Dual-comb spectroscopy (DCS) has widespread applications. It has become a more prominent spectroscopic tool because it has broad spectral coverage with high frequency resolution. We demonstrate the broadband and high resolution of DCS to probe transient events, showing the rst use of DCS of laser-induced plasmas (LIPs). Our measurements span absorption features 7 THz wide, simultaneously detecting Rb D2, K D1 and D2 absorption lines with the ability to resolve the isotope ratios in the Rb D2 line. This technique is more broadband and faster than tunable laser absorption spectroscopy because it eliminates the requirement to scan across transitions. Additionally, DCS makes higher resolution measurements than laser-induced break- down spectroscopy. Our ultimate goal is to use DCS as a technique to ascertain the chemical composition of unknown samples. Our rst demonstration of this technique illustrates that DCS makes broadband, high-resolution measurements with the ability to measure isotope ratios, which is necessary for determining sample composition.

<p>The spectroscopic, SEM and Raman measurements on carbon nanotubes under the exact conditions of which OES analysis were made showed that at<br /> a pressure of 400 Torr and a flow rate of 200 sccm, the quality and quantity of single-walled carbon nanotubes was the highest.</p>

<p>The spectroscopic, SEM and Raman measurements on carbon nanotubes under the exact conditions of which OES analysis were made showed that at<br /> a pressure of 400 Torr and a flow rate of 200 sccm, the quality and quantity of single-walled carbon nanotubes was the highest.</p>

Magister Scientiae - MSc / The spectroscopic, SEM and Raman measurements on carbon nanotubes under the exact conditions of which OES analysis were made showed that at a pressure of 400 Torr and a flow rate of 200 sccm, the quality and quantity of single-walled carbon nanotubes was the highest. / South Africa

<p>Laser-induced plasmas since their discovery in the 1960’s have found numerous applications in laboratories and industries. Their uses range from soft ionization source in mass spectroscopy, development of compact particle accelerator, and X-ray and deep UV radiation sources to diagnostic techniques such as laser-induced breakdown spectroscopy and laser electronic excitation tagging. In addition, the laser-induced plasma is important for studying of various nonlinear effects at beam propagation, such as laser pulse filamentation.</p> <p>This work deals with two challenging aspects associated with laser-induced plasmas. First is the study of Multi-Photon Ionization (MPI) as a fundamental first step in high-energy laser-matter interaction critical for understanding of the mechanism of plasma formation. The second is application of laser induced plasma for diagnostics of combustion systems.</p> <p>Numerous attempts to determine the basic physical constants of MPI process in direct experiments, namely photoionization rates and cross-sections of the MPI, were made; however, no reliable data was available until now, and the spread in the literature values often reached 2–3 orders of magnitude. This work presents the use of microwave scattering in quasi-Rayleigh regime off the electrons in the laser-induced plasma as method to measure the total number of electrons created due to the photoionization process and subsequently determine the cross-sections and rates of MPI. Experiments were done in air,<i> O₂, Xe, Ar, N₂, Kr</i>, and <i>CO</i> at room temperature and atmospheric pressure and femtosecond-laser pulse at 800 nm wavelength was utilized. Rayleigh microwave scattering (RMS) technique was used to obtain temporally resolved measurements of the electron numbers created by the laser. Numbers of electrons in the range 3 × 10⁸–3 × 10¹² were produced by the laser pulse energies 100–700 <i>μ</i>J and corresponding electron number densities down to about 10¹⁴ cm^-3 in the center of laser-induced spark were observed. After the laser pulse, plasma decayed on the time scale from 1 to 40 ns depending on the gas type and governed by two competing processes, namely, the creation of new electrons from ionization of the metastable atoms and loss of the electrons due to dissociative recombination and attachment to oxygen. </p> <p>Diagnostics of combustion at high pressures are challenging due to increased collisional quenching and associated loss of acquired signal. In this work, resonance enhanced multiphoton photon ionization (REMPI) in conjunction with measurement of generated electrons by RMS technique were used to develop diagnostics method for measuring concentration of a component in gaseous mixture at elected pressure. Specifically, the REMPI-RMS diagnostics was developed and tested in the measurements of number density of carbon monoxide (<i>CO</i>) in mixtures with nitrogen (<i>N₂</i>) at pressures up to 5 bars. Number of REMPI-induced electrons scaled linearly with <i>CO</i> number density up to about 5×10¹⁸ cm^-3 independently of buffer gas pressure up to 5 bar, and this linear scaling region can be readily used for diagnostics purposes. Higher <i>CO</i> number densities were associated laser beam energy loss while travelling through the gaseous mixture. Four (4) energy level model of <i>CO</i> molecule was developed and direct measurements of the laser pulse energy absorbed in the two-photon process during the passage through the <i>CO</i>/<i>N₂</i> mixture were conducted in order to analyze the observed trends of number of REMPI-generated electrons with <i>CO</i> number density and laser energy.</p>

<p>Constructive elastic microwave scattering, or, historically, coherent microwave scattering (CMS), refers to the inference of small plasma object characteristics via in-phase electromagnetic scattering – and has become a valuable technique in applications ranging from photoionization and electron-loss rate measurements to trace species detection, gaseous mixture and reaction characterization, molecular spectroscopy, and standoff measurement of local vector magnetic fields in gases through magnetically-induced depolarization. Notable advantages of the technique include a high sensitivity, good temporal resolution, low shot noise, non-intrusive probing, species-selectivity when coupled with resonance-enhanced multiphoton ionization (REMPI), single-shot acquisition, and the capability of time gating due to continuous scanning.</p> <p>Originally, the diagnostic was used for the measurement of electron total populations and number densities in collisional, weakly-ionized, and unmagnetized small plasma objects – so called collisional scattering. However, despite increased interest in recent years, the technique’s applicability to collisionless plasmas has remained relatively unexplored. This dissertation intends to expand upon the theoretical, mathematical, and experimental basis for CMS and demonstrate the constructive Thomson & Rayleigh scattering regimes for the first time. Furthermore, this work seeks to explore other novel and relevant capabilities of CMS including electron momentum-transfer collision frequency measurements via scattered phase information and spatially-resolved electron number characterizations of elongated plasma filament structures.</p> <p>This dissertation additionally leverages the technique to diagnose microplasmas and situations of particular interest. Primarily, photoionization (PI) – including UV resonance-enhanced multiphoton ionization, non-resonant visible PI, and mid-IR tunneling ionization in gaseous media. Such processes bear importance to studies on nonequilibrium plasmas, soft ionization in mass spectrometry, the development of compact particle accelerators, X-ray and deep UV radiation sources, laser-assisted combustion, laser-induced breakdown spectroscopy, species detection, mixture characterization and spectroscopy, studies on nonlinear beam propagation (filamentation, self-trapping and pulse splitting, dispersion, modulation instabilities), and so on. Finally, the application of CMS to ion thrusters is demonstrated.</p>

Après des décennies de développement, l'ablation laser est devenue une technique importante pour un grand nombre d'applications telles que le dépôt de couches minces, la synthèse de nanoparticules, le micro-usinage, l’analyse chimique, etc. Des études expérimentales ainsi que théoriques ont été menées pour comprendre les mécanismes physiques fondamentaux mis en jeu pendant l'ablation et pour déterminer l’effet de la longueur d'onde, de la durée d'impulsion, de la nature de gaz ambiant et du matériau de la cible. La présente thèse décrit et examine l'importance relative des mécanismes physiques qui influencent les caractéristiques des plasmas d’aluminium induits par laser. Le cadre général de cette recherche forme une étude approfondie de l'interaction entre la dynamique de la plume-plasma et l’atmosphère gazeuse dans laquelle elle se développe. Ceci a été réalisé par imagerie résolue temporellement et spatialement de la plume du plasma en termes d'intensité spectrale, de densité électronique et de température d'excitation dans différentes atmosphères de gaz inertes tel que l’Ar et l’He et réactifs tel que le N2 et ce à des pressions s’étendant de 10‾7 Torr (vide) jusqu’à 760 Torr (pression atmosphérique). Nos résultats montrent que l'intensité d'émission de plasma dépend généralement de la nature de gaz et qu’elle est fortement affectée par sa pression. En outre, pour un délai temporel donné par rapport à l'impulsion laser, la densité électronique ainsi que la température augmentent avec la pression de gaz, ce qui peut être attribué au confinement inertiel du plasma. De plus, on observe que la densité électronique est maximale à proximité de la surface de la cible où le laser est focalisé et qu’elle diminue en s’éloignant (axialement et radialement) de cette position. Malgré la variation axiale importante de la température le long du plasma, on trouve que sa variation radiale est négligeable. La densité électronique et la température ont été trouvées maximales lorsque le gaz est de l’argon et minimales pour l’hélium, tandis que les valeurs sont intermédiaires dans le cas de l’azote. Ceci tient surtout aux propriétés physiques et chimiques du gaz telles que la masse des espèces, leur énergie d'excitation et d'ionisation, la conductivité thermique et la réactivité chimique. L'expansion de la plume du plasma a été étudiée par imagerie résolue spatio-temporellement. Les résultats montrent que la nature de gaz n’affecte pas la dynamique de la plume pour des pressions inférieures à 20 Torr et pour un délai temporel inférieur à 200 ns. Cependant, pour des pressions supérieures à 20 Torr, l'effet de la nature du gaz devient important et la plume la plus courte est obtenue lorsque la masse des espèces du gaz est élevée et lorsque sa conductivité thermique est relativement faible. Ces résultats sont confirmés par la mesure de temps de vol de l’ion Al+ émettant à 281,6 nm. D’autre part, on trouve que la vitesse de propagation des ions d’aluminium est bien définie juste après l’ablation et près de la surface de la cible. Toutefois, pour un délai temporel important, les ions, en traversant la plume, se thermalisent grâce aux collisions avec les espèces du plasma et du gaz. / After decades of development, laser ablation has become an important technique for a large number of applications such as thin film deposition, nanoparticle synthesis, micromachining, chemical analysis, etc. Experimental and theoretical studies have been conducted to understand the physical mechanisms of the laser ablation processes and their dependence on the laser wavelength, pulse duration, ambient gas and target material. The present dissertation describes and investigates the relative importance of the physical mechanisms influencing the characteristics of aluminum laser-induced plasmas. The general scope of this research encompasses a thorough study of the interplay between the plasma plume dynamics and the ambient gas in which they expand. This is achieved by imaging and analyzing the temporal and spatial evolution the plume in terms of spectral intensity, electron density and excitation temperature within various environments extending from vacuum (10‾7 Torr) to atmospheric pressure (760 Torr), in an inert gas like Ar and He, as well as in a chemically active gas like N2. Our results show that the plasma emission intensity generally differs with the nature of the ambient gas and it is strongly affected by its pressure. In addition, for a given time delay after the laser pulse, both electron density and plasma temperature increase with the ambient gas pressure, which is attributed to plasma confinement. Moreover, the highest electron density is observed close to the target surface, where the laser is focused and it decreases by moving away (radially and axially) from this position. In contrast with the significant axial variation of plasma temperature, there is no large variation in the radial direction. Furthermore, argon was found to produce the highest plasma density and temperature, and helium the lowest, while nitrogen yields intermediate values. This is mainly due to their physical and chemical properties such as the mass, the excitation and ionization levels, the thermal conductivity and the chemical reactivity. The expansion of the plasma plume is studied by time- and space-resolved imaging. The results show that the ambient gas does not appreciably affect plume dynamics as long as the gas pressure remains below 20 Torr and the time delay below 200 ns. However, for pressures higher than 20 Torr, the effect of the ambient gas becomes important and the shorter plasma plume length corresponds to the highest gas mass species and the lowest thermal conductivity. These results are confirmed by Time-Of-Flight (TOF) measurements of Al+ line emitted at 281.6 nm. Moreover, the velocity of aluminum ions is well defined at the earliest time and close to the target surface. However, at later times, the ions travel through the plume and become thermalized through collisions with plasma species and with surrounding ambient gas.