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Since the Eu3+ ion can occupy different cation sites in a host material, it can serve as a useful probe of nanocrystalline structures to gain more insight into the structural changes that can occur when the particle size is reduced from the bulk to nanometer regime. The use of laser spectroscopy to probe two nanocrystalline structures, Eu3+:Y2O3 and Eu3+:CaO, was investigated. The nanocrystalline structures were prepared by the laser-vaporization-gas-phase condensation of the bulk oxides. The particle size distribution and dominant particle diameters of the nanocrystals were determined by transmission electron microscopy. The particle size dependency results of Eu3+:Y2O3 revealed three distinct phases: (1) the sharp lines of the monoclinic Y2O3 were dominant in the larger particles; (2) the C2 site of the cubic phase, which appears in the smaller particles; and (3) the amorphous phase that increases in intensity as the particle size decreases. The observation of distinct spectral lines from the monoclinic phase confirms the presence of a crystalline phase for all of particle sizes studied. The site-selective results of various concentrations of 13-nm Eu3+:CaO showed that the laser-vaporization-gas-phase condensation method of preparation produced two europium-containing phases at most concentrations: cubic CaO and monoclinic Eu2O3. Results showed that the monoclinic Eu2O3 phase could be reduced by 95% by annealing at 800 0C for 30 minutes without particle growth. Since the Eu3+ ion and the Y3+ ion are isovalent, the substitution of a Eu3+ ion into Y2O3 is considered a trivial case of extrinsic disorder since the impurity is neutral relative to a perfect crystal1. As a result, it is not necessary to have any other defects present in the crystals to maintain charge neutrality. With Eu3+:CaO, the dopant and host cation charges are different and therefore the dopant distribution can be investigated by site-selective spectroscopy. Since the experimental dopant distribution results for nanocrystalline Eu3+:CaO were inconclusive, a model to predict the theoretical change in the dopant distribution in Eu3+:CaO as a function of particle size was developed. The model predicts that the defect chemistry is affected when the particle size is approximately 50 nm and smaller. / Ph. D.

The detection and measurement of gas properties has become essential to meet rigorous criteria of environmental unfriendly emissions and to increase the energy production efficiency. Although low cost devices such as pellistors, semiconductor gas sensors or electrochemical gas sensors can be used for these applications, they offer a very limited lifetime and suffer from cross-response and drift. On the contrary, gas sensors based on optical absorption offer fast response, zero drift, and high sensitivity with zero cross response to other gases. Hence, over the last forty years, diode laser spectroscopy (DLS) has become an established method for non-intrusive measurement of gas properties in scientific as well as industrial applications. Wavelength modulation spectroscopy (WMS) is derivative form of DLS that has been increasingly applied for making self-calibrated measurements in harsh environments due to its improved sensitivity and noise rejection capability compared to direct absorption detection. But, the complexity in signal processing and higher scope of error (when certain restrictions on operating conditions are not met), have inhibited the widespread use of the technique. This dissertation presents a simple and novel strategy for practical implementation of WMS with commercial diode lasers. It eliminates the need for pre-characterization of laser intensity parameters or making any design changes to the conventional WMS system. Consequently, sensitivity and signal strength remain the same as that obtained from traditional WMS setup at low modulation amplitude. Like previously proposed calibration-free approaches, this new method also yields absolute gas absorption line shape or absorbance function. Residual Amplitude Modulation (RAM) contributions present in the first and second harmonic signals of WMS are recovered by exploiting their even or odd symmetric nature. These isolated RAM signals are then used to estimate the absolute line shape function and thus removing the impact of optical intensity fluctuations on measurement. Uncertainties and noises associated with the estimated absolute line shape function, and the applicability of this new method for detecting several important gases in the near infrared region are also discussed. Absorbance measurements from 1% and 8% methane-air mixtures in 60 to 100 kPa pressure range are used to demonstrate simultaneous recovery of gas concentration and pressure. The system is also proved to be self-calibrated by measuring the gas absorbance for 1% methane-air mixture while optical transmission loss changes by 12 dB. In addition to this, a novel method for diode laser absorption spectroscopy has been proposed to accomplish spatially distributed monitoring of gases. Emission frequency chirp exhibited by semiconductor diode lasers operating in pulsed current mode, is exploited to capture full absorption response spectrum from a target gas. This new technique is referred to as frequency chirped diode laser spectroscopy (FC-DLS). By applying an injection current pulse of nanosecond duration to the diode laser, both spectroscopic properties of the gas and spatial location of sensing probe can be recovered following traditional Optical Time Domain Reflectometry (OTDR) approach. Based on FC-DLS principle, calibration-free measurement of gas absorbance is experimentally demonstrated for two separate sets of gas mixtures of approximately 5% to 20% methane-air and 0.5% to 20% acetylene-air. Finally, distributed gas monitoring is shown by measuring acetylene absorbance from two sensor probes connected in series along a single mode fiber. Optical pulse width being 10 nanosecond or smaller in the sensing optical fiber, a spatial resolution better than 1 meter has been realized by this technique. These demonstrations prove that accurate, non-intrusive, single point, and spatially distributed measurements can be made in harsh environments using the diode laser spectroscopy technology. Consequently, it opens the door to practical implementation of optical gas sensors in a variety of new environments that were previously too difficult. / Ph. D.

The major thrusts of this work have been: 1) To develop a high-efficiency low-power TM-010 microwave cavity for nitrogen support gas at atmospheric pressure, 2) To discover and physically characterize potential laser and emission spectroscopic applications of this atom source, with a particular emphasis on laser-induced fluorescence. The result is the most efficient microwave-induced plasma cavity for nitrogen at one atmosphere that exists to date, giving stable and analytically useful molecular plasmas with only 50 Watts applied power. It is called the “High-Efficiency Molecular Microwave Plasma" (HEMMP) cavity. The HEMMP possesses excellent vaporization and atomization properties. It can handle aqueous sample flows of around 1 mL/min, introduced as an aerosol from a nebulizer. A detection system and sampling system were designed and an analytical instrument was built around the HEMMP cavity. Details of construction, operating conditions and operation of the instrument are described. Applications investigated include laser-induced fluorescence (LIF), atomic emission spectroscopy (AES), and laser-enhanced ionization (LEI) [also known as the opto-galvanic effect (OGE)]. The major emphasis of the application work has been physical characterization of the low-power nitrogen plasma as an atom source for LIF. This is the first time that either laser-induced fluorescence or laser-enhanced ionization have been observed and extensively characterized in any microwave-induced plasma (MIP). This is also the first time that atomic emission has been studied in a low-power N₂-MIP. LIF, AES, and LEI signal intensities were studied as a function of applied microwave power, support gas flow rate, signal observation height, and support gas composition using nitrogen and argon mixtures. Results for LIF yielded detection limits in the very low parts per billion range, and for AES in the low parts per billion range. Limit of detection (LOD) and background noise studies were done for all 3 techniques. Signal intensities were measured as a function of laser light intensity for LIF and LEI. Laser saturation was not observed with 300 mW power from the CW dye laser. The effects of electrode geometry and applied electrode voltage on LEI signals were also studied. Extensive background spectral studies were done for the nitrogen plasma. Analytical feasibility has been demonstrated for AES, LIF, and LEI in the low-power nitrogen MIP. The results presented provide the background physical investigations required for a full-scale development of these techniques for chemical analysis. / Ph. D.

This thesis presents applications of near infrared diode lasers to high resolution spectroscopy of transient radical species. Firstly, time resolved near infrared laser gain versus absorption is utilised in Chapter 2 to determine the I∗ quantum yield following ultraviolet photolysis of iodobenzene and its fluorinated analogues. The experimental method is first confirmed by comparison with literature values of the quantum yield for iodomethane photolysis, returning a quantum yield of Φ(I∗) = 0.71 ± 0.04 in good agreement with the literature, before being applied to determine the I∗ quantum yield following 248 nm and 266 nm photolysis of iodobenzene (Φ(I^∗) = 0.28 ± 0.04) and pentafluoroiodobenzene (Φ(I^∗) = 0.32 ± 0.05). The I^∗ quantum yields for 4-fluoroiodobenzene, 2,4-difluoroiodobenzene and 3,5-difluoroiodobenzene are also reported in order to determine the effect of selective fluorination on the dynamics of the photodissociation process. This work complements velocity-map ion imaging studies and spin-orbit resolved ab initio calculations of the ultraviolet photolysis of these compounds. Chapter 3 details the development of a narrow-bandwidth tunable continuous wave ultraviolet radiation source, through sum frequency mixing of tunable near infrared diode lasers with a fixed frequency, high powered, solid state laser. The application of the UV radiation source to spectroscopy of the A¹A₂ − X¹A₁ electronic band of formaldehyde is explored, where absolute absorption cross sections are determined for rotational transitions within the 220410 and 220430 vibronic bands. The sub-Doppler resolution has allowed refinement of the rotational constants for the slowly predissociating excited state of the 2²₀4³₀ vibronic band. The lifetimes of several rotational levels is determined to be in the range 0.74 ns to 1.46 ns. In Chapter 4 the UV radiation source developed in Chapter 3 is applied to the A ²Σ⁺ − X ²Π electronic band of the OH radical. Firstly, this source is utilised to probe a continuous supply of hydroxyl radicals using cavity-enhanced absorption spectroscopy and wavelength modulation spectroscopy. Pressure induced broadening parameters for the Q₁(2) rotational transition for He, Ne, Ar and N₂ buffer gases are also measured. Following the successful application of this source to probe a continuous OH source at atmospheric pressure, the UV spectrometer is used to probe OH radicals from nitric acid photolysis at 193 nm, where the nascent speed distribution and Doppler lineshape is shown to be in excellent agreement with the literature. Time resolved absorption spectroscopy of the nascent OH fragment also returns a translational relaxation constant of k_trans = (3.85±1.06)×10⁻¹⁰cm³molecule⁻¹s⁻¹, which is in good agreement with literature values. These preliminary results indicate the potential of this narrow-bandwidth tunable UV source as an absorption-spectroscopy-based probe of nascent Doppler profiles. Chapter 5 presents the application of frequency-modulated radiation from a near infrared diode laser as a probe of the angular momentum polarisation of the nascent CN fragments, produced by 266 nm photolysis of ICN. These CN fragments are probed in the high rotational states of both the ground and first excited vibrational level on the A ²Π − X ²Σ⁺ electronic transition; in particular these constitute the first measurements of alignment and orientation in the first excited vibrational level at this photolysis wavelength. The alignment parameters reported for both vibrational levels are comparable, indicating that the incoherent dynamics contributing to their formation are the same. In contrast, the orientation of the v = 1 CN fragment is shown to be of opposite sign to that of v = 0 at this photolysis wavelength, although the absolute differences in their orientation parameters are similar to that observed for photolysis at 248 nm. This observation is consistent with coherent orientation arising from phase differences between wavepackets propagating on multiple excited potential energy surfaces.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 1975 / Vita. / Includes bibliographical references. / Ph. D. / Ph. D. Massachusetts Institute of Technology, Department of Physics

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemistry, 1979 / Includes bibliographical references. / Ph. D. / Ph. D. Massachusetts Institute of Technology, Department of Chemistry

Applications of laser absorption spectroscopy for trace gas detection are many and diverse, ranging from the environmental and atmospheric to the medical and industrial. The aim of creating a spectrometer which combines high sensitivities and selectivities (in order to measure small amounts of absorbers or species that are only weakly absorbing, in a complex background matrix) with a wide spectral coverage (to allow broadband absorbers or multi-component samples to be studied) can be realised by implementing three separate concepts: the exploitation of the strong, fundamental transitions of the mid-infrared; the use of sensitive spectroscopic techniques; and the selection of a widely tunable laser source. In this thesis, these ideas are investigated individually and in combination in order to achieve such a goal. Laser spectroscopic techniques based on optical cavities are used to build a high resolution spectrometer covering a large spectral range capable of selectively detecting low levels of gaseous compounds of interest, especially those of medical or environmental significance. Work in both the near- and mid-infrared is presented, including much of the initial, developmental work which was conducted in the former region. The thesis begins with an overview of both narrowband and broadband near-infrared radiation sources, with a particular emphasis on commonly available diode lasers (DLs). A novel laser source, the digital supermode distributed Bragg reector (DS-DBR) laser, is introduced as a useful laser source for spectroscopy, combining the usual benefits of telecom DLs with a wide tunability (1563 – 1613 nm). The laser can be operated in an internal or external ramping mode, allowing the output wavelength to be scanned or stepped across a desired region. The observation of mode-hopping during the application of the scanning methodology is examined and rationalised. The ability of the DS-DBR laser to perform high resolution spectroscopy over its entire spectral coverage is demonstrated by recording spectra of carbon dioxide (CO₂) over this range, covering transitions from two of the four Fermi resonance components of the 3ν₁ + ν₃ combination band. The results of conducting wavelength modulation spectroscopy on CO₂ are also reported. A system developed for performing cavity ring-down spectroscopy (CRDS), capable of the real-time retrieval of ring-down times (RDTs), is presented and discussed. The outcomes of initial tests performed with a conventional DL at 1557 nm, to study a calibrated mixture of CO₂ in air at various pressures, are given. In addition, the results of combining this system with the DS-DBR laser are discussed. The bandwidth of the DS-DBR laser was found to be larger than that of a standard DFB DL, resulting in the presence of noisy cavity modes. Despite this, the acquisition of reproducible RDTs is demonstrated, with single wavelength studies of an evacuated cavity at 1605.5 nm yielding a RDT of 24.54 ± 0.04 µs and Allan variance calculations signalling an attainable minimum detectable absorption coefficient, α_min, of 2.8 x 10^-10 cm^-1 over 20 s. The ability to perform CRDS across the whole DSDBR laser wavelength range without the need for cavity re-alignment is illustrated, and studies conducted on CO₂ in air, calibrated mixtures and breath are reported. Investigations are also described into the accurate determination of the ¹³C/¹²C ratio in exhaled CO₂ undertaken using CRDS and cavity enhanced absorption spectroscopy (CEAS) on CO₂ isotopologues, an approach which can be utilised as a diagnostic aid in determining Helicobacter pylori infection. The focus of the thesis then moves to the mid-infrared, to describe quasi phase matching difference frequency generation (QPM-DFG) and its use to generate laser light at 3 µm by optically mixing near-infrared DLs. The theory behind this non-linear optical interaction is outlined, and the construction of a free-space QPM-DFG system using periodically poled lithium niobate is detailed and characterised. This DL-based QPM-DFG arrangement has been coupled with the CRDS system developed to create a mid-infrared CRD spectrometer. The results of single wavelength studies indicate RDTs of ~ 6 µs and an achievable αmin of 2.9 x 10^-9 cm^-1 over 44 s for an evacuated cavity. Spectroscopic investigations carried out on methane (CH₄), acetone and deuterium are documented; for the latter species, Dicke narrowing of the electric quadrupole ν(1←0) Q(2) transition at 2987.29 cm^-1 is observed and the integrated absorption cross-section for the same transition measured as 2.29 ± 0.03 x 10^-27 cm²cm^-1molec^-1. The results of modifications made to the system, namely the use of a more powerful Nd:YAG laser as the pump radiation source, as well as a faster detector combined with a variable amplifier, are presented; these include the observation of an improved optimal α_min of 6.4 x 10^-10 cm^-1 over 151 s for an empty cavity. Finally, work utilising the DS-DBR laser as one of the near-infrared sources for the QPM-DFG set-up is presented. This configuration generates radiation covering a wide mid-infrared range (3130 – 3330 nm) and has been used to perform direct absorption and wavelength modulation spectroscopy on ro-vibrational transitions within the fundamental ν₃ (F₂) band of CH₄. The spectrum of methanethiol (CH₃SH) over this region has also been investigated, with preliminary studies identifying a feature at 3040 cm^-1 as a potential indicator for monitoring this biomarker in breath. The results of coupling this mid-infrared radiation with an optical cavity to perform CEAS combined with phase sensitive detection are subsequently reported. Studies were conducted on calibrated CH₄ mixtures and ambient air to examine two transitions of the fundamental ν₃ (F₂) band of CH₄ in order to characterise the system: effective path lengths of ~ 700 m and α_min of 6.2 x 10^-8 cm^-1 over 8 s were found. The ^RQ₄ CH₃SH absorption feature at 3040 cm^-1 was also further studied with this system using prepared samples of CH₃SH in N₂ at different concentrations, yielding a CH₃SH detection limit of 2.4 ppm at 19 Torr. The potential of such a cavity-based, DS-DBR sourced, QPM-DFG mid-infrared spectrometer for trace gas sensing having thus been demonstrated, possible improvements that could be implemented to increase the sensitivity of the system are then discussed.

Thesis (MSc (Physics))--University of Stellenbosch, 2009. / An ultrafast pump-probe spectroscopy experiment was developed in order to investigate the fast photoinduced isomerization reaction of the molecule resveratrol. Characteristics of the resveratrol molecule are discussed, including the photoisomerization reaction from trans- to cis-resveratrol. The experimental setup for the conventional spectroscopy measurement was developed and characterized in order to investigate and understand the conventional absorption and uorescence spectroscopy of resveratrol thoroughly. The absorption spectra for both trans- and cis-resveratrol, as well as the uorescence spectra were measured, discussed and explained. This therefore forms a foundation and serves as an initial step to develop a pump-probe spectroscopy experiment for resveratrol. A general overview of ultrafast pump-probe spectroscopy is presented, as well as an explanation of the nal developed experimental setup. The principles and characteristics of the chirped pulse ampli cation (CPA) femtosecond laser source and the tunable noncollinear optical parametric ampli er (NOPA) employed as the pump pulse are discussed. The process of white light continuum (WLC) generation was investigated to utilize as the ultrashort probe pulse. Two white light continuum generation experimental setups were developed and characterized for WLC generation in a transparent medium with the fundamental CPA laser light at 775 nm (in sapphire) and with the second harmonic (SH) of the CPA light at 387 nm (in quartz). A spectrometer was designed, built and characterized in conjuction with a line focus, for simultaneous measurement of the absorption in the pumped, unpumped and reference regions in the sample. In this way the photoisomerization of resveratrol could be measured with temporal resolution as a transient absorption signal. A 420 μg/ml resveratrol solution in ethanol was investigated in this pump-probe spectroscopy experiment and the results obtained are discussed accordingly.

Thesis (MSc)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: The large band gap (approximately 11.5-12.1 eV) and high transmission of calcium fluoride (CaF2) crystal in the ultraviolet (UV) and vacuum ultraviolet (VUV) region makes it an important material for optics for laser applications in UV. However, CaF2 degrades during long exposure to UV irradiation due to defect generation. The formation of selftrapped excitons (STE) is considered the first step in defect generation. In this project the possibility of observing STE states in CaF2 using a narrow bandwidth tunable VUV laser source is investigated. This is the first spectroscopy study of an alkaline earth fluoride using VUV tunable laser radiation instead of a fixed wavelength laser. The use of a VUV tunable laser source has potential for determining the energies of the STE states, which are unknown. Our main objective is addressed by developing techniques to measure absorption spectra of pure and doped CaF2 samples, using a VUV scanning monochromator and a tunable VUV laser, and by doing a literature study. The results obtained with the scanning monochromator show absorption features in 126-180 nm range of all our samples. These vary for different samples and correlate with information from the supplier on the samples’ fluorescence spectra. Total absorption of the VUV light by CaF2 in the 115-126 nm range is observed. With the narrow bandwidth tunable laser light, absorption spectra were obtained in the range of 143-146.7 nm of all our CaF2 samples. No significance peaks which can be related to the STE states in CaF2 were observed in the VUV laser absorption spectra, but the results are valuable to improve the technique. The conclusion is that either a different spectral range or fluorescence detection can be investigated in future. / AFRIKAANSE OPSOMMING: Die groot bandgaping (ongeveer 11.5-12.1 eV) en hoë transmissie van kalsiumfluoried (CaF2) kristal in die ultraviolet (UV) en die vakuum ultraviolet (VUV) gebied maak dit ’n belangrike materiaal vir optika vir laser toepassings in die UV. CaF2 degradeer egter gedurende langdurige blootstelling aan UV lig as gevolg van die generering van defekte. Die vorming van ’n elektron-holte paar wat deur die kristalstuktuur gestabiliseer word teen rekombinasie (self-trapped excitons, afgekort STE) word beskou as die eerste stap in defek generering. In hierdie projek word die moontlikheid ondersoek om STE toestande in CaF2 waar te neem deur die gebruik van ’n afstembare VUV laserbron met emissie in ’n smal spektrale band. Dit is die eerste spektroskopiese studie van ’n aardalkali-fluoried deur die gebruik van afstembare VUV laserlig in plaas van ’n vaste golflengte laser. Die gebruik van ’n afstembare VUV laserbron het potensiaal vir die bepaling van die energieë van die STE teostande, wat onbekend is. Ons hoofdoel word aangespreek deur die ontwikkeling van tegnieke vir die meet van absorpsie spektra van suiwer en gedoteerde CaF2 monsters met behulp van ’n VUV skanderende monochromator en ’n afstembare VUV laser, en deur ’n literatuurstudie. Die resultate wat behaal is met die skanderende monochromator toon die absorpsieprofiele van al ons monsters in die 126-180 nm spektrale gebied. Die absopsieprofiele varieer vir die verskillende monsters en korreleer met die inligting van die verskaffer oor die fluoressensie spektra van die monsters. Totale absorpsie van die VUV lig deur CaF2 in die 115-126 nm gebied is waargeneem. Met die smalband afstembare laserlig is absorpsie spektra in die 143-146.7 nm gebied vir al ons CaF2 monsters verkry. Geen beduidende pieke wat verband hou met die STE toestande in CaF2 is waargeneem in die VUV laser absorpsie spektra nie, maar die resultate is waardevol vir die verbetering van die tegniek. Die gevolgtrekking is dat of ’n ander spektraalgebied of fluoressensiedeteksie in die toekoms ondersoek kan word.