Further studies on Norrish type II reactions including a reaction in the first excited singlet state and cyclization of 1:4 biradicals

The Norrish type II processes of methyl-2,2-dimethyl- cyclopropyl ketone, alpha-alkoxy acetones and alkyl pyruvates have been examined using the AM1 semi-empirical molecular orbital method with complete geometry optimization at the partial configuration interaction level in the restricted Hartree-Fock (RHF) frame. The results reveal that the methyl-substituted cyclopropyl ketone has a constrained geometry favourable for hydrogen abstraction from the gamma-position relative to the carbonyl group in the excited singlet state. The presence of the ether oxygen atom in the beta-position relative to the carbonyl group in alkoxy acetones and alkyl pyruvates leads to increased reactivity relative to alkyl monoketones and diketones respectively. The cyclization of 1:4 biradicals has been studied in the unrestricted Hartree-Fock (UHF) frame, and the results reveal that the 1:4 biradical derived from alkoxy acetones readily cyclizes to form oxetanols. On the other hand, in the 1:4 biradicals derived from methyl-substituted cyclopropyl ketone, the three-membered ring breaks readily to form an enol intermediate. Delocalization of an odd electron in 1:4 biradicals derived from alkyl pyruvates is thought to make cyclization difficult.

Spin and charge density waves in the extended Hubbard model: A slave-boson approach

We use the extended Hubbard model to investigate the properties of the charge- and spin-density-wave phases in the presence of a nearest-neighbors repulsion term in the framework of the slave-boson technique. We show that, contrary to Hartree-Fock results, an instablity may occur for sufficiently high values of the Hubbard repulsion, both in the spin- and charge-density-wave phase, which makes the system discontinuously jump to a phase with a smaller or zero wave amplitude. The limits of applicability of our approach are discussed and our results are compared with previous numerical analysis. The phase diagram of the model at half-filling is determined.

Goldstone modes and bifurcations in phase-separated binary condensates at finite temperature

We show that the third Goldstone mode, which emerges in binary condensates at phase separation, persists to higher interspecies interaction for density profiles where one component is surrounded on both sides by the other component. This is not the case with symmetry-broken density profiles where one species is entirely to the left and the other is entirely to the right. We, then, use Hartree-Fock-Bogoliubov theory with Popov approximation to examine the mode evolution at T not equal 0 and demonstrate the existence of mode bifurcation near the critical temperature. The Kohn mode, however, exhibits deviation from the natural frequency at finite temperatures after the phase separation. This is due to the exclusion of the noncondensate atoms in the dynamics.

Asymptotic behavior and interpretation of virtual states: The effects of confinement and of basis sets

A real-space high order finite difference method is used to analyze the effect of spherical domain size on the Hartree-Fock (and density functional theory) virtual eigenstates. We show the domain size dependence of both positive and negative virtual eigenvalues of the Hartree-Fock equations for small molecules. We demonstrate that positive states behave like a particle in spherical well and show how they approach zero. For the negative eigenstates, we show that large domains are needed to get the correct eigenvalues. We compare our results to those of Gaussian basis sets and draw some conclusions for real-space, basis-sets, and plane-waves calculations. (C) 2016 AIP Publishing LLC.

Dynamics of rotationally resolved multiphoton ionization processes in molecules

This dissertation presents the results of studies of several rotationally- resolved resonance enhanced multiphoton ionization (REMPI) processes in some simple molecular systems. The objective of these studies is to quantitatively identify the underlying dynamics of this highly state-specific process which utilizes the narrow bandwidth radiation of a laser to ionize a molecule by first preparing an excited state via multiphoton absorption and subsequently ionizing that state before it can decay. Coupled with high-resolution photoelectron spectroscopy, REMPI is clearly an important probe of molecular excited states and their photoioniza tion dynamics.

A key feature of our studies is that they are carried out using accurate Hartree-Fock orbitals to describe the photoelectron orbitals of the molecular ions. The use of such photoelectron orbitals is important in rotationally-resolved studies where the angular momentum coupling in the photoelectron orbital plays a significant role in the photoionization dynamics. In these studies the Hartree-Fock molecular molecular photoelectron orbitals are obtained by numerical solution of a Lippmann-Schwinger integral equation.

Studies reported here include investigations of (i) ionic rotational branching ratios and their energy dependence for REMPI via the A^2Σ^+(3sσ) and D^2Σ^+(3pσ)states of NO, (ii) the influence of angular momentum constraints on branching ratios at low photoelectron energies for REMPI via low-J levels of the resonant intermediate state, (iii) the strong dependence of photoelectron angular distributions on final ionic rotational state and on the alignment in REMPI of the A^2Σ^+ state of NO, (iv) vibrational state dependence of ionic rotational branching ratios arising from rapid orbital evolution in resonant states (E'^2Σ^+(3pσ) of CH), (v) the influence of rovibronic interactions on the rotational branching ratios seen in REMPI via the D^2Σ^+(3pσ) state of NO, and (vi) effects of laser intensity on the photoionization dynamics of REMPI.

Fermionic quantum systems. Part I: Phase transitions in quantum dots. Part II: Nuclear matter on a lattice

In the first part I perform Hartree-Fock calculations to show that quantum dots (i.e., two-dimensional systems of up to twenty interacting electrons in an external parabolic potential) undergo a gradual transition to a spin-polarized Wigner crystal with increasing magnetic field strength. The phase diagram and ground state energies have been determined. I tried to improve the ground state of the Wigner crystal by introducing a Jastrow ansatz for the wave function and performing a variational Monte Carlo calculation. The existence of so called magic numbers was also investigated. Finally, I also calculated the heat capacity associated with the rotational degree of freedom of deformed many-body states and suggest an experimental method to detect Wigner crystals.

The second part of the thesis investigates infinite nuclear matter on a cubic lattice. The exact thermal formalism describes nucleons with a Hamiltonian that accommodates on-site and next-neighbor parts of the central, spin-exchange and isospin-exchange interaction. Using auxiliary field Monte Carlo methods, I show that energy and basic saturation properties of nuclear matter can be reproduced. A first order phase transition from an uncorrelated Fermi gas to a clustered system is observed by computing mechanical and thermodynamical quantities such as compressibility, heat capacity, entropy and grand potential. The structure of the clusters is investigated with the help two-body correlations. I compare symmetry energy and first sound velocities with literature and find reasonable agreement. I also calculate the energy of pure neutron matter and search for a similar phase transition, but the survey is restricted by the infamous Monte Carlo sign problem. Also, a regularization scheme to extract potential parameters from scattering lengths and effective ranges is investigated.

Stopping power of gases for heavy ions

Pulse-height and time-of-flight methods have been used to measure the electronic stopping cross sections for projectiles of ¹²C, ¹⁶O, ¹⁹F, ²³Na, ²⁴Mg, and ²⁷Al, slowing in helium, neon, argon, krypton, and xenon. The ion energies were in the range 185 keV ≤ E ≤ 2560 keV.

A semiempirical calculation of the electronic stopping cross section for projectiles with atomic numbers between 6 and 13 passing through the inert gases has been performed using a modification of the Firsov model. Using Hartree-Slater-Fock orbitals, and summing over the losses for the individual charge states of the projectiles, good agreement has been obtained with the experimental data. The main features of the stopping cross section seen in the data, such as the Z₁ oscillation and the variation of the velocity dependence on Z₁ and Z₂, are present in the calculation. The inclusion of a modified form of the Bethe-Bloch formula as an additional term allows the increase of the velocity dependence for projectile velocities above v_o to be reproduced in the calculation.

Spin-related phenomena in magnetic and spin-orbit couppled bose-einstein condensates

129 p.

Comparison of quantum chemistry calculation methods of nucleic acid base-quartets

Nine base-quartets were calculated by six semi-empirical methods and ab initio Hartree-Fork method using STO-3G basis set. The results showed that PM3 method can be use to calculate base quartets, the results of PM3 calculations are close to the ab initio

Reactive Many-Body Expansion for a Protonated Water Cluster.

We generalize the standard many-body expansion technique that is used to approximate the total energy of a molecular system to enable the treatment of chemical reactions by quantum chemical techniques. By considering all possible assignments of atoms to monomer units of the many-body expansion and associating suitable weights with each, we construct a potential energy surface that is a smooth function of the nuclear positions. We derive expressions for this reactive many-body expansion energy and describe an algorithm for its evaluation, which scales polynomially with system size, and therefore will make the method feasible for future condensed phase simulations. We demonstrate the accuracy and smoothness of the resulting potential energy surface on a molecular dynamics trajectory of the protonated water hexamer, using the Hartree-Fock method for the many-body term and Møller-Plesset theory for the low order terms of the many-body expansion.

Coherent tunnelling in coupled quantum wells under a uniform magnetic fields

Coherent tunnelling is studied in framework of the effective mass approximation for an asymmetric coupled quantum well. The Hartree potential due to the electron-electron interaction is considered in our calculation. The effects of the longitudinal and transverse magnetic field on coherent tunnelling characteristics are discussed. It has been found that the external field plays an important role in modulating the electron states.

Mode softening in charge-density excitations of a two-dimensional electron gas driven by Rashba spin-orbit interaction

The electron density response of a uniform two-dimensional (2D) electron gas is investigated in the presence of a perpendicular magnetic field and Rashba spin-orbit interaction (SOI). It is found that, within the Hartree-Fock approximation, a charge density excitation mode below the cyclotron resonance frequency shows a mode softening behavior, when the spin-orbit coupling strength falls into a certain interval. This mode softening indicates that the ground state of an interacting uniform 2D electron gas may be driven by the Rashba SOI to undergo a phase transition to a nonuniform charge density wave state.

Electronic states of a two-dimensional electron system in a lateral superlattice and a perpendicular magnetic field

The electronic state of a two-dimensional electron system (2DES) in the presence of a perpendicular uniform magnetic field and a lateral superlattice (LS) is investigated theoretically. A comparative study is made between a LS induced by a spatial electrostatic potential modulation (referred to as a PMLS) and that induced by a spatial magnetic-field modulation (referred ro asa MMLS). By utilizing a finite-temperature self-consistent Hartree-Fock approximation scheme; the dependence of the electronic state on different system parameters (e.g., the modulation period, the modulation strength, the effective electron-electron interaction strength, the averaged electron density, and the system temperature) is studied in detail. The inclusion of exchange effect is found to bring qualitative changes to the electronic state of a PMLS, leading generally to a nonuniform spin splitting, and consequently the behavior of the electronic state becomes similar to that of a MMLS. The Landau-level coupling is taken into account, and is found to introduce some interesting features not observed before. It is also found that, even in the regime of intermediate modulation strength, the density dependence of the spin splitting of energy levels, either for a PMLS or a MMLS, can be qualitatively understood within the picture of a 2DES in a perpendicular magnetic field with the modulation viewed as a perturbation. [S0163-1829(97)02248-0].

笼内金属富勒烯Gd@C_(82)的分离和质谱研究

由于其独特的电子结构，笼内金属富勒烯（碳笼内包容金属原子）的研究一直吸引着人们极大的兴趣，目前已经合成并分离出的笼内金属富勒烯主要包括碳笼内包容铱、钪、镧系原子等。在本论文中我们利用质谱技术对笼内金属富勒烯Gd_2@C_(82)气相离子行为和C_(60)与丙烯酸甲酯气相离子反应进行研究，X光电子能谱对碳笼内轧原子的价态进行研究。笼内金属富勒烯Gd_2@C_(82)是利用电弧放电法合成的。将碳份与三氧化二钆 （C：Gd=1：100, 摩尔比）灌入碳棒，将为棒前处理后，在氦气份保护下进行放电。通过对空心富勒烯在不同沸点的溶解情况进行了研究，发现大碳数的空心富勒烯在高沸点容剂中有较大的溶解度。利用两步高温高压法，既先利用1，2，4－三甲2基苯对大碳数的空心富勒烯进行分离，然后利用吡啶对笼内金属富勒烯进行提取。利用三种不同的质谱电离方式：电喷雾 （ESI），激光解析飞行时间（LDI-TOF），共振电子捕获技术（REC）来研究笼内金属富勒烯的气相离子特征。实验结果表明LDI-TOF和REC电离技术得到的质谱谱图中，笼内金属富勒烯和空心富勒烯均出现分子离子峰，但在电喷雾质谱谱图中只有笼内金属富勒烯出现分子离子峰。结合这一实验结果和理论分析说明碳笼内钆离子偏离碳中心位置。通过对比笼内金属富勒烯Gd@C_(82)和三氧化二钆中钆离子的光电子能谱谱图，Gd@C_(82)的电子结构应表示为Gd~(3+)@C_(82)~(3-)。利用四极杆质谱仪对C_(60)分子[C_(60)H]~+和加成产物为[C_(60)C_4H_3O]~+，后者是丙烯酸甲酯分子经过α短裂后与质子化[C_(60)H]~+ 发生加成反应所得的产物，在限制性Hartree-Fock条件下，采用AM1半经验量子化学计算方法对加成产[C_(60)C_3H_3O]~+八种可能结构的产物进行计算，得出了三种可能的最稳定的环加成同分异构体。

Competitions of magnetism and superconductivity in FeAs-based materials

Using the numerical unrestricted Hartree-Fock approach, we study the ground state of a two-orbital model describing newly discovered FeAs-based superconductors. We observe the competition of a (0, π) mode spin-density wave and the superconductivity as the doping concentration changes. There might be a small region in the electron-doping side where the magnetism and superconductivity coexist. The superconducting pairing is found to be spin singlet,orbital even, and coexisting sxy + dx~2-y~2 wave (even parity).