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We demonstrate experimentally that the multiphoton ionization rate in gallium arsenide depends on the alignment of the laser polarization with respect to the crystal axis. We show real-time modulation of 1900nm laser ionization rate, through viewing transmission, which mimics the symmetry of the semiconductor crystal. We propose that the modulation in the ionization rate arises because the varying reduced effective carrier mass, as predicted by Keldysh theory. We show direct comparison of the experimental transmission modulation depth with that predicted by Keldysh theory. This opens up a novel method for real-time non-invasive crystallography of crystalline materials.

We demonstrate experimentally that the multiphoton ionization rate in gallium arsenide depends on the alignment of the laser polarization with respect to the crystal axis. We show real-time modulation of 1900nm laser ionization rate, through viewing transmission, which mimics the symmetry of the semiconductor crystal. We propose that the modulation in the ionization rate arises because the varying reduced effective carrier mass, as predicted by Keldysh theory. We show direct comparison of the experimental transmission modulation depth with that predicted by Keldysh theory. This opens up a novel method for real-time non-invasive crystallography of crystalline materials.

We demonstrate experimentally that the multiphoton ionization rate in gallium arsenide depends on the alignment of the laser polarization with respect to the crystal axis. We show real-time modulation of 1900nm laser ionization rate, through viewing transmission, which mimics the symmetry of the semiconductor crystal. We propose that the modulation in the ionization rate arises because the varying reduced effective carrier mass, as predicted by Keldysh theory. We show direct comparison of the experimental transmission modulation depth with that predicted by Keldysh theory. This opens up a novel method for real-time non-invasive crystallography of crystalline materials.

Advancements in spectroscopy rely on the improvement of two fundamental characteristics: spatial and temporal resolutions. High harmonic spectroscopy (HHS) is an emerging technology that promises the capability of studying the fastest processes that exist today: electronic motion with angstrom spatial and attosecond temporal resolution. HHS is based on the process of high harmonic generation (HHG) which arises from the nonlinear interaction between an intense, infrared laser pulse and an atomic or molecular gaseous medium, producing coherent, attosecond-duration bursts of extreme ultraviolet (XUV) light. In order to utilize the attosecond pulses for spectroscopic measurements, it is necessary to improve the conversion efficiency of HHG. Chapter 2 of this thesis describes the improvements we make to the HHG source in order to obtain high XUV photon flux and we report on the nonlinear ionization of atomic systems using these pulses in Chapter 6. In Chapters 3 - 5, we describe several HHG experiments in complex, polyatomic molecules in order to promote the use of HHS as a general spectroscopic tool. Amplitude modulations in high harmonic spectra of complex molecules can be attributed to several types of interference conditions that depend on a system's molecular or electronic structure such as recombination with multiple centres or dynamical interference from multi-orbital contributions to ionization. Our results demonstrate the capability of HHS to extract useful information on molecular and electronic structure from large, polyatomic molecules directly from their high harmonic spectra. Furthermore, we use HHS to investigate the suppression of ionization in complex molecules due to quantum destructive interference during ionization as well as the distinguishability of emitted harmonic spectra from molecular isomers. Chapter 6 explores the study of multi-electron dynamics in complex molecules using XUV multiphoton ionization of atoms and molecules as well as the ionization and fragmentation of C60 which has hundreds of delocalized valence electrons. This thesis also describes the author's role in the design and fabrication of a time-of- flight mass spectrometer (Section 6.1) as well as an HHG detector system (Appendix A).

<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>

Neste trabalho investigamos a dinâmica de formação de nanopartículas de Au por pulsos de femtossegundos formatados (800 nm, 30 fs, 1 kHz e 2 mJ), induzida pela ionização da molécula de quitosana. Inicialmente desenvolvemos um sistema de formatação de pulsos ultracurtos que faz uso de um modulador espacial de luz, constituído por um arranjo linear de cristais líquidos, com o qual somos capazes de impor distintas modulações de fase ao pulso laser. Para monitorar o processo de produção de nanopartículas, montamos um sistema de excitação (pulsos de femtossegundos) e prova (luz branca), o qual permite a observação em tempo real do aparecimento da banda de plásmon e, consequentemente, da dinâmica de formação das nanopartículas. Resultados obtidos para pulsos não formatados (limitados por Transformada de Fourier) demonstraram que a formação de nanopartículas deve-se à ionização não linear da quitosana, a qual está relacionada à oxidação do grupo hidroxila para o grupo carbonila. Medidas de microscopia eletrônica de transmissão forneceram os tamanhos (entre 20 e 100 nm) e formatos (esferas, pirâmides, hexágonos, bastões, etc) das nanopartículas geradas. Ainda, nossos resultados revelaram que esta ionização é iniciada por absorção multifotônica, mais especificamente por absorção de 4 fótons. Utilizando pulsos formatados com fase espectrais constante, degrau e cossenoidal com diferentes frequências, investigamos a influência destes na formação de nanopartículas. Concluímos que os pulsos mais longos são mais favoráveis ao processo de ionização, e consequente redução dos íons de Au para a formação de nanopartículas metálicas. Este comportamento se deve, provavelmente, à redistribuição da energia absorvida para os modos vibracionais, o que é mais provável para pulsos mais longos. Assim, o método apresentado pode abrir novas maneiras para a formação de nanopartículas de metálicas, as quais podem ser mais exploradas dos pontos de vista aplicado e fundamental. / In this work we have studied the synthesis of Au nanoparticles using shaped ultrashort pulses (800 nm, 30 fs, 1 kHz and 2 mJ), induced by the ionization of the chitosan. Initially we developed a pulse shaping setup that uses a spatial light modulator (liquid crystals array), with which we are able to impose distinct phase mask to the laser pulse. In order to monitor the nanoparticles production process, we used a pump-probe system, consisting of femtosecond pulses (pump) and white light (probe), which allows the observation of the plasmon band enhancement and hence the nanoparticles formation dynamics. The results obtained by Fourier Transform limited pulses have shown that the nanoparticles formation is due to the nonlinear ionization of chitosan, which is related to hydroxyl group oxidation to the carbonyl group. Transmission electron microscopy measurements provided the sizes (20-100 nm) and shapes (spheres, pyramids, hexagons, rods, etc.) of the produced nanoparticles. Moreover, our results revealed that ionization is initiated by multiphoton absorption, more specifically by four photons absorption. Using pulses shaped with constant, step and cossenoidal (with different frequencies) spectral phase masks, we investigated their influence in the nanoparticles formation. We conclude that longer pulses are more favorable to the ionization process and, consequently, to the gold ions reduction for the synthesis of the metallic nanoparticles. This behavior is probably due to the redistribution of the absorbed energy to the vibrational modes, which is more likely for longer pulses. Therefore, the approach presented here can open new ways to produce metallic nanoparticles, which can be further explored from applied and fundamental points of view.

A new instrument using a time-of-flight (TOF) mass filter (MF) for resonance enhanced multiphoton ionization (REMPI) studies of gas phase molecules was developed. This instrument was designed to make use of the selectivity of the REMPI process and the sensitivity of a TOF-MF with a microchannel plate detector. A pulsed valve inlet system was tested to determine its effectiveness in “cooling” molecules but it was not used for the bulk of the studies performed. The instrument was tested using molecular and atomic bromine, atomic carbon, and atomic iodine. The atomic bromine, carbon, and iodine, were generated by the photodissociation of molecular bromine, hydrocarbons, and methyl iodide respectively. Nitrogen gas in air was used to test the pulsed valve system. The instrument was then used to conduct REMPI studies of five dichlorotoluene (DCT) isomers (2,4-DCT; 2,5-DCT; 3,4-DCT; 2,6-DCT; and 2,3-DCT). REMPI studies of six dichloroaniline (DCA) isomers, including 2,5-DCA; 3,4-DCA; 3,5-DCA; 2,6-DCA; 2,4-DCA; and 2,3-DCA were conducted. Six isomers of dichlorophenol (DCP) were studied (2,5-DCP; 3,4-DCP; 3,5-DCP; 2,3-DCP; 2,4-DCP; and 2,6-DCP). It was determined that the 2,3-DCP; 2,4-DCP; and 2,6-DCP isomers photodissociated to form CCl, which itself could be observed through a REMPI process. The results from the REMPI studies of the dichloroaromatics and data from previous ultraviolet, infrared, and RAMAN studies of these molecules was used in order to assign the observed peaks. The observed 0,0 π→π* transition energies of the dichloroaromatics studied were used along with ultraviolet 0,0 π→π* transition energies from previous works in order to discuss substituent effects. A qualitative method of predicting the relative location of 0,0 π→π* transition energies of dichloroaromatics was developed.