Negative muons were brought to rest in a gas mixture of 30 torr CH$\sb4$ and 570 torr D$\sb2$, using the cyclotron trap at PSI. The muons formed muonic deuterium atoms which diffused through the mixture and transferred their muons to the carbon of the CH$\sb4$ molecules. A planar germanium detector and a silicon detector were used to observe x-rays from the initial muon cascade in the deuterium, and from subsequent cascade in the muonic carbon after transfer. A transfer rate of (4.5 $\pm$ 1.8) $\times$ 10$\sp{10}$/sec was found which agrees well with a previous result measured at 50 bar of (5.1 $\pm$ 1.0) $\times$ 10$\sp{10}$/sec. Transfer was found to occur predominantly to the n = 4 state in $\mu$C. The initial angular momentum state distribution in the $\mu$C was constructed using the cascade program of V. Markushin, and found to be consistent with any combination of two possible initial distributions (I 0.252 (4s) + 0.409 (4f) + 0.339 (4p)) and (II 0.284 (4d)+ 0.377 (4f) + 0.339 (4p)). The transfer theories of Gershtein and that of Holtzwarth and Pfeifer both agree well with the measured transfer rate and initial energy state, but not with the initial angular momentum distributions. The 2S population in $\mu$C was also determined to lie between 5% and 11%, which is higher than the 3% population in direct capture.

Ubiquitous in nature, light-matter interactions constitute the pervasive foundation for many physical phenomena with application in optical and atomic physics, electrical and communication engineering, medicine, and biology. In many situations of interest, only the aftermath of light-matter interaction is experimentally accessible. The properties of light and matter are both encoded in the characteristics of this secondary source of radiation. In most cases, these field and intensity characteristics carrying information about the initial source of radiation or the specific material system are statistical in nature. A typical light-matter interaction experiment involves an initial source of radiation that interacts with a material system from which a secondary emission occurs. Measurements are usually performed on this secondary radiation, whose properties depend on both the characteristics of the primary source and the specifics of light-matter interaction. When the scope is to determine certain material properties, different approaches can be taken. For instance, one can actively modify the properties of the primary source of radiation to control different aspects of the interaction. This active modality offers great versatility in sensing applications. In many practical situations, however, accessing directly the primary source is simply not possible. In such circumstances, one is limited to so-called passive approaches where the sole source of information lies in the measurable properties of the secondary radiation. Contingent upon the intrinsic structural properties, the material system can be static or dynamic during the process of light-matter interaction and the experimental approaches may vary accordingly. In this thesis, we explore three different passive sensing scenarios based on different types of light-matter interactions. First, we examine a situation where the secondary radiation is the result of the continuous interaction between optical radiation and material systems with random structures that do not vary in time. In this case, we will discuss how the spatial coherence of the secondary emission can be used to extract information about either the static matter or the initial light source. Next, we study the cases when the secondary radiation originates from dynamic material systems in both steady-state and transient conditions. In the first case, when the matter is continuously excited, the intensity fluctuations are used to quantify the structural dynamics of the medium and retrieve its complex mechanical properties. As a particular application, we address the viscoelastic properties of blood, a typical example of a dynamic, optically-dense random medium. In the case of transient interactions, a low-intensity decaying signal is measured after the primary source shuts off. We discuss how subtle structural properties of matter can influence, even in these conditions, the rate of secondary emission.

This study investigates the minority carrier transport properties of wide bandgap semiconductors, primarily gallium oxide (Ga2O3) and gallium nitride (GaN). Ga2O3 is an emerging ultra-wide bandgap semiconductor with applications in high temperature electronics and sensors for use in extreme environments. Ga2O3 is a suitable material for devices deployed in the lower Earth satellite orbits due to its intrinsic radiation hardness, applications in solar-blind ultraviolet (UV) detection, and high power/high frequency electronics. The main factor limiting Ga2O3 technology so far is the reliable high mobility p-type Ga2O3; however, recent advances have shown a promising future for developments in this direction. Minority carrier transport properties such as minority carrier diffusion length (L) and lifetime (t) are of vital importance with the advent of p-type conductivity, as they are the limiting factor in the performance of bipolar devices. In this thesis, a comparison of the temperature dependence of L, t, and CL emission in n-type Si-doped Ga2O3 Schottky rectifiers, exposed to 18 MeV alpha particles and 10 MeV protons is presented. Additionally, the effect of electron injection, a countermeasure to in-situ mitigates the radiation damage, is studied in these structures. Electron injection has also been found to enhance L and t in unintentionally doped GaN. Lastly, the temperature dependence of minority carrier diffusion length and CL emission is presented in the novel p-type Ga2O3.

Optical beam steering is a key component in any application that requires dynamic (i.e. realtime control) of beam propagation through free-space. Example applications include remote sensing, spectroscopy, laser machining, targeting, Lidar, optical wireless communications (OWC) and more. The pointing control requirements for many of these applications can be met by traditional mechanical steering techniques; however, these solutions tend to be bulky, slow, expensive, power hungry and prone to mechanical failures leading to short component lifetimes. Two emerging applications, Lidar imaging and OWC, truly need improved beam-steering capabilities to flourish and support the advancement of self-driving cars or relieve the congestion in radio-frequency wireless networks, respectively. We consider the novel requirements of these applications during development of a new beam-steering technology. We introduce imaging-based beam steering (IBBS) that uses an imaging transform between spatial and directional domains to implement a new method of electronic beam-steering. We introduce this concept while focusing on transmitters (Tx) for OWC but the pointing control mechanism is bi-directional supporting both transmit and receive functionality, even out of the same aperture; likewise, features that make this solution compelling for OWC are also great for Lidar imaging. In IBBS, an array of high-speed sources are positioned at the focal plane of a lens and the lens passively collects, collimates and steers the beam into a conjugate direction. "Steering" is accomplished by selecting which source to use for an OWC link. This gives a coarse, pixelated beam-steering control that is well-suited for short-range OWC such as indoor communications and we present a prototype bulb for this application. Notably, multiple sources can be utilized at once with each steered into its conjugate directions and this presents the first beam-steering technology that supports multiple beams out of a single aperture; this feature uniquely supports multiplexed communications and fast, high-resolution Lidar imaging.

Fundamental studies of material surfaces are of continued interest to the development and improvement of many modern technologies, e.g. catalysis, energy efficient electronics, and high-capacity batteries etc. This dissertation targets two distinct sets of molecule-surface interactions relevant to the continued development of structure-property correlations using tools from Density Functional Theory with added verification from ultrahigh vacuum surface-science experiments. These include Haber-Bosch interactions at molybdenum-nitride surfaces and separation-dependent interactions between simple aromatics and Ru(0001) used to model a metal contact of Organic Electronic Devices (OEDs). In the first study, we focus on computational modelling of nitrogen fixation reactions on Mo- and N-terminated δ-MoN(0001). A comparative analysis to analogous predictions reported for Mo-terminated γ-Mo2N(111) sites demonstrates a near-total dependence on the atomic surface-structure with little to no impact from changes in sub-surface stoichiometry. Changing from Mo- to N-terminated surface drastically changes the reaction barriers such that the rate-limiting-step in the overall ammonia evolution reaction changes from NHx hydrogenation to N2 dissociative adsorption. In the second one, we explored the effect of changing metal-organic molecule separation on charge-transfer across the interface and the electronic properties of organic matter pertinent to OEDs. We studied various computational models of benzene and pyridine molecules held at fixed distances from Ru(0001) by introducing two-dimensional hexagonal SiO2 thin-films between molecules and the metal. Substantial metal-to-molecule charge-transfer is noted when molecules bind directly to the Ru interface, but virtually no interaction is noted when increasing metal-molecule separations up to ~12 Å. An analogous series of experiments investigating pyridine-Ru interactions introduced after exposing SiO2/Ru(0001) thin-films to varied doses of pyridine exhibits behavior similar to that predicted by theory.

The last two decades have witnessed the emergence of attosecond science, a new discipline that leverages light pulses with duration at the same characteristic time scale of electronic motion in atoms, molecules, and condensed matter. Attosecond spectroscopy has proven a particularly useful tool to monitor and control the evolution of transiently bound electronic states. These states, also known as autoionizing states, play a fundamental role in the ionization and charge transfer processes in matter. This thesis is a theoretical study on the role of such autoionizing states in the ionization of polyelectronic atoms by ultrashort pulses, which has been instrumental to four joint experimental and theoretical studies: i) with the group of Zenghu Chang, at UCF, concerning the core ionization of the argon atom at the L2,3 edge; ii) with the group of Arvinder Sandhu, at the University of Arizona, on the control of the lifetime of autionizing states in the argon atom, as well as iii) on the measurement of the lifetime of argon's dark autoionizing states via non-colinear four-wave mixing; iv) with the group of Louis DiMauro, at the Ohio State University, on the verification of the Kramers-Krönig relations in the ionization of laser-dressed argon. We have adopted several complementary theoretical approaches. In solving the time-dependent Schroedinger equation, we introduced a novel "essential states" procedure, which drastically reduces the cost of ab initio calculations. We developed a model that shows how destructive interference between autoionization and radiative ionization channels can stabilize transient states, explaining experimental observations and providing the first evidence for a phenomenon first predicted four decades ago. We have devised the formalism needed to extrapolate, from the single-atom response, the off-axis dipolar emission from an extended sample, and implemented it to theoretically reproduce, for the first time and with semi-quantitative accuracy, four-wave mixing experimental spectra in argon. Lastly, we developed a model for propagating light through a laser-dressed sample, demonstrating its use by predicting the pressure dependence of transient absorption spectra in model systems.

The motion of electrons in atoms, molecules, and in condensed matter unfolds on a time scale from the attosecond (1 as = 10-18 s) to several femtoseconds (1 fs= 10-15 s). The advent of attosecond light pulses has opened the way to the time-resolved study of electronic motion and of ionization processes using pump-probe spectroscopy. In this thesis, we examine three aspects of the ionization dynamics in poly-electronic atoms: i) the coherence of the emerging charged fragments, ii) two-photon ionization pathways, and iii) the reconstruction of photoionization amplitudes from experimental observable. We focus on the role of autoionizing states and of inter-channel coupling, which are pervasive features in the spectra of atoms and molecules. Autoionizing states are localized electronic configurations with energy above the ionization threshold. Due to electronic motion correlated character, the excess energy in these configuration can be transferred to a single electron, which is emitted to the continuum. Here, we use state-of-the-art techniques to accurately reproduce these processes. In photoionization, the photoelectron and its parent ion form an entangled pair. Even if the initial state is pure, therefore, either of the two fragments is only partially coherent. We simulate ab initio XUV-pump IR-probe experiment on helium to control this loss of coherence. To a first approximation, the states of the He+ ion with the same principal quantum number n are degenerate. On a femtosecond timescale, the partially coherent population of nl ionic states results in a stationary, delay-dependent dipole, controllable through the intermediate nln'l' autoionizing states of the atom. Fine-structure corrections cause picosecond real-time fluctuations of this dipole, from which we reconstruct the coherence in the residual ion at its inception. In argon, resonant phase structures below the (3s-1) threshold are revealed using the Reconstruction of Attosecond Beating by Interference of Two-Photon Transition (RABBITT) technique. Our calculations, which are in excellent agreement with experimental measurements, mark a significant improvement over previous models

One of the most fundamental goals of attosecond science is to observe and to control the dynamic evolutions of electrons in matter. The attosecond transient absorption spectroscopy is a powerful tool to utilize attosecond pulse to measure electron dynamics in quantum systems directly. In this work, isolated single attosecond pulses are used to probe electron dynamics in atoms and to study dynamics in hydrogen molecules using the attosecond transient absorption spectroscopy technique. The target atom/molecule is first pumped to excited states and then probed by a subsequent attosecond extreme ultraviolet (XUV) pulse or by a near infrared (NIR) laser pulse. By measuring the absorbed attosecond XUV pulse spectrum, the ultrafast electron correlation dynamics can be studied in real time. The quantum processes that can be studied using the attosecond transient absorption spectroscopy include the AC stark shift, multi-photon absorption, intermediate states of atoms, autoionizing states, and transitions of vibrational states in molecules. In all experiments, the absorption changes as a function of the time delay between the attosecond XUV probe pulse and the dressing NIR laser pulse, on a time scale of sub-cycle laser period, which reveals attosecond electron dynamics. These experiments demonstrate that the attosecond transient absorption spectroscopy can be performed to study and control electronic and nuclear dynamics in quantum systems with high temporal and spectral resolution, and it opens door for the study of electron dynamics in large molecules and other more complex systems.

The main objective of this dissertation is to provide better understanding of the atomic configurations, electronic structure, vibrational properties, and thermodynamics of transition metal nanoparticles and evaluate the intrinsic (i.e. size and shape) and extrinsic (i.e. ligands, adsorbates, and support) effects on the aforementioned properties through a simulational approach. The presented research provides insight into better understanding of the morphological changes of the nanoparticles that are brought about by the intrinsic factors as well as the extrinsic ones. The preference of certain ligands to stabilize specific sizes of nanoparticles is investigated. The intrinsic and extrinsic effects on the electronic structure of the nanoparticles are presented. The physical and chemical properties of the nanoparticles are evaluated through better understanding of the above effects on the experimentally observed properties as well as the applied techniques. The unexpected experimental results are tested and interpreted by deconvolution of the affecting factors. The application of Debye model to nanoparticles is tested and its shortcomings at nanoscale are discussed. Predictions which can provide insight into intelligent choice of candidates to cater to certain properties are provided. The results of this thesis can be used in the future in design and engineering of functionalized materials. We use ab initio calculations based on Density Functional Theory (DFT) to obtain information about the energetics, atomic configuration, and electronic structure of the nanoparticles. Ab initio Molecular Dynamics (MD) is used to study the evolution of the structures of the nanoparticles. To calculate vibrational frequencies, the finite displacement method is employed.

Accurate rate coefficients of atomic and molecular processes allow us to probe the conditions in space and understand the history of the Universe. Although experimental rate coefficients are the most desirable, availability of accurate rate coefficients of some processes depends on rigorous theoretical studies. In this dissertation, theoretical tools for collisions involving three bodies are discussed and applied to three different reactions. Rate coefficient of the reactive scattering of H2 + D- is computed using the ABC program. The present results are about ten times smaller than the experimental upper limit, suggesting that a further improvement of the sensitivity of the signal in the experiment may lead to the observation of the H- ions produced from this reaction. For the isotopic exchange reaction of 16O16O + 18O, the multiconfiguration time-dependent Hartree method was used to model the time evolution of the reaction. The results suggest that distribution of final reaction products is highly anisotropic, and simplified statistical approaches, sometime used for this kind of processes, are not applicable to this reaction. The three-body recombination of hydrogen atoms is investigated using the hyperspherical adiabatic approach with the R-matrix method at zero total angular momentum. It is found that Jahn-Teller effect contributes less than 15% increase in the total three-body recombination rate. The nascent population of the H2 molecules, formed in the recombination process, is found to be dominated by highly excited rovibrational levels, which could have substantial impacts in some astrophysical models. In addition, a novel simplified approach for dissociative electron attachment to polyatomic molecules is developed and applied to H2CN. The estimated rate coefficient is found to be too small to contribute to formation of CN- in the interstellar medium. An accurate theoretical rate coefficient of dissociative recombination of prebiotic molecule CH2NH2+ is also reported. The reported value is consistent with databases for astrochemistry, but it is much smaller than the value used in photochemical models of the upper atmosphere of Titan, which has an impact on Titan ammonia abundance in the models.