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.

With the ever growing number of proposed desnity functional theory (DFT) functionals it becomes necessary to thoroughly screen any new method to determine its merit. Especially relevant methods include a proper description of the van der Waals (vdW) interaction, which can prove vital to a correct description of a myriad of systems of technological importance. The first part of this dissertation explores the utility of several vdW-inclusive DFT functionals including optB86b-vdW, optB88-vdW, optPBE-vdW, revPBE-vdW, rPW86-vdW2, and SCAN+rVV10 by applying them to model systems of small organic molecules, pyridine and thiophene, on transition metal surfaces. Overall, we find the optB88-vdW functional gives the best, most balanced description of both thiophene and pyridine on transition metal surfaces while revPBE-vdW, rPW86-vdW2, and SCAN+rVV10 functionals perform especially poorly for these systems. In the second part of this dissertation we change our focus to potential applications of DFT. Specifically, we study the hydrodesulfurization (HDS) process and molecules that could be used in molecular electronics. The removal of sulfur containing molecules from petrochemicals through HDS is an exceptionally important process economically, and the field of molecular electronics is rapidly developing with hopes of competing with and replacing their silicon analogues. First we investigate the hydrodesulfurization of thiophene. In this dissertation we manage to map the HDS rate of thiophene in realistic reaction conditions to the charge transfer and adsorption energy of thiophene on bare transition metal surfaces in hopes of predicting ever more active HDS catalysis. Finally we look at the adsorption of polythiophenes and 5,14-dihydro-5,7,12,14-tetraazapentacene (DHTAP) on Au(111) and Cu(110). We find that polythiophenes may dissociate of Au(111), presenting an issue for their use in molecular electronics. DHTAP, in contrast, proves to a suitable candidate for use practical devices.

The composition and morphology of mineral surfaces is known to play an important role in various phenomena relevant to planetary science. For example, the synthesis and processing of complex organics likely occurs at mineral surfaces strongly affected by the space environment. Furthermore, the dissipative and adhesive properties of dust grains may depend strongly on the chemical state of the surface including the presence of dangling bonds, adsorbates, and radicals. In this dissertation, experimental results are first presented which demonstrate that mineral grains subjected to high temperatures in a reducing environment lead to iron nanoparticles which are strongly catalytic for the formation of complex organic species. Next, results obtained using molecular-dynamics simulations demonstrate that uncoordinated surface atoms in metallic nanoparticles result in plastic deformation, strong dissipation and adhesion during collisions. This can be contrasted with previous simulations which demonstrate significantly weaker dissipation when surface atoms are passivated. Calculations of critical sticking velocities demonstrate that simple coarse- grain models are insufficient for predicting the adhesive behavior of sub-micron sized grains. Next, results are presented describing a computational study illuminating the role of surface chemistry on adhesion and dissipation for iron nanoparticle collisions, which in the case of free radical adsorbates may also contribute to the creation of more complex species. Lastly, to further elucidate dissipation, the direct coupling of harmonic vibrational modes in the dissipation process is established. The results demonstrate broad participation of low and high-frequency modes during a collision during a timescale less than time required for particles to rebound. Hence, our results demonstrate extremely strong likelihood of adhesion during collisions. This approach provides a way to use density-functional theory calculations to directly compute dissipative couplings at mineral interfaces.

There has been significant interest in spin systems involving two or more coupled spins as a single logical qubit, particularly for scalable quantum computing architectures. Recent realizations include the so-called singlet-triplet qubits and coupled magnetic molecules. An important class of coupled-spin systems, the three-spin paradigm for spin greater than 1/2, has not yet been fully realized in scalable qubit architectures. In this thesis, I develop the theoretical framework to investigate a class of tripartite spin models for realistic systems. First, I model a spin 1/2 particle (e.g., an electron) and two spin 1 particles (in a dimer arrangement) coupled with an exchange interaction. I find that if the two spin particles possess zero-field magnetic anisotropy, there exists resonance conditions that enable read, manipulate, and write operations on the representative qubit using the electron. Next, I generalize this result for any spin S, and describe how the resonance conditions change based on the type of exchange coupling, magnetic anisotropy, and magnitude of applied magnetic fields. The rest of the thesis is dedicated to utilizing the tools described in the framework to uncover the properties of potential scalable quantum architectures. To guide the correspondence between experiment and model Hamiltonians of effective tripartite spin systems connected to leads, I investigate the transport properties of a three-terminal quantum dot coupled to a magnetic molecular dimer using the generalized master equation. I then model both steady state and transient phenomena using equilibrium and non-equilibrium Green's functions (NEGF), and comment on the applicability of a newly-developed NEGF-derived quantum master equation. Finally, I characterize two examples of novel quantum systems: the spin qubit candidate h-BN VB- and the thin film FeBipy spin-crossover molecule.

By applying a dc voltage V across the stack of intrinsic Josephson junctions naturally present in the high-temperature superconductor consisting of two parts bismuth, two parts strontium, one part calcium, two parts copper, and a bit more than eight parts oxygen, several groups have been able to obtain coherent THz emission at output powers in the µW range. In order to enhance the output power well into the mW range suitable for many applications, we have studied a compact design of a stand-alone mesa array with gold layers on the top and bottom of the superconductor. In this design, an array of twelve stand-alone mesas consisting of six identical interior pie-shaped wedge mesas and six identical exterior slitted annular mesas, all produced from an original stand-alone disk mesa of outer radius by one circular cut at smaller inner radius and three linear cuts through their center all mutually rotated by sixty degrees. We studied the cavity mode frequencies of the pie-shaped and six-fold slitted annular mesas with various ratios between inner and outer radii, in order to obtain a set of several matching cavity mode frequencies of the two different mesa shapes. The cavity mode frequencies of the six-fold slitted annular mesas have been compared with those obtained from the pie-sliced wedge mesas, in the quest to obtain at least two or three closely matching resonance frequencies. The matching wave functions for the two mesa shapes and the predictions for the angular distributions of the high-power output THz emission will be presented. Increasing the output power from the µW into the mW range will make a unique leap in the THz emission field which is needed for many industrial and medical uses.

Space-time wave packets (STWP) constitute a broad class of pulsed optical fields that are rigidly transported in linear media without diffraction or dispersion, and are therefore propagation-invariant in absence of optical nonlinearities or waveguiding structures. Such wave packets exhibit unique characteristics, such as controllable group velocities in free space and exotic refractive phenomena. At the root of these behaviors is a fundamental feature underpinning STWP: their spectra are not separable with respect to the spatial and temporal degrees of freedom. Indeed, the spatio-temporal structure is endowed with angular dispersion, in which each spatial frequency is associated with a single prescribed wavelength. Although the basic concept of STWPs has been known since the 1980s, only very recently has rapid experimental development emerged. These advances are made possible by innovations in spatio-temporal Fourier synthesis, thereby opening a new frontier for structured light at the intersection of beam optics and ultrafast optics. In this dissertation, I investigate the physics underlying STWPs and their applications in various fields of optics. Namely, I study crucial parameters that determine physical characteristics of STWPs, such as group velocity, access to low spatial frequencies, and classify STWPs into 10 distinct classes. In addition, I study the finite-energy STWP in the presence of finite apertures and find that the propagation-invariant distance is determined by the 'fuzziness' in space-time correlation underlying the field. Moreover, in the context of 2D STWPs (light sheets), I present an application where an in-line optical delay line is constructed based on tunability of group velocities of STWPs over a wide range of values from subluminal to superluminal velocities, demonstrating a delay-bandwidth product of ~100 for pulses of width of ~1 ps. Finally, by introducing a strategy capable of sculpting an arbitrary spatio-temporal spectral profile I extend the experimental technique to synthesize 3D STWPs localized in all dimensions with tunable group velocity in the range from 0.7c to 1.8c in free space, and endowed with prescribed orbital angular momentum. By providing unprecedented flexibility in sculpting the three-dimensional structure of pulsed optical fields, the new experimental strategy presented in the last part of the dissertation promises to be a versatile platform for the emerging enterprise of space-time optics.

We formulate a quantum Monte Carlo (QMC) method for calculating the ground state of many-boson systems. The method is based on a field-theoretical approach, and is closely related to existing fermion auxiliary-field QMC methods which are applied in several fields of physics. The ground-state projection is implemented as a branching random walk in the space of permanents consisting of identical single-particle orbitals. Any single-particle basis can be used, and the method is in principle exact. We apply this method to an atomic Bose gas, where the atoms interact via an attractive or repulsive contact two-body potential parametrized by the s-wave scattering length. We choose as the single-particle basis a real-space grid. We compare with exact results in small systems, and arbitrarily-sized systems of untrapped bosons with attractive interactions in one dimension, where analytical solutions exist. Our method provides a way to systematically improve upon the mean-field Gross-Pitaevskii (GP) method while using the same framework, capturing interaction and correlation effects with a stochastic, coherent ensemble of non-interacting solutions. to study the role of many-body correlations in the ground state, we examine the properties of the gas, such as the energetics, condensate fraction, and the density and momentum distributions as a function of the number of particles and the scattering length, both in the homogenous and trapped gases. Results are presented for systems with up to 1000 bosons. Comparing our results to the mean-field GP results, we find significant departure from mean field at large positive scattering lengths. The many-body correlations tend to increase the kinetic energy and reduce the interaction energy compared to GP. In the trapped gases, this results in a qualitatively different behavior as a function of the scattering length. Possible experimental observation is discussed.