Thesis (Ph. D.)--Carleton University, 2003. / Includes bibliographical references (p. 262-286). Also available in electronic format on the Internet.

Thesis (Ph. D.)--University of Chicago, Dept. of Chemistry, August 1999. / Includes bibliographical references. Also available on the Internet.

Thesis (Ph. D.)--Physics, Georgia Institute of Technology, 2007. / de Heer, Walter A., Committee Chair ; Chou, Meiyin, Committee Member ; Whetten, Robert L., Committee Member ; First, Phillip N., Committee Member ; Pummer, Earl Ward, Committee Member.

This thesis was directed at learning more about the unusual electronic environment near hydrogen within strong hydrogen bonds. "Strong" hydrogen bonds are unique in that the hydrogen atom is symmetrically located, or nearly so, between two electronegative atoms; the bond energies are relatively large. In a "normal" hydrogen bond the hydrogen atom is bonded to, and thus physically closer to, a parent atom, and only weakly attracted to another electronegative atom; bond energies are typically small. To examine these bonds, deuterium was substituted for hydrogen and the electric quadrupole coupling constant (QCC) of deuterium was measured using field cycling nuclear magnetic resonance. The electric quadrupole moment of deuterium is sensitive to changes in the surrounding electric field gradient, and is thus a good probe of the immediate electronic structure. The results show that the temperature dependence of the QCC is opposite to, and much larger than, what one would normally expect to observe for deuterium. The QCC is found to decrease strongly with decreasing temperature. This project was the first to study in detail the temperature dependence of deuterium QCCs in strong hydrogen bonds. The magnitude of the deuterium QCCs for the diacetates was found to be strongly depressed relative to typical values for deuterium. These results parallel large shifts in the infrared vibrational frequencies observed in many molecules which contain strong hydrogen bonds. The asymmetry parameter, which is a measure of the departure from axial symmetry of the electric field gradient (EFG) at deuterium, was found to be unusually large for what are known to be linear, or nearly linear, three-center bonds. Based on ab initio Hartree-Fock calculations aimed at determining the EFG at H in the archetypal bifluoride ion, F-H-F$\sp-$, the electronic charge density is drastically depleted at H. It is believed that the large reduction in the charge density allows the deuterium EFG to be highly sensitive to the shape of the charge distribution on the atoms to which deuterium is bonded. If these atoms are at points of low crystallographic symmetry, the polarization of these adjacent atoms by other nearby atoms may cause the EFG to depart substantially from being axially symmetric. Also obtained from the molecular orbital calculations for bifluoride ion were the total electronic energy and the electric field gradient at H. From these calculations potential function models for the asymmetric stretch and the bend were constructed. An attempt was made to correlate the predictions made by these models for the temperature dependence of the deuteron quadrupole coupling constant in bifluoride ion with the experimentally observed results for the diacetates.

We have demonstrated a means of quantum control by all-optical control of valence electron spin polarization in the lithium dimer. The energy levels of interest are two closely spaced rovibrational levels (the G1Πg (12, 21f) and 13Sg- (1, 21f) states separated by 720 MHz). In the absence of any optical fields, the spin-orbit interaction couples the two states, resulting in each state being a mixture of the pure singlet and pure triplet states. The initial mixing coefficients are governed by the strength of the spin-orbit coupling between the two levels. In lithium dimer, this mixing is very weak; the nominal singlet (triplet) state is initially 87% singlet (triplet) and 13% triplet (singlet). When a strong coupling field is applied to the nominally singlet state, an Autler-Townes (AT) split pair is created. Since one of the AT components is pushed closer in energy to the nearby triplet state, the triplet state gains more singlet character. Since the AT splitting is dependent upon the magnitude of the applied coupling field Rabi frequency, the mixing coefficients of the perturbed pair can therefore be coherently controlled. Such control may be useful for applications in cold molecule formation and control of predissociation rates, and may also provide insight into the role that valence electron spin polarization plays in reactive collisions. / Physics

Doctor of Philosophy / Department of Physics / Igor V. Litvinyuk / The goal of this dissertation project was to study the dynamics of nuclear motion in diatomic (H[subscript]2, N[subscript]2, O[subscript]2, CO) and triatomic (CO[subscript]2) molecules initiated by the ionization and/or excitation of these molecules with near-IR few-cycle laser pulses. This dynamics includes vibrational and rotational motion on the electronic potential surfaces of the molecules and their molecular ions. The experimental techniques used included the pump-probe approach, laser Coulomb explosion imaging and the COLTRIMS technique. The results are presented in four chapters. A study of rotational and vibrational nuclear dynamics in H[subscript]2 and D[subscript]2 molecules and ions initiated by 8 fs near-IR pulses is presented in Chapter 4. Transient alignment of the neutral molecules was observed and simulated; rotational frequency components contributing to the rotational wavepacket dynamics were recovered. Chapter 5 is dedicated to revealing the contribution of excited dissociative states of D[subscript]2[superscript]+ ions to the process of fragmentation by electron recollision. It was shown that it is possible to isolate the process of resonant excitation and estimate the individual contributions of the [superscript]2sigma[subscript]u[superscript]+ and [superscript]2pi[subscript]u states. In Chapter 6 the subject of investigation is the nuclear dynamics of N[subscript]2, O[subscript]2 and CO molecules initiated by ionization of a neutral molecule by a short intense laser pulse. It was shown that the kinetic energy release of the Coulomb explosion fragments, measured as a function of the delay time between pump and probe pulses, reveals the behavior of nuclear wave packet evolution on electronic states of the molecular ions. It was shown that information on the dissociation and excitation pathways can be extracted from the experimental spectra and the relative contributions of particular electronic states can be estimated. Chapter 7 is focused on studying the fragmentation of CO[subscript]2 following the interaction of this molecule with the laser field. The most important result of this study was that it presented direct experimental evidence of charge-resonant enhanced ionization (CREI), a phenomenon well-studied for diatomic molecules and predicted theoretically for triatomic molecules. The critical internuclear distance, the relevant ionic charge state and a pair of charge-resonant states responsible for the CREI were also found.

Doctor of Philosophy / Department of Physics / Zenghu Chang / The observation and control of dynamics in atomic and molecular targets requires the use of laser pulses with duration less than the characteristic timescale of the process which is to be manipulated. For electron dynamics, this time scale is on the order of attoseconds where 1 attosecond = 10[superscript]-18 seconds. In order to generate pulses on this time scale, different gating methods have been proposed. The idea is to extract or “gate” a single pulse from an attosecond pulse train and switch off all the other pulses. While previous methods have had some success, they are very difficult to implement and so far very few labs have access to these unique light sources. The purpose of this work is to introduce a new method, called double optical gating (DOG), and to demonstrate its effectiveness at generating high contrast single isolated attosecond pulses from multi-cycle lasers. First, the method is described in detail and is investigated in the spectral domain. The resulting attosecond pulses produced are then temporally characterized through attosecond streaking. A second method of gating, called generalized double optical gating (GDOG), is also introduced. This method allows attosecond pulse generation directly from a carrier-envelope phase un-stabilized laser system for the first time. Next the methods of DOG and GDOG are implemented in attosecond applications like high flux pulses and extreme broadband spectrum generation. Finally, the attosecond pulses themselves are used in experiments. First, an attosecond/femtosecond cross correlation is used for characterization of spatial and temporal properties of femtosecond pulses. Then, an attosecond pump, femtosecond probe experiment is conducted to observe and control electron dynamics in helium for the first time.

The first Hohenberg-Kohn theorem is extended to noninteger particle numbers. As a result of this extension; one can show that there exists a noncrossing theorem for ground ensembles of different particle numbers. This theorem excludes some densities as possible ground-state densities of a given system, provided the ground-state densities of this and other systems are known at a neighboring particle number. This theorem produces inequalities that functionals modelling the exchange-correlation energy and the noninteracting kinetic energy must fulfill It was proved in the early 1980's that the exchange-correlation potential undergoes a discontinuous positive jump as the particle number crosses an integer. This explained significant discrepancies in Local Density Approximation calculations of the fundamental band gap. The proof relies on a physically reasonable assumption that the shape of the exchange-correlation potential of the interacting system changes continuously with respect to the particle number. We prove that the noninteracting kinetic energy varies continuously with respect to the particle number and argue that this strongly indicates that the assumption is correct. In the special case where the particle number crosses 1, a rigorous proof is presented for the existence and size of the discontinuity of the exchange-correlation potential. The ensemble-search noninteracting kinetic energy of a 1- or 2-particle system is shown to be given by the von Weizsacker functional A new density scaling is developed under which the first terms of the gradient expansion for the noninteracting kinetic energy and the exchange energy become exact as the scaling constant goes to infinity. This is a limit that should be fulfilled by approximate functionals for these quantities. The successful Generalized Gradient Approximations for exchange do not, and we explain why / acase@tulane.edu

Kinetic investigation of the polymerization initiation process, predictive control of average molar mass and qualitative control of composition distributions, which can be controlled by selected feed of reagents into the reactor ('semi-batch' operation) obtained with appropriate computations, and phenomenon of counterion condensation accompanying charged polymers are investigated based on detailed kinetics obtained by Automatic Continuous Online Monitoring of Polymerization reactions (ACOMP) / acase@tulane.edu

This dissertation examines the electronic structure and magnetic properties of II-VI and IV-VI dilute magnetic semiconductors (DMS). Properties that are investigated include the exchange energy, magnetic moment, density of states, sources of the magnetic coupling, and the effect that crystal disorder has on the aforementioned parameters. The computational methods employed are the Vienna ab-initio Simulation Package (VASP), and the Layered Korringa-Kohn-Rostoker (LKKR) method. These two methods are based upon density functional theory. VASP relies on the construction of a pseudopotential and a plane wave expansion to model the charge density and wavefunction. LKKR uses multiple scattering theory to find the Green's function and electronic structure. The coherent potential approximation (CPA) can be readily incorporated into the LKKR approach, resulting in a first principle technique that can study a substitutionally disordered random alloy We have studied how the double-exchange, super-exchange, and inter-band exchange are effected by the crystal symmetry of the host, the electronic structure of the transition metal, and geometry of the impurities d-shell. We observed in a few materials that a competition between exchange mechanism is possible. When the sign of the interactions are the same, the result is an unambiguous magnetic ground state. However, when the sign of the competing exchange mechanisms are opposite, the material is expected to have a weaker, often oscillating, magnetic coupling, as a result of magnetic frustration and sensitivity to transition metal spacing and orientation. We have also examined how the chemical interactions may be coupled to the magnetic interactions. This becomes important at high impurity concentrations when the transition metal impurity cannot participate effectively in crystal bonding. In these cases, the transition metal d-orbitals that reside in the gap, and are involved in the exchange, are forced to initiate bonding with the host. This will result in an unexpected magnetic coupling. We note that most models of the transition metal coupling are formulated in the dilute limit The goal of this study was to discover, theoretically, a DMS structure that is both half-metallic and ferromagnetic at room temperature. The Cr doped compounds, and Ni II-VI compounds were found to be the most likely candidates to exhibit these properties. We also seek to establish systematic trends of how the electronic structure and magnetic properties vary as a function of crystal disorder. This is relevant since disorder is always present to some degree in these types of materials as a consequence of the growth techniques used in their fabrication / acase@tulane.edu