No description available.

No description available.

Advanced technologies for mixed gas detection are discussed. A calorific measurement technique for hydrogen-natural gas mixtures using ultrasonic transducers is examined. Measuring the speed of sound in the gas medium enables an accurate composition testing of mixed gas. At the beginning, different ultrasonic transducers are tested and a suitable one for gas testing is chosen. A jig is designed to conduct the testing with nitrogen/oxygen mixtures in a proof of principle experiment. Another jig is designed and manufactured to test a transit time ultrasonic method for flow rate calculation in order to obtain a full energy flow measurement. A mixed gas leak detection technique based on laser spectroscopy is also studied. A Mid-Wave Infrared (MWIR) laser is implemented to be used as a source in a direct absorption measurement for methane detection. The implemented MWIR laser uses nonlinear optics to generate a MWIR output. A novel intracavity structure using periodically poled lithium niobate as the nonlinear crystal is implemented, and the highest blackbox efficiency for continuous wave difference frequency generation in the MWIR region is reported, to the best of our knowledge. Currently the output power is around 8.1 mW at 3.5 μm with a 1.058% W-1 blackbox efficiency. Watt level MWIR generation is expected using an optimized setup. At last, a second laser source that operates in the long-wave infrared (LWIR) region was also studied. The discussed laser setup for LWIR generation is similar to the MWIR one with different pump and signal wavelengths and an orientation patterned gallium phosphide (OP-GaP) as the nonlinear crystal. Due to the absorption loss of GaP at the pump wavelength, only mW power level is expected out of the intracavity structure. Some alternative approaches for LWIR generation are discussed. / Thesis / Master of Applied Science (MASc)

Aqueous mineral oxide surfaces are ubiquitous in nature, where they play an important role in soil erosion, delta formation etc. Understanding the interfacial solvent environment at mineral oxide surfaces is important as many reactions, e.g., mineral dissolution, heterogeneous catalysis, and electrochemical water splitting occur at interfaces.Vibrational sum frequency generation (vSFG), a second-order nonlinear spectroscopic technique, inherently surface specific under the electric dipole approximation, serves as an excellent tool to study aqueous interfaces. vSFG is forbidden in centrosymmetric environments under the electric dipole approximation, making vSFG inherently specific to non-centrosymmetric environments such as surfaces, where the centrosymmetry is broken. vSFG is capable of measuring interfacial structure and dynamics without contributions from the bulk. Though vSFG has been extensively used to study aqueous interfaces yet there remain fundamental questions that need to be addressed. Is the interface capable of perturbing the environment of a centrosymmetric molecule to render it vSFG active? What higher order multipole terms contribute to vSFG? What are the vibrational energy relaxation pathways and mechanisms at oxide/water interfaces? In this dissertation, we have employed Stark active IR probe molecules (SCN-, N3-), that are sensitive to the local environment and whose frequency shifts depend on the localized electrostatic potential, to understand the interfacial solvent environment and measure the electrostatic potential associated with the charged sites at the aqueous Al2O3(0001) surface. The vibrational lifetime of IR probe molecules sheds information on solvent polarity, H-bonding network, and applied external electric fields. Hence, measuring the vibrational dynamics, whose timescales are comparable to the vibrational lifetime of the IR probe molecules, is a useful tool to understand vibrational energy relaxation (VER) pathways and mechanisms, specific solute-solvent interactions, and localized solvent environment. Though IR probe molecules have been employed to study bulk solvents, the literature for interfaces/surfaces is limited to reverse micelles, air/water interfaces and metal electrode surfaces. The VER rates of IR probe molecules (charged solutes) in bulk solvent and confined solvent environments are significantly different, which reflects the different local properties. The aim of this dissertation is to understand the localized solvent environment as well as the VER pathways and mechanisms of the IR probe molecule (SCN-) at the aqueous mineral oxide interfaces using IR pump-vSFG probe spectroscopy. Bulk H2O and D2O are similar in terms of H-bonding capability, static dielectric constant, and dipole moment. The FTIR spectra of the CN stretch of SCN- in bulk H2O and D2O share a similar central frequency, yet the measured vibrational lifetimes of SCN- reveal accelerated vibrational energy relaxation in bulk H2O vs. bulk D2O, indicating fundamental differences between the two solvent environments. This reflects distinct vibrational energy relaxation pathways. Probing the vibrational lifetime of the CN stretch of SCN- at the alumina(0001)/H2O and alumina(0001)/D2O interfaces enabled us to understand the effect of the interfacial solvent density of states on the solute-solvent vibrational coupling at interfaces. We observed three times faster vibrational energy relaxation (VER) for interfacial D2O (T1 ~7 ps) compared to bulk D2O (T1 ~22 ps). The lifetime of the CN stretch at the α-Al2O3(0001)/H2O interface (T1 ~3 ps) is, however, similar to the dynamics in bulk H2O (T1 ~ 2.7 ps) where effective coupling with the solvent combination band (water bending + librational modes) provides an efficient pathway for intermolecular vibrational energy transfer. Ab-initio simulations show that there is an increase in the vibrational density of states (VDOS) at the interface in the low-frequency region of the O-D stretch, resulting in greater overlap between SCN- and D2O vibrational modes compared to the bulk D2O. The VDOS is not the only factor determining VER. At the interface, there are likely enhanced solute-solvent interactions due to increased transition dipole – transition dipole coupling, as a result of reduced dielectric constant and more net oriented molecules. The two factors (a) availability of accessible energy-accepting states of the solvent and (b) increased solute-solvent coupling, cause acceleration in the vibrational relaxation at the alumina/D2O interface. This work provides insight into the vibrational relaxation pathways and coupling between solute and solvent vibrational modes, which is essential for understanding fundamental condensed phase phenomena in the bulk as well as at interfaces. Our research suggests that VER dynamics cannot be generalized for all interfaces as there are significant differences between how charged solutes behave within confined reverse micelles, at the air/water interface, and at solid/water interfaces. In this dissertation, the basic question of the origin of non-centrosymmetry is also addressed by studying the steady state vSFG response from the azido stretch of N3-, a centrosymmetric molecule, at the α-Al2O3 (0001)/H2O interface. We observed the azide asymmetric stretch peak at the aqueous alumina interface demonstrating that the interface sufficiently perturbs the centrosymmetric environment of the azide ion to make it vSFG active, thereby re-emphasizing the surface-specificity of the vSFG technique. DFT calculations revealed that the application of an external electric field (in the range 0.1 - 0.5 V/Å, similar to the ones typically observed at interfaces), 1-3 the centrosymmetry of the azide ion is broken, introducing Raman activity to the previously IR only active mode (asymmetry azide stretch) thereby making the mode vSFG active. Unlike metal surfaces, where the electrostatic potential is homogeneously distributed over the surface, mineral oxide surfaces have localized and spatially heterogeneous charged sites depending on the bulk pH solution, due to protonation/deprotonation of terminal hydroxyl groups. We employed the asymmetric stretching frequency of N3, an IR probe molecule, that is sensitive to the local solvent environment and applied electric potential to determine the localized interfacial electrostatic potential. Having demonstrated that the interface perturbs the centrosymmetry of N3-, shifts in the central frequency of its asymmetric stretch mode can be used to report on the interfacial localized surface potential of the Al2O3 surfaces. Our previous work using Stark active SCN- to probe the localized charged sites of the alumina (0001)/H2O interface led to the foundation of vSFG spectroscopy as a probe of the local electrostatic potential. Using the N3- Stark tuning rate, the localized electrostatic potential at the negatively charged Al-O- sites was measured to be -170 mV, similar to the one measured by SCN- (-154 mV). In this dissertation, we expand the library of nitrile groups that can be used to measure the interfacial electrostatic potential by using N3-, another Stark active IR molecule, while probing the origin of non-centrosymmetry in this centrosymmetric molecule at mineral oxide/water interfaces. / Chemistry

No description available.

No description available.

No description available.

Over the past two decades, vibrational sum-frequency generation (VSFG) has been applied as a versatile technique for probing the structure and dynamics of molecules at surfaces and interfaces. The excellent surface specificity of the SFG allows for probing different kinds of liquid interfaces with no or negligible contribution from adjacent and much deeper bulk phase. VSFG spectroscopy has provided evidence that the structure of the water at interfaces is different from the bulk. With the ultrafast pulses, VSFG can also be used as a probe of ultrafast vibrational dynamics at interfaces. However, apart from a few pioneering studies, the extension of VSFG into time domain has not been explored extensively. Here VSFG is used as a probe of ultrafast vibrational dynamics of water at silica interfaces. Silica is an excellent model system for the solid phase where one can systematically vary the surface charge via bulk pH adjustment. The extension of the surface electric field, the interfacial thickness and surface accumulation of ions at a charged silica surface were studied using IR pump-VSFG probe spectroscopy. A vibrational lifetime (T1) of about 250 fs, similar to bulk H2O, was observed for the O-H stretch of H2O/silica interface when the silica surface is negatively charged. At the neutral surface, where the thickness of interfacial water is smaller than at the charged surface, the vibrational lifetime of O-H stretch becomes more than two times longer (T1~ 600 fs) due to the decreased number of neighboring water molecules, probed by SFG. The fast T1 at negatively charged surface begins to slow down by screening of the penetration of surface electric field via adding salt which suggests the primary reason for similar vibrational dynamics of water at charged interface with bulk water is the penetration of electric field. By decoupling of OH of HDO in D2O, a frequency dependent vibrational lifetime is observed with faster T1 at the red compared to the blue side of the hydrogen bond spectral region. This correlates with the redshift of the SFG spectra with increasing charged surface and is consistent with a theoretical model that relates the vibrational lifetime to the strength of the hydrogen bond network. / Chemistry

The distinct structure and dynamics of interfacial water are due to a break in the extended hydrogen bonding network present in bulk water. At solid-aqueous interfaces, the presence of surface charge, which induces a static electric field, and the electrolytes, which are present in most naturally relevant systems, can additionally perturb the hydrogen bonding environment due to polarization. The interplay between the surface-charge-induced electric field and the ions in changing the structure of interfacial water has important consequences in the chemistry of processes ranging from protein-water interactions to mineral-water reactivity in oil recovery. Accessing information about the first few layers of water at buried interfaces is challenging. Vibrational sum-frequency generation (vSFG) spectroscopy is a powerful technique to study exclusively the interfacial region and is used here to investigate the role of interfacial solvent structure on surface reactivity. It is known that the rate of quartz dissolution increases on addition of salt at neat water pH. The reason for this enhancement was hypothesized to be a consequence of perturbations in interfacial water structure. The vSFG spectra, which is a measure of ordering in the interfacial water structure, shows an enhanced effect of salt (NaCl) at neat pH 6~8. The trend in the effect of salt on vSFG spectra versus the bulk pH is remarkably consistent with the enhancement of rate of quartz dissolution, providing the first experimental correlation between interfacial water structure and silica dissolution. If salt alters the structure of interfacial water, it must affect the vibrational energy transfer pathways of water, which is extremely fast in bulk water (~130 fs). Thus far, the role of ions on the vibrational dynamics of water at charged surfaces has been limited to the screening effects and reduction in the depth of the region that contributes to vSFG. Here, we measure the ultrafast vibrational relaxation of the O-H stretch of water at silica at different bulk pH, using time-resolved (TR-vSFG). The fast vibrational dynamics of water (~200 fs) observed at charged silica surfaces (pH 6 and pH 12), slows down (~600 fs) on addition of NaCl only at pH 6 and not at pH 12. On the other hand at pH 2 (neutral surface), the vibrational relaxation shows an acceleration at high ionic strengths (0.5 M NaCl). The TR-vSFG results suggest that there is a surface-charge dependence on the sensitivity of the interfacial dynamics to ions and that reduction in the probe depth of vSFG alone cannot explain the changes in the vibrational lifetime of interfacial O-H. This is further supported by the cation specific effects observed in the TR-vSFG of the silica/water interface. While the vibrational relaxation of O-H stretch slows on addition of all salts (LiCl, NaCl, RbCl, and CsCl), the degree of slowing down is sensitive to the cation identity. The vibrational lifetime of O-H stretch in the presence of different cations follows the order: Li+ < Na+ < Rb+, consistent with previous Hofmeister effect reported in vSFG spectroscopy as well as AFM measurements at silica/water interface. To provide molecular insight on the effect of surface charge density and ionic strength on the changes in interfacial water structure, Molecular Dynamics (MD) simulations were performed on water at different types of surfaces. It was shown that the properties of water near the interface, e.g., a net orientation and the depth to which this persists, depend on the degree of specific adsorption of the counter ions. Our vSFG results, along with the insights from MD simulations, highlight the importance of considering the role of ions on the solvent structure within the electric double layer region, beyond the screening effects predicted by classical electrochemical models. / Chemistry

<p> In this work, tunable Mid-Infrared (IR) light sources based on quasi-phase matched (QPM) difference frequency generation (DFG) by periodically poled lithium niobate (PPLN) crystals are studied. The theory of DFG and the characteristics of lithium niobate crystals are described and analyzed. Characteristics of the wavelength tuning of QPM DFG by PPLN crystals are studied. In order to analyze in detail, simulation and experimental data of the widely tunable mid-IR laser source around 2 1- μm to 5 1- μm wavelength are presented. The simulations of DFG process by PPLN are conducted based on the nonlinear optics reported. In the experiment, a 1.064 μm Nd:YAG laser and a tunable Ti:sapphire laser are employed as the signal and pump lasers, respectively. Based on the studies of the wavelength tuning characteristics at different temperatures, an optimization procedure to achieve a maximum wavelength tuning range is proposed. The potential applications in gas detection of the mid-IR source are also described briefly. Recommendation for future works and potential applications of the PPLN DFG based mid-IR lasers are discussed. </p> / Thesis / Master of Applied Science (MASc)