B. Mischler

Determination of rotational constants in a molecule by femtosecond four-wave mixing

Femtosecond time-resolved DFWM experiments were performed on CHCl3 and SO2. The recurrences observed originate from the preparation of rotational coherences within the sample. The determination of rotational constants for a symmetric and an asymmetric top are shown. The simulation of the transients for the symmetric top allows the extraction of the main moment of inertia B and the centrifugal terms perpendicular to the molecule axis. For the asymmetric top, all three rotational constants can be derived. In addition to the transients at 1/2(B + C), two additional types of asymmetry transients at 1/4C and 1/4A arise from this experiment. The first origins from the levels at low K and the second from the levels at high K in the high-J limit. Copyright

Stimulated emission pumping of OH and NH in flames by using two-color resonant four-wave mixing

Abstract In this work we examine the analytical potential of two-color resonant four-wave mixing for the determination and characterization of trace elements in a combustion environment. Experimental results for NH and OH in flames at atmospheric pressure are presented. The selectivity of the technique is used to simplify the Q-branch region of the (0-0) A3Π−X3Σ− vibronic transition of NH. Furthermore, substantial signal-to-noise ratios in the (0-0) A2Σ+−X2Π system of OH are achieved. The high sensitivity is applied to perform stimulated emission pumping involving the weak (0–1) vibrational band. In addition, we demonstrate that the technique is sensitiive to state changing collisions.

Femtosecond photoionization of (H2O)n and (D2O)n clusters

Cluster ion distributions of water in a molecular beam are investigated by femtosecond ionization at 780 nm and reflectron time-of-flight mass spectrometry. The electric field strength generated by the ultrashort laser pulses is sufficient to efficiently ionize most of the molecules that are present in the molecular beam. In this work ion signals of large water clusters containing up to 60 monomers are reported. Upon ionization rapid proton transfer is observed, leading to the formation of protonated water cluster ions. Unprotonated clusters (H2O)n+(n>2) are not observed in the mass spectra. The configurational energy imparted to the protonated clusters induces unimolecular dissociation on the μs time scale. These metastable reactions are characterized by modeling the ion trajectories in the mass spectrometer. The numerical procedure in conjunction with the integrated parent and daughter intensities results in unimolecular dissociation rates as a function of cluster size. Additional information about prot...

Absolute concentration measurements using DFWM and modeling of OH and S2 in a fuel-rich H2/air/SO2 flame

Concentration profiles of OH and S{sub 2} in a sulfur-containing premixed H{sub 2}/air flame were obtained and compared to numerical calculations. The measurements utilized degenerate four-wave mixing and absorption spectroscopy in parallel (MULTIPLEX spectroscopy) and were numerically simulated for a one-dimensional flat flame. An established reaction mechanism for hydrogen oxidation was extended by reactions for the sulfur chemistry drawn from the literature. The experimental results for the OH and S{sub 2} concentration profiles are in good qualitative agreement with the simulations. However, the model calculations underestimate the S{sub 2} concentrations by two orders of magnitude, indicating that important intermediates and reactions could have been omitted.

Degenerate Four-Wave Mixing of S2 and OH in Fuel-Rich Propane/Air/ S02 Flames

Absolute concentration profiles of S 2 and OH in a premixed propane/air/SO 2 flame at atmospheric pressure are determined by DFWM and absorption spectroscopy in the wavelength range from 308.5 to 310.5 nm. The OH radical is monitored via the well known (0,0) band of the electronic A( 2 Σ + )-X( 2 Π) system. Upon addition of SO 2 to the fuel stream numerous transitions of sulfur containing species are observed in addition. Recent molecular parameters for the B( 3 Σ u - )-X( 3 Σ g - ) transition of S 2 are used to compute a synthetic spectrum of the molecule within the accuracy of the laser system (0.15 cm -1 ) that overlaps favorably with major features in the experimental spectra. In spite of the dense spectrum, isolated transitions are observed that are suitable for concentration measurements. Relative concentrations of OH and S 2 are mapped by taking advantage of the high sensitivity of the DFWM technique. Absorption spectroscopy, on the other hand, is used to obtain an absolute number density of OH at positions in the flame where a significant level of the radical occurs. These measurements are linked to the relative OH profile and yield the absolute concentration of the hydroxyl as a function of height above the burner surface. Furthermore, the relative S 2 profile obtained by DFWM is put on an absolute scale by quantitative comparison with the OH profile. The required Einstein B coefficients for the B-X transition of S 2 are obtained from a calculation of Honl-London factors and taking into account the Franck-Condon factors and the electronic transition moment from the literature. The measured profiles of S 2 and OH are in good qualitative agreement with a recent theoretical model of the sulfur chemistry in flames.

Phase-conjugate resonant holographic interferometry applied to NH concentration measurements in a two-dimensional diffusion flame

Resonant holographic interferometry is a diagnostic technique based on the dispersion of light having a frequency close to that of an electronic transition of a molecule. We propose a novel single-laser, two-color setup for the recording of resonant holograms and apply it to two-dimensional (2D) species concentration measurements in a combustion environment. The generation of the second color is achieved by optical phase conjugation from stimulated Brillouin scattering in a cell. The frequency shift of ~8.5 GHz introduced by the phase conjugation matches approximately the linewidth of many molecular transitions at typical flame temperatures and can be implemented to produce holograms of good contrast and diffraction efficiency. Phase-conjugate resonant holographic interferometry is demonstrated in a 2D NH(3) -O(2) flame, yielding interferograms containing information on the NH radical concentration distribution in the flame. Experimental results are quantified by application of a numerical computation of the complex refractive index.

Imaging of the reaction zone in a 100 kW oil-burning furnace by use of a broad-band excimer laser

The reaction zone in the hostile combustion environment of a 100 kW oil-burning furnace has been imaged by laser-induced fluorescence using a broad-band XeCl-excimer laser. Upon excitation, the averaged images obtained by using an interference filter around 320 nm (FWHM of 10 nm) show three distinct areas along the direction of the gas flow. An intense emission spreads around the spray axis and is attributed to the fluorescence of large hydrocarbons in the unburned fuel. Approximately 12 cm downstream of the nozzle, a narrow dark region is displayed suggesting the preheat zone of the combustion process where large hydrocarbons are considerably degraded. The third distinct region is characterized by a strong onset of the fluorescence intensity localized downstream of the dark region. This feature is strongly suppressed by replacing the interference filter by a broad-band transmission filter passing light from 350 to 500 nm. Since OH strongly absorbs at the laser wavelength and its fluorescence is significantly lower above 345 nm, the findings imply that the major contribution to the observed intensity in this region originates from the OH radical. This molecule reaches its maximum concentration immediately downstream of the flame front. However, a contribution from other flame species fluorescing around 320 nm cannot be ruled out. Nevertheless, the combined spatial and spectral information obtained imply that the reaction zone of the combustion process can be localized accurately. The results are compared with simultaneously performed numerical simulations of the burner and are in reasonable agreement.

Numerical Simulation of the Oil Combustion Process in a 100 kW Serially Produced Low NOx Combustion System

Abstract Modern low NOx domestic oil and gas burners use the effect of flue gas recirculation to lower combustion temperature and hence to reduce the amount of NO being formed. Because these burners do take flue gas from the combustion chamber and mix it with primary air, a strong interaction is possible between the burner and the boiler. This interaction has to be taken into account particularly for the jet pump type of flue gas recirculators, because it can lower the rate of recirculation and hence increase the NOx emissions drastically. Numerical simulations (CFD) of the oil combustion process within a commercially available domestic burner have been carried out. Extensive comparison with experimental data (LDV, PDPA, LIF, probe) shows the achievable accuracy of numerical simulations for such complex geometrys nowadays. The final discussion gives reasons for the differences between simulations and measurements and shows the necessariness of further developments concerning the oil model.