibility of the peptide change significantly upon antibody binding. Furthermore, we demonstrate that the strong fluorescence increase upon binding can also be used to directly detect p53 autoantibodies from human blood serum samples in fluorescence intensity time traces. Our data demonstrate that new refined single-molecule fluorescence techniques in combination with quenched peptide epitopes open new possibilities for the reliable detection of antibody binding events in homogeneous solution.

We have applied time-resolved picosecond linear laser-induced fluorescence to obtain spatially resolved profiles of NO in laminar methane/air and methane/nitric oxide/air counterflow diffusion flames at atmospheric pressure. Temperature profiles have been measured with broadband CARS of nitrogen. Flames at strain rates from 59 s −1 to 269 s −1 were studied to characterize the strain rate dependence of the NO concentration. The work shows that NO concentrations decrease with increasing strain rate. Comparisons have been made with predicted NO levels obtained using two different chemical kinetic mechanisms (Lindstedt and co-workers and GRI-Mech. 3.0). Computed concentrations are shown to be in good agreement with experimental data. The addition of up to 600 ppm NO to the fuel did permit an assessment of differences in the reburn chemistry between the two mechanisms.

The distribution of translational, rotational, and vibrational energies in the fragments (benzylium ions and ethyl radicals) of the dissociation of n -propylbenzene cations has been determined by statistical adiabatic channel model/classical trajectory (SACM/CT) calculations. The reaction was treated by CT calculations of capture processes for transitional modes, starting with specified fragment energies. A short-range valence/long-range ion-induced dipole potential model for the transitional modes was employed. The derived distributions approach the results from phase space theory (PST) at small energies and angular momenta. At larger energies and angular momenta, the shapes of the distribution functions remain similar to those from PST; however, the average translational, rotational, and vibrational energies of the fragments increasingly differ from PST predictions. The present results are consistent with separate SACM/CT calculations on the same potential of specific rate constants k ( E , J ) and thermally averaged rate constants k ∞ ( T ) of the dissociation/recombination reaction.

Specific rate constants, k ( E , J ), and thermal capture rate constants, k cap ( T ), are determined by statistical adiabatic channel model/classical trajectory (SACM/CT) calculations for unimolecular dissociation and the reverse association reactions of representative polyatomic molecular ions. Simple short-range valence/long-range ion-induced dipole model potentials without reverse barriers have been employed, using the reactions C 8 H 10 + ⇔ C 7 H 7 + + CH 3 and C 9 H 12 + ⇔ C 7 H 7 + + C 2 H 5 as illustrative examples. Simplified representations of k ( E ) and k cap ( T ) from rigid activated complex Rice–Ramsperger–Kassel–Marcus (RRKM) theory are compared with the SACM/CT treatment and with experimental results. The Massey parameters of the transitional mode dynamics, for the systems considered, are smaller than unity such that their dynamics is nonadiabatic while the dynamics of the conserved modes is adiabatic. Because of the long-range/short-range switching character of the potential, simple rigid activated complex RRKM theory cannot be used without modifications. The effects of a shifting of the effective bottle-neck of the dynamics with increasing energy towards smaller interfragment distances in the present cases are amplified by a shift into a range of increasing anisotropy of the potential. As a consequence, the thermal capture rate constants markedly decrease with increasing temperature.