Laser diagnostics and minor species detection in combustion using resonant four-wave mixing

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Application of Resonant and Non-Resonant Laser-Induced Plasmas for Quantitative Fuel-to-Air Ratio and Gas-Phase Temperature Measurements

In this work, two laser-induced plasma techniques are used for gas-phase chemical and temperature measurements. The first technique, laser-induced breakdown spectroscopy (LIBS) is applied for fuel-to-air ratio (FAR) measurements in a well calibrated Hencken flame. In Chapter I, relevant technical and background information for each technique is provided. In Chapter II, measurements are first performed for high-pressure (1-11 Bar) methane-air flames, for which calibration curves are generated using the emission ratio of hydrogen at 656 nm and ionic nitrogen at 568 nm. The effect of pressure on the sensitivity and precision of the resulting calibrated curves is evaluated. Results indicate a degradation of measurement precision as environmental pressure increases, with data indicating that fluctuations of the plasma play a major part in this behavior. Expanding upon this work with LIBS, a comparison of FAR calibration curve results for atmospheric methane-air Hencken flame using three different laser pulse widths, femto-, pico-, and nanosecond regimes, is done in Chapter III. The results are discussed in the context of potential advantages for high-pressure LIBS-based FAR measurements. Results indicate that while nanosecond duration pulses provide better precision at 1 Bar conditions, femtosecond duration pulses might be better suited for high-pressure measurements.In Chapter IV, the radar REMPI technique, which uses microwave scattering from a plasma created by selective multiphoton ionization of molecular oxygen, is used for gas-phase temperature measurements through the wall of ceramic-enclosed environments. Specifically, measurements are done through the wall of a heated laboratory flow reactor and through the wall of a ceramic well-stirred reactor. Results show good agreement with thermocouple and/or computational modeling and the effectiveness of radar REMPI for through-the-wall measurements.In Chapter V, a new technique is discussed, namely acoustic REMPI, which utilizes the pressure wave generated from the creation of the REMPI plasma for diagnostics. The acoustic emission from the plasma is characterized and used for gas-phase temperature measurements. Comparison, with radar REMPI shows a high-level of agreement.Finally, in Chapter VI, a summary of the work in this dissertation is provided along with a discussion of potential for work in the future

Femtosecond real-time probing of reactions. VIII. The bimolecular reaction Br+I2

In this paper, we discuss the experimental technique for real-time measurement of the lifetimes of the collision complex of bimolecular reactions. An application to the atom–molecule Br+I_2 reaction at two collision energies is made. Building on our earlier Communication [J. Chem. Phys. 95, 7763 (1991)], we report on the observed transients and lifetimes for the collision complex, the nature of the transition state, and the dynamics near threshold. Classical trajectory calculations provide a framework for deriving the global nature of the reactive potential energy surface, and for discussing the real-time, scattering, and asymptotic (product-state distribution) aspects of the dynamics. These experimental and theoretical results are compared with the extensive array of kinetic, crossed beam, and theoretical studies found in the literature for halogen radical–halogen molecule exchange reactions

Optical Characterization of Anisotropic Interfaces

The understanding of optical properties of surfaces and interfaces are critical for the development of new technologies ranging from photonics to molecular electronics. Knowing for example the orientation of molecules functionalized onto a surface yields valuable information about the macroscopic properties of the resulting surface. The optical properties can be tuned using various strategies such as molecular functionalization or patterning of meta-structures that ultimately interact with light in a rational way. Advanced optical and spectroscopy methods allowing one to probe such surfaces are therefore key to correlate properties and surface functionalization. In this thesis, multiple approaches have been taken to understand the optical response of anisotropic interfaces. First, polarization-modulation infrared linear dichroism is used to characterize thin films of azobenzene-containing glasses and follow the dynamic of their orientation. Because of the high charge-transfer of these molecules their nonlinear properties are also investigated using second harmonic generation microscopy under microscopy conditions. The coupling of nonlinear active molecules with metallic nanostructures is also investigated using SHG to evaluate the plasmonic enhancements from the nanostructures alone or functionalized with molecules with high hyperpolarizability

Superfast Dynamics of Bipyridinium Ions at Interfaces and Polar Solutions.

The super-fast dynamics of solutes in polar solvents and at solid-liquid interface is investigated by femtosecond time-resolved Raman scattering (RRS) and computer simulation of molecular dynamics (MD). Femtosecond RRS is used to investigate four bipyridinium radicals in aqueous solution: methylviologen monocation MV+, benzylviologen monocation, 4,4\u27-bipyridinium-N,N\u27-di(propylsulfonate) monoanion and N,N\u27-ethylene-2,2\u27 -bipyridinium monocation. Time-resolution of the dynamics of the four radicals is carried out by a pump and probe technique using the time-dependent transient intensity of Stokes and anti-Stokes RRS of C-C stretching mode. It is found that the lifetime of the electronic excited state B3u is less than 350 fs and the vibrational relaxation rates in the electronic ground state are some 2--5 ps. The possible vibrational relaxation mechanisms including the radical structure and charge effect on the vibrational relaxation are discussed. The photo-induced interfacial electron transfer from colloidal CdS particles to an adsorbed methylviologen is studied by femtosecond RRS. It is found by RRS spectra that part of the electron transfer and the accompanying aromatic-to-quinoid structure change of methylviologen occurs within the laser pulse width 350 fs. Time-resolving the photo-induced MV+ RRS band by a pump-probe scheme shows that the photo-induced MV+ dynamics is a double-exponential consisting of two components: 270fs and 6.8ps. The 270fs fast component is assigned to electron transfer from shallow traps and accounts for the part of MV+ produced within the pulse width. However the 6.8 ps slow component is assigned to electron transfer from relatively deep traps. The solvation dynamics upon solute ionization in bulk Stockmayer fluids and at surface is studied using MD simulation. For 20--40 selected thermodynamics states, the non-equilibrium solvation, starting from a neutral polar solute in bulk or adsorbed to a surface is studied by investigating the (complementary) solvent response function, solvent numbers in the first solvent shell, spatial solvent distribution, and pair distribution function. The dependence of the solvation dynamics on solute charge, solvent dipole moment, and surface parameters is studied. A mechanism with two kinds of transient states is proposed to explain the 3-mode dynamics. The computer simulation compliments the solvation dynamics upon electron transfer to a solute