The main aim of this thesis has been to contribute to the ongoing work with development of new diode-laser-based spectroscopic techniques and metho­do­lo­gies for sensitive detection of molecules in gas phase. The techniques under scrutiny are tunable diode laser absorption spectrometry (TDLAS) and Faraday modulation spectrometry (FAMOS). Conventional distributed-feedback (DFB) tele­communication diode lasers working in the near-infrared (NIR) region have been used for detection of carbon monoxide (CO) and temperature in hot humid media whereas a unique frequency-quadrupled external-cavity diode laser producing mW powers of continuous-wave (cw) light in the ultra violet (UV) region have been used for detection of nitric oxide (NO). A methodology for assessment of CO in hot humid media by DFB-TDLAS has been developed. By addressing a particular transition in its 2nd overtone band, and by use of a dual-fitting methodology with a single reference water spectrum for background correction, % concentrations of CO can be detected in media with tens of percent of H2O (≤40%) at T≤1000 °C with an accuracy of a few %. Moreover, using an ordinary DFB laser working in the C-band, a technique for assessment of the temperature in hot humid gases (T≤1000 °C) to within a fraction of a percent has been developed. The technique addresses two groups of lines in H2O that have a favorable temperature dependence and are easily accessed in a single scan, which makes it sturdy and useful for industrial applications. A technique for detection of NO on its strong electronic transitions by direct absorption spectrometry (DAS) using cw UV diode laser light has been deve­loped. Since the electronic transitions are ca. two or several orders of magnitude stronger than of those at various rotational-vibra­tional bands, the system is capable of detecting NO down to low ppb∙m concentrations solely using DAS. Also the FAMOS technique has been further developed. A new theoretical description expressed in terms of both the integrated line strength of the transition and 1st Fourier coefficients of a magnetic-field-modulated dispersive lineshape functions is presented. The description has been applied to both ro-vib Q-transitions and electronic transitions in NO. Simulations under different pressures and magnetic field conditions have been made that provide the optimum conditions for both cases. A first demonstration and characteri­zation of FAMOS of NO addressing its electronic transitions in the UV-region has been made, resulting in a detection limit of 10 ppb∙m. The characterization indicates that the technique can be significantly improved if optimum conditions can be obtained, which demonstrates the high potential of the UV-FAMOS technique.

This thesis focuses on the utilisation of mid-infrared techniques in technological atmospheric pressure, non-thermal plasma (NTP) diagnostics. Two mid-infrared techniques were demonstrated in this work namely laser absorption and Fourier transform infrared (FTIR) spectroscopy. The performance of external-cavity quantum cascade laser (EC-QCL), a relatively new laser type with broad tuning capability was also demonstrated as potential diagnostics tool for technological NTP applications. A dual plate dielectric barrier discharge (DBD) and a packed-bed NTP reactor were designed and fabricated to perform plasma process. Quantitative analysis of the laser absorption and FTIR spectroscopy techniques for gas detection were validated by using standard gas samples. Real-time CO monitoring by means of in-situ laser absorption spectroscopy measurements were performed for gas phase diagnostics in the decomposition of TEOS by means of plasma-enhanced chemical vapour deposition (PE-CVD) and in CO2 reforming of CH4 by means of NTP. In-line FTIR measurements simultaneously recorded the gas spectrum at the exhaust of the plasma reactors. Information from both measurements was found to provide useful information on the plasma processes and chemistry for the NTP applications. Finally, wavelength stability and linearity performance of a broad tuning range EC-QCL were evaluated by using the Allan variance technique. (LOD) at SNR = 1 was estimated to be ~ 2 ppm, achieved under atmospheric pressure, at the room temperature, and a path length of 41 cm for NO detection produced from the decomposition of dichloromethane (DCM) by means of NTP.

The work presented in this thesis is concerned with development of spectroscopic detection methods based on absorption spectroscopy using semiconductor lasers, with particular ref- erence to the field of medical diagnostics through breath analysis. The first part of this thesis deals with the design and testing of a prototype analyser for simultaneous monitoring of the exchange gases O₂ , CO₂ and H₂O in breath. The aim of this analyser is to provide information required to monitor respiration, with potential use in intensive care monitoring or during anaesthesia. The relatively high concentrations of these gases in breath and read- ily available diode laser sources make detection in the near-infrared (NIR) ideal. However, the relatively weakly absorbing A-band O₂ transitions at 760 nm require the application of a sensitive spectroscopic method, cavity enhanced absorption spectroscopy (CEAS). In contrast, CO₂ and H₂O are monitored using direct single pass absorption spectroscopy, with transitions arising from the 2ν₁ + ν₃ band at 2 μm and ν₁ + ν₃ band at 1.3 μm, respectively. It has been demonstrated that these gases can be detected simultaneously over a short pathlength (2.74 - 4 cm) in the respiratory flow by combining various spectroscopic methodologies and real-time data analysis. This analyser is shown to offer a viable alter- native for monitoring respiration, exhibiting absolute detection limits of changes of 0.26 % O₂ , 0.02 % CO₂ and 0.003 % H₂O with a 10 ms time resolution, which are comparable to current mass spectrometry based methods, but without their inherent delays. Following this, investigations into the detection of the main gas constituents in breath in the NIR employing noise-reduction modulation based spectroscopic techniques, namely wavelength and frequency modulation (WMS and FMS respectively) are also reported. The described WMS studies on water at 1.37 μm provide a demonstration of conventional WMS detection, as well as a “proof-of-principle” example of a relatively new approach to calibrating the non-absolute information obtained from a WMS absorption signal. Typically WMS spectra are calibrated using mixtures of known gas concentrations or an absolute direct absorption spectrum where possible. In this work however, a self-calibrating method, the phasor decomposition method (PDM), is employed and the returned concentration from this calibration is compared to direct absorption measurement. From this, the calculated concentration using the PDM is found to differ by 9 % from the concentration value obtained by direct absorption, providing an alternative method of calibration for when direct absorption measurements are not possible. The use of FMS in the NIR is also demonstrated as a potential alternative to CEAS for monitoring O₂ at 760 nm. FMS detection is performed on atmospherically broadened O₂ and a time-normalised α_min(t) of 2.45 ×10⁻⁶ cm⁻¹ s^1/2 is obtained, which is two orders of magnitude less sensitive than the value of α_min(t) = 2.35 ×10⁻⁸ cm⁻¹ s^1/2 obtained with CEAS. This combined with the experimental requirements of an FMS system, make its use for detection of O₂ a less practicable option compared to CEAS for real-time breath analysis. The latter work in this thesis involves a change in focus to detection of trace gases in breath in the mid-infrared (MIR). The move of spectroscopic detection to the MIR exploits the larger absorption cross-sections available in this region, and to achieve this, a relatively new form of semiconductor laser, the quantum cascade laser (QCL) is used. The design of a continuous wave QCL spectrometer at 8 μm and its operating characteristics are demon- strated and improvements in its performances are also discussed. This QCL system is then utilised to demonstrate the potential of monitoring species in breath, namely the narrow- band absorber methane and the broadband absorber acetone, taking into consideration the potential interference from other absorbing species in breath and the different spectroscopic characteristics exhibited by these molecules. Finally, the potential to further improve the sensitive detection of trace gases in breath in the MIR is also investigated with studies on the use of CEAS and multipass cells. In this work, the molecule of interest is the biomarker OCS, using transitions of the 2ν₂ band at 1031 cm⁻¹ , that are probed using a 10 μm QCL. The application of CEAS in the MIR is not as well developed as in the NIR, and the experimental consequences of using optical cavities at these wavelengths, where equipment tends to be more limited, are investigated and sensitivities discussed in the context of other literature. The experimental procedure of optimising a cavity for CEAS using the off-axis alignment method is also studied in detail, as well as the addition of WMS to further improve the signal quality. An effective absorption pathlength of ∼ 100 m was achieved in the cavity, with a bandwidth reduced α_min(BW) of 1.7 ×10⁻⁷ cm⁻¹ Hz^−1/2 using WMS CEAS achieved. With the poorer quality optics and limitations in equipment in the MIR for CEAS experiments, the use of a multipass cell, a 238 m Herriott cell, is also investigated as an alternative to the use of an optical cavity at 10 μm. Detection of OCS using direct absorption and WMS is demonstrated in the Herriott cell, achieving α_min(BW) = 2.03×10⁻⁸ cm⁻¹ Hz^−1/2 using WMS. This shows an improvement in sensitivity compared to WMS CEAS, and also shows the potential for future work on biomarker detection, as it approaches the ∼ ppb levels required for breath analysis.

Tunable diode laser absorption spectroscopy using an external cavity diode laser operating in the infra-red has been developed to monitor CO2 in the freestream of the Longshot hypervelocity facility at the Von Karman Institute for Fluid Dynamics. The Longshot facility offers a unique European facility for ground testing and numerical validation applications, however, some of the traditional data rebuilding aspects are in question. A non-intrusive absorption<p>sensor could significantly aid in improving the knowledge of freestream static values thereby improving the models used in data rebuilding and numerical simulation. The design of such a sensor also expands the spectroscopic capabilities of the Von Karman Institute.<p><p>The absorption sensor is designed around the single P12 (00001)-(30013) rovibrational transition near 1.6µm (6218.09cm-1 specifically) which yields relatively weak direct absorption levels at about 3.5% per meter for typical Longshot freestream conditions. However, when handled carefully, adequate signal-to-noise can be acquired to exploit significant flow information. By being able to operate in this range, total sensor cost can be easily an a factor of two or more cheaper than sensors designed for the deeper infrared. All sensor elements were mounted to a compact portable optics bench utilizing single-mode optical fibers to allow for quick installation at different facilities by eliminating tedious optical realigning. Scans at 600Hz were performed over 20ms of the 40ms test time to extract core static temperature, pressure and velocity.<p><p>These results are compared with the current state of the Longshot data rebuild method. The non-uniform flow properties of the shear layer and test cabin rested gas accumulation was of an initial concern. The temperature and density gradients along with significant radial velocity components could result in DLAS temperature, pressure and velocity that are significantly different than that of the target freestream inviscid core values. Fortunately, with the proper selection of the P12 rotational number, this effect could be more or less ignored as the higher temperature and lower density gas of this region is relatively transparent.<p><p>Ultimately, acquired temperature and density were moderately accurate when compared to Longshot rebuilt results owing primarily to the baseline extraction which poses issues for such low absorption signals. However, the extracted velocity data are quite accurate. This is a definite puls for the sensor as the freestream enthalpy of cold hypersonic facilities is dictated primarily by the kinetic energy contribution. Being able to compare velocity gives insight to the level of vibration non-equilibrium in the flow. The velocity of the DLAS and the Longshot rebuild are quite close. This adds more weight to the argument that vibrational excitation is very low (if present at all) in the free stream and that the van de derWaals equation of state usage and constant specific heat assumption might be an adequate model for the data rebuild after all. / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished

No description available.

<p>This work presents the development and/or application of several laser diagnostics for studying the flame structure of composite propellant flames. These studies include examining the flame structure of novel energetic materials with potential as propellant ingredients, the near-surface flame structure of basic composite propellants, and the global flame structure of propellants containing metal additives.<br></p><p><br></p><p>First, the characterization of the deflagration of various novel energetic cocrystals is presented. The synthesis and development of novel energetic materials is a costly and challenging process. Rather than synthesizing new materials, cocrystallization provides the potential opportunity to achieve improved properties of existing energetic materials. This work presents the characterization of the effect of cocrystallization on the deflagration of a 2:1 molar cocrystal of CL-20 and HMX as well as a 1:1 molar cocrystal of CL-20 and TNT. A hydrogen peroxide (HP) solvate of CL-20 as well as a polycrystalline composite of HMX and ammonium perchlorate (AP) were also studied. A physical mixture of each material was also tested for comparison. The burning rate of each material was measured as a function of pressure. Flame structure during self-deflagration was examined using planar laser-induced fluorescence (PLIF) of CN and OH. The burning rate of the HMX/CL-20 cocrystal and the CL-20/HP solvate closely matched that of CL-20, but the burning rate of the TNT/CL-20 cocrystal was between the burning rate of its coformers. All HMX/AP materials had a higher burning rate than either HMX or AP individually and the burning rate of a physical mixture was found to be a function of particle size. The differences in the burning rate of the physical mixtures and composite crystal of HMX/AP can be explained by changes in the flame structure observed using PLIF. Burning rates and flame structure of the cocrystals were found to closely match those of their respective physical mixtures when smaller particle sizes were used (approx. less than 100 um). The results obtained demonstrate that the deflagration behavior of the coformers is not indicative of the deflagration behavior of the resulting physical mixture or cocrystal. However, changes in the resulting flame structure greatly affect the burning rate.</p><p><br></p><p>Next, PLIF of nitric oxide (NO) was utilized to characterize the near surface flame structure of composite propellants of AP and hydroxyl-terminated polybutadiene (HTPB) containing varying particle sizes of AP burning at 1 atm in air. In all propellants, the NO PLIF signal was strongest close to the burning propellant surface and fell to a non-zero constant value within ~1 mm of the surface where it remained throughout the remainder of the flame. Distinct diffusion-flame-like structure was observed above large individual burning AP particles in the propellant containing a bimodal distribution of 400 and 40 um AP. In contrast, the flame of a propellant containing only fine AP (40 um) behaved like a homogeneous, premixed flame. The flame of the propellant containing a bimodal distribution of 200 and 40 um AP also showed similar behavior to a premixed flame with some heterogeneous structure indicating that, at this pressure, the propellant is approaching a limit where the particle sizing is small enough that the flame behaves like a homogeneous, premixed flame. Additionally, propellants containing aluminum were tested. No significant differences were observed in the NO PLIF behavior between the propellants with and without aluminum suggesting that, at these conditions, the aluminum does not have a significant effect on the AP/HTPB flame structure near the burning surface.</p><p><br></p><p>The effect of aluminum particle size on the temperature of aluminized-composite-propellant flames burning at 1 atm is also presented. In this work, measurements of 1) the temperature of CO (within the flame bath gas) and 2) the temperature of AlO (located primarily within regions surrounding the burning aluminum particles) within aluminized, AP-HTPB-propellant flames were performed as a function of height above the burning propellant surface. Three aluminized propellants with varying aluminum particle size (nominally 31 um, 4.5 um, or 80 nm) and one non-aluminized AP-HTPB propellant were studied while burning in air at 1 atm. A wavelength-modulation-spectroscopy (WMS) diagnostic was utilized to measure temperature and mole fraction of CO via mid-infrared wavelengths and a conventional AlO emission-spectroscopy technique was utilized to measure the temperature of AlO. The bath-gas temperature varied significantly between propellants, particularly within 2 cm of the burning surface. The propellant with the smallest particles (nano-scale aluminum) had the highest average temperatures and far less variation with measurement location. At all measurement locations, the average bath-gas temperature increased as the initial particle size of aluminum in the propellant decreased, likely due to increased aluminum combustion. The results support arguments that larger aluminum particles can act as a heat sink near the propellant surface and require more time and space to ignite and burn completely. On a time-averaged basis, the temperatures measured from AlO and CO agreed within uncertainty at near 2650 K in the nano-aluminum propellant flame, however, AlO temperatures often exceeded CO temperatures by ~250 to 800 K in the micron-aluminum propellant flames. This result suggests that in the flames studied here, and on a time-averaged basis, the micron-aluminum particles burn in the diffusion-controlled combustion regime, whereas the nano-aluminum particles burn within or very close to the kinetically controlled combustion regime.</p><p><br></p><p>The study of the effect of aluminum particle size on the temperature of aluminized, composite-propellant flames was then extended to characterize the same propellants burning at elevated pressures ranging from 1 to 10 atm. A novel mid-infrared scanned-wavelength direct absorption technique was developed to acquire measurements of temperature and CO in particle-laden propellant flames burning at up to 10 atm. The results from the application of this diagnostic are among the very first measurements of gas properties in aluminized composite propellant flames burning at pressures above atmospheric pressure. In all propellants, the flame temperature and combustion efficiency of the propellant flames increased with an increase in pressure. In addition, the propellants with smaller aluminum particle sizes achieved higher flame temperatures as the particles were able to ignite and react faster. However, the propellants containing nano-scale and the smallest micron-scale aluminum powders had similar global flame temperatures suggesting that at some point a decrease in particle size results in minimal gains in the overall flame temperature. The results demonstrate how well measurements of gas properties can be used to understand the behavior of the aluminum particle combustion in the flame.</p><p><br></p><p>Last, the design, development, and application of a laser-absorption-spectroscopy diagnostic capable of providing quantitative, time-resolved measurements of gas temperature and HCl concentration in flames of aluminized, composite propellant flames is presented. This diagnostic utilizes a quantum-well distributed-feedback tunable diode laser emitting near 3.27 um to measure the absorbance spectra of one or two adjacent HCl lines using a scanned-WMS technique which is insensitive to non-absorbing transmission losses caused by metal particulates in the flame. This diagnostic was applied to characterize the spatial and temporal evolution of temperature and/or HCl mole fraction in small-scale flames of AP-HTPB composite propellants containing either an aluminum-lithium alloy or micron-scale aluminum. Experiments were conducted at 1 and 10 atm. At both pressures, the flame temperature of the aluminum-lithium propellant, on a time-averaged basis, was 80 to 200 K higher than that of the aluminum-propellant (depending on location in the flame) indicating more complete combustion. In addition, the mole fraction of HCl in the aluminum-lithium propellant flame reached values 65-70% lower than the conventional aluminum-propellant flame at the highest measurement location in the flame. The measurements at both pressures showed similar trends in the reduction of HCl in the aluminum-lithium propellant flame but at 10 atm this occurred on a length scale an order of magnitude smaller than the flame at atmospheric pressure. The results presented further support that the use of an aluminum-lithium alloy is effective at reducing HCl produced by the propellant flame without compromising performance, thereby making it an attractive additive for solid rocket propellants.</p>

<p dir="ltr">Emission and absorption spectroscopy diagnostics are useful for providing non-invasive,<br>quantitative measurements of various gas properties in combustion environments, including<br>temperature and species concentrations. These measurements become even more useful<br>when they are applied with high spatial and temporal resolution. This dissertation describes<br>several ways that both emission and absorption diagnostics were advanced through leveraging<br>improvements in mid-IR camera and laser technology and through refining the use of existing<br>techniques.<br>A literature review is provided for both laser absorption and emission spectroscopy. Previous advancements in spatially resolved techniques are explained. The fundamental equations<br>of spectroscopic diagnostics are reviewed, starting from statistical mechanics.<br>A spectrally-resolved emission imaging diagnostic is presented. This diagnostic provided<br>1-dimensional measurements of gas temperature and relative mole fraction of CO₂ and HCl<br>in flames. An imaging spectrometer and a high-speed mid-infrared camera were used to<br>provide 1D measurements of CO₂and HCl emission spectra with a spectral resolution of<br>0.46 cm^-1 at rates up to 2 kHz. Measurements were acquired in HMX and AP-HTPB flames<br>burning in air at 1 atm. This diagnostic was applied to characterize how the path-integrated<br>gas temperature of HMX flames varies in time and with distance above the burning surface.<br>Additionally, Abel inversion with Tikhonov regularization was applied to determine the radial<br>distribution of temperature and relative concentration of CO₂ and HCl within the core of<br>AP-HTPB flames.<br>Next, a similar emission imaging diagnostic is presented which uses spectrally-resolved<br>measurements of emission spectra at visible wavelengths, unlike the mid-infrared measure-<br>ments in the rest of this dissertation. This diagnostic provided 1D temperature measure-<br>ments of aluminum oxide (AlO), an intermediate product of aluminum combustion. While<br>this author created the AlO diagnostic, these measurements were performed alongside a CO<br>absorption diagnostic used by a different researcher to compare the flame bath gas (via CO)<br>and the region immediately around aluminum particles (via AlO) when varying forms of<br>aluminum powder were used in a propellant. This comparison allows analysis of the burning regime of aluminum particles. Evidence was found that nano-aluminum particles burn in<br>the kinetically controlled combustion regime, while micron-aluminum particles burn in the<br>diffusion-controlled regime.<br>Multi-spectral emission imaging of hypergolic ignition of ammonia borane (AB) is then<br>presented. Three high-speed cameras with multiple optical filters were used to capture<br>infrared and visible wavelength videos of four individual species during AB ignition: BO,<br>BO₂, HBO₂, and the B-H stretch mode of AB were imaged. The ignition process was<br>observed to act in two steps: gas evolution and then propagation of a premixed flame. The<br>evolution of the species and flame front revealed that boranes may continue to complete<br>combustion to a further degree than other boron fuels. This author performed the infrared<br>camera imaging and also ran infrared spectrograph measurements to confirm which species<br>were viewed through the optical filters.<br>Next, a scanned-wavelength direct-absorption diagnostic for directly measuring NH₃ in<br>high-temperature combustion environments is presented. A quantum cascade laser (QCL)<br>was scanned at 5 kHz over multiple NH₃ transitions between 959.9 cm⁻¹ and 960.3 cm⁻¹ to<br>measure path-integrated NH₃ temperature and mole fraction. Many NH₃ transitions overlap<br>with high-temperature water lines at commonly used diagnostic frequencies, severely limiting<br>those diagnostics’ capabilities in water-rich, high-temperature environments that are typical<br>of combustion applications. The optical frequencies used in this diagnostic are insensitive<br>to water absorption and thus remedy this issue. This diagnostic was demonstrated within<br>the flame of ammonia borane. AB-based fuels were burned in ambient air and translated<br>vertically to effectively scan the measurement line-of-sight vertically through the flame. Ad-<br>ditionally, flames of these fuels were characterized at a stationary height in an opposed-flow<br>burner (OFB) under O₂ flow.<br>The final chapter presents scanned-wavelength direct-absorption measurements of path-<br>integrated temperature and CO mole fraction in opposed-flow diffusion flames of hydroxyl-<br>terminated polybutadiene (HTPB). HTPB strands were held in an opposed-flow burner<br>under an opposed flow of O2 or 50/50 O₂/N₂ to create quasi-steady and quasi-1D diffusion<br>flames above the fuel strand. The opposed-flow burner was translated vertically to effectively<br>scan the measurement line-of-sight vertically through the flame. A quantum-cascade laser (QCL) was scanned across the P(2,20), P(0,31), and P(3,14) absorption transitions in CO’s<br>fundamental vibration bands near 2008 cm⁻¹ at 10 kHz to determine the path-integrated<br>temperature and CO mole fraction. The laser beam was passed through sapphire rods<br>held close to the flame edge to bypass the flame boundary and provide a well defined path<br>length for mole fraction measurements. The measured profiles and fuel regression rates<br>were compared to predictions produced by a steady opposed-flow 1D diffusion flame model<br>produced by researchers at the Army Research Lab. The model was generated with chemical<br>kinetics mechanisms employing two different assumptions for the nascent gaseous product of<br>HTPB pyrolysis: C₄H₆ or C₂₀H₃₂. It was found that the C₂₀H₃₂ model produced temperature<br>and CO profiles along with regression rates that agreed more closely with the measured<br>results.<br></p>

<p dir="ltr">The objective of this research is to advance the development and application of laser diagnostics in gas phase medium, which ranges from atmospheric non-reacting flows to turbulent reacting flows in high-pressure, high-temperature environments. Laser diagnostic techniques are powerful tools for non-intrusive and in-situ measurements of important chemical parameters, such as temperature, pressure, and species mole fractions, in harsh environments. These measurements significantly advance the knowledge across various research disciplines, such as combustion dynamics, chemical kinetics, and molecular spectroscopy. In this thesis, detailed theoretical models and experimental analysis are presented for three different techniques: 1. Chirped-probe-pulse femtosecond coherent anti-Stokes Raman scattering (CPP fs CARS); 2. Two-color polarization spectroscopy (TCPS); 3. Ultrafast-laser-absorption-spectroscopy (ULAS). The first chapter provides a brief survey of laser diagnostics, including both linear and nonlinear methods. The motivations behind the three studies covered in this dissertation are also discussed. </p><p dir="ltr">In the second chapter, single-shot CPP fs CARS thermometry is developed for the hydrogen molecule at 5 kHz. The results are divided into two parts. The first part concentrates on the development of H₂ CPP fs CARS thermometry for high-pressure and high-temperature conditions. The second part demonstrates the application of H₂ CPP fs CARS in a model rocket combustor at pressures up to 70 bar. In the first part, H₂ fs CARS thermometry was performed in Hencken burner flames up to 2300 K, as well as in a heated gas-cell at temperatures up to 1000 K. It was observed that the H₂ fs CARS spectra are highly sensitive to the pump and Stokes chirp. Chirp typically originates from optical components such as windows and polarizers. As a result, the pump delay is modeled to provide a shift to the Raman excitation efficiency curve. With the updated theoretical model, excellent agreement was found between the simulated and experimental spectra. The averaged error and precision are 2.8% and 2.3%, respectively. In addition, the spectral phase and pump delay determined from the experimental spectra closely align with the theoretical predictions. It is also found that pressure does not have significant effects on the H₂ fs CARS spectra up to 50 bar at 1000 K. The collision model provides excellent agreement with the experiment. This allows the use of low-pressure laser parameters for high-pressure thermometry measurements. In the second part, spatially resolved H₂ temperature was measured in a rocket chamber at pressures up to 70 bar. This is the first demonstration of fs CARS thermometry inside a high-pressure rocket combustor. These results highlight the potential of using H₂ CPP fs CARS thermometry to provide quantitative data in high-pressure experiments for the study of combustion dynamics and model validation efforts at application relevant operating conditions.</p><p dir="ltr">The third chapter presents the development of a TCPS system for the study of the NO (<i>A</i>²Σ⁺-<i>X</i>²Π) state-to-state collision dynamics with He, Ar, and N₂. Two sets of TCPS spectra for 1% NO, diluted in different buffer gases at 295 K and 1 atm, were obtained with the pump beam tuned to the R₁₁(11.5) and ^OP₁₂(1.5) transitions. The probe was scanned while the pump beam was tuned to the line center. Collision induced transitions were observed in the spectra as the probe scanned over transitions that were not coupled with the pump frequency. The strength and structure of the collision induced transitions in the TCPS spectra were compared between the three colliding partners. Theoretical TCPS spectra, calculated by solving the density matrix formulation of the time-dependent Schrödinger wave equation, were compared with the experimental spectra. A collision model based on the modified exponential-gap law was used to model the rotational level-to-rotational level collision dynamics. An unique aspect of this work is that the collisional transfer from an initial to a final Zeeman state was modeled based on the difference in the cosine of the rotational quantum number <i>J</i> projection angle with the z-axis for the two Zeeman states. Rotational energy transfer rates and Zeeman state collisional dynamics were varied to obtain good agreement between theory and experiment for the two different TCPS pump transitions and for the three different buffer gases. One key finding, in agreement with quasi-classical trajectory calculations, is that the spin-rotation changing transition rate in the <i>A</i>²Σ⁺ level of NO is almost zero for rotational quantum numbers ≥ 8. It was necessary to set this rate to near zero to obtain agreement with the TCPS spectra. </p><p dir="ltr">The fourth chapter presents the development and application of a broadband ULAS technique operating in the mid-infrared for simultaneous measurements of temperature, methane (CH₄), and propane (C₃H₈) mole fractions. Single-shot measurements targeting the C-H stretch fundamental vibration bands of CH₄ and C₃H₈ near 3.3 μm were acquired in both a heated gas cell up to ~650 K and laminar diffusion flames at 5 kHz. The average temperature error is 0.6%. The average species mole fraction error are 5.4% for CH₄, and 9.9% for C₃H₈. This demonstrates that ULAS is capable of providing high-fidelity hydrocarbon-based thermometry and simultaneous measurements of both large and small hydrocarbons in combustion gases. </p>

<p dir="ltr">Laser absorption spectroscopy (LAS) is a widely used technique to acquire path-integrated measurements of gas properties such as temperature and mole fraction. Although extremely useful, the application of LAS to study heterogeneous combustion environments can be challenging. For example, beam steering can be one such challenge that arises during measurements in heterogeneous combustion environments such as metallized propellant flames or measurements at high-pressure conditions. The ability to only obtain path integrated measurements has been a major challenge of conventional LAS techniques, especially in characterizing combustion environments with a non-uniform thermo-chemical distribution along the line of sight (LOS). Additionally, simultaneous measurements of multiple species using LAS with narrow-bandwidth lasers often necessitates employing multiple light sources. Aerospace applications, such as characterizing hypersonic flows may require ultrashort time resolution to study fast-evolving chemistry. Similarly, atmospheric entry most often requires measurements of atoms and molecules that absorb at wavelengths ranging from ultraviolet to mid-infrared. The availability of appropriate light sources for such measurements has been limited. In the past, several researchers have come up with diagnostic techniques to overcome the above-mentioned challenges to a certain extent. Most often, these solutions have been need-based while compromising on other diagnostic capabilities. Therefore, LAS diagnostics capable of acquiring broadband measurements with ultrafast time resolution and the ability to acquire measurements at wavelengths in ultraviolet through mid-infrared is required to study advanced combustion systems and for the development of advanced aerospace systems for future space missions. Ultrafast laser absorption spectroscopy is one such technique that provides broadband measurements, enabling simultaneous multi-species and high-pressure measurements. The light source utilized for ULAS provides the ultrafast time resolution necessary for resolving fast-occurring chemistry and more importantly the ability to acquire measurements at a wide range of wavelengths ranging from ultraviolet to far-infrared. The development and application of ULAS for characterizing propellant flames and hypersonic flows under non-equilibrium conditions by overcoming the above-mentioned challenges is presented here. </p><p>This work describes the development of a single-shot ultrafast laser absorption spectroscopy (ULAS) diagnostic for simultaneous measurements of temperature and concentrations of CO, NO, and H₂O in flames and aluminized fireballs of HMX (C₄H₈N₈O₈). Ultrashort (55 fs) pulses from a Ti:Sapphire oscillator emitting near 800 nm were amplified and converted into the mid-infrared through optical parametric amplification (OPA) at a repetition rate of 5 kHz. Ultimately, pulses with a spectral bandwidth of ≈600 cm^-1 centered near 4.9 µm were utilized in combination with a mid-infrared spectrograph to measure absorbance spectra of CO, NO, and H₂O across a 30 nm bandwidth with a spectral resolution of 0.3 nm. The gas temperature and species concentrations were determined by least-squares fitting simulated absorbance spectra to measured absorbance spectra. Measurements of temperature, CO, NO, and H₂O were acquired in an HMX flame burning in air at atmospheric pressure and the measurements agree well with previously published results. Measurements were also acquired in fireballs of HMX with and without 16.7 wt% H-5 micro-aluminum. Time histories of temperature and column densities are reported with a 1-σ precision of 0.4% for temperature and 0.3% (CO), 0.6% (NO), and 0.5% (H₂O), and 95% confidence intervals (C.I.) of 2.5% for temperature and 2.5% (CO), 11% (NO), and 7% (H₂O), thereby demonstrating the ability of ULAS to provide high-fidelity, multi-parameter measurements in harsh combustion environments. The results indicate that the addition of the micron-aluminum increases the fireball peak temperature by ≈100 K and leads to larger concentrations of CO. The addition of aluminum also increases the duration fireballs remain at elevated temperatures above 2000 K.</p><p dir="ltr">Next, the application of ULAS for dual-zone temperature and multi-species (CO, NO, H₂O, CO₂, HCl, and HF) measurements in solid-propellant flames is presented. ULAS measurements were acquired at three different central wavelengths (5.121 µm, 4.18 µm, and 3.044 µm) for simultaneous measurements of temperature and: 1) CO, NO, and H₂O, 2) CO₂ and HCl, and 3) HF and H₂O. Absorption measurements with a spectral resolution of 0.35 nm and bandwidth of 7 cm^-1, 18 cm^-1, and 35 cm^-1, respectively were acquired. In some cases, a dual-zone absorption spectroscopy model was implemented to accurately determine the gas temperature in the hot flame core and cold flame boundary layer via broadband absorption measurements of CO₂, thereby overcoming the impact of line-of-sight non-uniformities. Results illustrate that the hot-zone temperature of CO₂ agrees well with the equilibrium flame temperature and single-zone thermometry of CO, the latter of which is insensitive to the cold boundary layer due to the corresponding oxidation of CO to CO₂.</p><p dir="ltr">The initial development and implementation of an ultraviolet and broadband ultrafast-laser-absorption-imaging (UV-ULAI) diagnostic for one dimensional (1D) imaging of temperature and CN via its <i>B</i>²Σ⁺←<i>X</i>²Σ⁺absorption bands near 385 nm. The diagnostic was demonstrated by acquiring single-shot measurements of 1D temperature and CN profiles in HMX flames at a repetition rate of 25 Hz. Ultrashort pulses (55 fs) at 800 nm were generated using a Ti:Sapphire oscillator and then amplification and wavelength conversion to the ultraviolet was carried out utilizing an optical parametric amplifier and frequency doubling crystals. The broadband pulses were spectrally resolved using a 1200 l/mm grating and imaged on an EMCCD camera to obtain CN absorbance spectra with a resolution of ≈0.065 nm and a bandwidth of ≈4 nm (i.e. 260 cm^-1). Simulated absorbance spectra of CN were fit to the measured absorbance spectra using non-linear curve fitting to determine the gas properties. The spatial evolution of gas temperature and CN concentration near the burning surface of an HMX flame was measured with a spatial resolution of ≈10 µm. 1D profiles of temperature and CN concentration were obtained with a 1-σ spatial precision of 49.3 K and 4 ppm. This work demonstrates the ability of UV-ULAI to acquire high-precision, spatially resolved absorption measurements with unprecedented temporal and spatial resolution. Further, this work lays the foundation for ultraviolet imaging of numerous atomic and molecular species with ultrafast time resolution.</p><p dir="ltr">Ultraviolet ULAS was applied to characterize the temporal evolution of non-Boltzmann CN (<i>X</i>²Σ⁺) formed behind strong shock waves in N₂-CH₄ mixtures at conditions relevant to entry into Titan's atmosphere. An ultrafast (femtosecond) light source was utilized to produce 55 fs pulses near 385 nm at a repetition rate of 5 kHz and a spectrometer with a 2400 lines/mm grating was utilized to spectrally resolve the pulses after passing through the Purdue High-Pressure Shock Tube. This enabled broadband single-shot absorption measurements of CN to be acquired with a spectral resolution and bandwidth of ≈0.02 nm and ≈6 nm (≈402 cm^-1 at these wavelengths), respectively. A line-by-line absorption spectroscopy model for the <i>B</i>²Σ⁺←<i>X</i>²Σ⁺ system of CN was developed and utilized to determine six internal temperatures (two vibrational temperatures, four rotational) of CN from the (0,0), (1,1), (2,2) and (3,3) absorption bands. Measurements were acquired behind reflected shock waves in 5.65% CH₄ and 94.35% N₂ with an initial pressure of 1.56 mbar and incident shock speed of ≈2.1 km/s. For this test condition, the chemically and vibrationally frozen temperature of the mixture behind the reflected shock was 5000 K and the pressure was 0.6 atm. The high repeatability of the shock-tube experiments (0.3% variation in shock speed across tests) enabled multi-shock time histories of CN mole fraction and six internal temperatures to be acquired with a single-shot time resolution of less than 1 ns. The measurements revealed that CN <i>X</i>²Σ⁺ is non-Boltzmann rotationally and vibrationally for greater than 200 µs, thereby strongly suggesting that chemical reactions are responsible for the non-Boltzmann population distributions. </p><p><br></p>

The effect of a controlled oxygen admixture to a plasma nitrocarburizing process using active screen technology and an active screen made of carbon was investigated to control the carburizing potential within the plasma-assisted process. Laser absorption spectroscopy was used to determine the resulting process gas composition at different levels of oxygen admixture using O2 and CO2, respectively, as well as the long-term trends of the concentration of major reaction products over the duration of a material treatment of ARMCO® iron. The short-term studies of the resulting process gas composition, as a function of oxygen addition to the process feed gases N2 and H2, showed that a stepwise increase in oxygen addition led to the formation of oxygen-containing species, such as CO, CO2, and H2O, and to a significant decrease in the concentrations of hydrocarbons and HCN. Despite increased oxygen concentration within the process gas, no oxygen enrichment was observed in the compound layer of ARMCO® iron; however, the diffusion depth of nitrogen and carbon increased significantly. Increasing the local nitrogen concentration changed the stoichiometry of the ε-Fe3(N,C)1+x phase in the compound layer and opens up additional degrees of freedom for improved process control.