Fully differential cross sections for four-body scattering processes

While the original concept of the atom can be traced back to the ancient Greeks, current knowledge of the atom is due largely to the study of atomic collisions. The structure of atoms is now fairly well understood, but the understanding of their interactions remains incomplete. In atomic collisions, the particles involved in the collision interact through the Coulomb force, which is known exactly. However, for Coulomb forces, the solution of the Schrödinger equation can only be obtained analytically for two mutually interacting particles. As a result, when more than two particles are involved, theory must resort to approximations. The validity of these approximations is then determined by comparison with experiment. Three new fully quantum-mechanical models that include all relevant two-particle interactions are presented here, and used to study fully differential cross sections (FDCS) of four-body collisions. In particular, this work focuses on electron-impact excitation-ionization of helium, as well as single charge transfer, transfer-excitation, and double charge transfer in proton + helium collisions. The calculations required for this work result in nine-dimensional integrals that are performed numerically. For excitation-ionization, the projectile-ejected electron interaction is found to be important in correctly predicting the shape of the FDCS. However, the projectile-atom and projectile-ion interactions play a much smaller role in this process. For single charge transfer and transfer-excitation, the current model does a reasonable job of predicting the shape and magnitude of experiment. However, for double charge transfer, the theoretical results overestimate experiment by several orders of magnitude. For all of the charge transfer collisions, calculations show that the interaction of the electrons within the target atom has little effect on the FDCS --Abstract, page iii

Theoretical Fully Differential Cross Sections for Double-Charge-Transfer Collisions

We present a four-body model for double charge transfer, called the four-body double-capture model. This model explicitly treats all four particles in the collision, and we apply it here to fully differential cross sections (FDCSs) for proton+helium collisions. The effects of initial- and final-state electron correlations are studied, as well as the role of the projectile-nucleus interaction. We also present results for proton+helium single capture, as well as single-capture:double-capture ratios of FDCSs

Four Body Charge Transfer Process in Proton Helium Collision

Recent advancements in experimental techniques now allow for the study of fully differential cross sections for 4-body collisions. Theoretical fully differential cross sections will be presented and compared with absolute experimental data for transfer-excitation in proton-helium collisions. The role of different scattering mechanism will be discussed

Four-Body Charge Transfer Processes in Heavy Particle Collisions

Fully differential cross sections (FDCS) for proton + helium single capture and transfer-excitation collisions are presented using the Four-Body Transfer-Excitation (4BTE) model. This is a first order perturbative model that allows for any two-particle interaction to be studied. For single capture, the effect of the projectile-nuclear term in the perturbation is examined. It is shown that inclusion of this term results in an unphysical minimum in the FDCS, but is required to correctly predict the magnitude of the experimental results. For transfer-excitation, the role of electron correlation in the target helium atom is studied, and shown to be unimportant in the calculation of the FDCS

Four-Body Model for Transfer Excitation

We present here a four-body model for transfer-excitation collisions, which we call the four-body transfer-excitation (4BTE) model. Each two-body interaction is explicitly included in the 4BTE model, allowing us to study the effects of individual two-body interactions. We apply our model to fully differential cross sections for proton+helium collisions, and study the effect of the incident projectile-atom interaction, the scattered projectile-ion interaction, the projectile-nuclear interaction, and electron correlation within the target atom

Fully Differential Cross Section for Four Body Charge Transfer Process

Recently experimental fully differential cross sections (FDCS) have been reported for double capture in proton helium collisions which disagree with existing theoretical calculations by two orders of magnitude. We introduce here a theoretical model for charge transfer processes which is fully quantum mechanical and takes all post collision interactions (PCI) between the particles into account exactly. The results of this model are in much better agreement with experimental data

Nanostructured Polymer Lithography for Photovoltaic Applications

The self-assembly of diblock copolymers into ordered domains holds great potential to furthering the efficiency of photovoltaic devices. Solutions containing polystyrene-block-poly(ethylene oxide) (PS-b-PEO) and poly(methyl methacrylate) (PMMA) were applied to silicon wafers from toluene solutions. Hexagonally ordered domains, with pore sizes ranging from 10-30 nm, were obtained by annealing films in solvent vapor, with the best results produced from a humidified benzene environment. Exposing the films to UV light cross-linked the polystyrene matrix and degraded the PMMA. Removal of the PMMA and PEO produced an ordered polystyrene template, which can be used for nanolithography for the deposition of quantum dots onto the wafers. Details of the film preparation, annealing times and conditions, and characterization will be presented

Effects of the Final-State Electron-ion Interactions on the Fully Differential Cross Sections for Heavy-Particle-Impact Ionization of Helium

Three-dimensional fully differential cross sections for heavy-particle-impact ionization of helium are examined. Previously, the three-body distorted-wave (3DW) model has achieved good agreement with experiment in the scattering plane for small momentum transfers, but poor agreement for large momentum transfers. Poor agreement was also observed outside the scattering plane for all momentum transfers. In particular, the 3DW calculations predicted cross sections that were too small both perpendicular to the scattering plane and for large momentum transfers. The important unanswered question concerns the physical effects that cause the significant disagreement between experiment and theory. In previous works, the role of the projectile-ion interaction has been examined. Although the importance of exchange between the ejected electron and the residual bound electrons has been well established, and frequently studied, for electron-impact ionization, the importance of this effect has not been examined for heavy-particle scattering. In this paper we examine the role of this effect for heavy-particle scattering

Experimental and Theoretical Cross Sections for Molecular-frame Electron-impact Excitation-ionization of D₂

We present both experimental and theoretical results for the dissociative ionization of D2 molecules induced by electron impact. Cross sections are determined in the molecular frame and are fully differential in the energies and emission angles of the dissociation fragments. Transitions are considered from the X1Σg+ electronic ground state of D2 to the 2sσg, 2pπu and 2pσu excited states of D2+. The experimental results are compared to calculations performed within the molecular four-body distorted-wave framework to describe the multicenter nature of the scattering process. The cross sections reveal a dramatic dependence on both the alignment of the internuclear axis with respect to the direction of the projectile momentum and on the symmetry of the excited dissociating state which is energetically resolved

Ionization and Ionization-Excitation of Helium to the n=1-4 States of He⁺ by Electron Impact

We present experimental and theoretical results for the electron-impact-induced ionization of ground-state helium atoms. Using a high-sensitivity toroidal electron spectrometer, we measured cross-section ratios for transitions leading to the first three excited states of the residual helium ion relative to the transition leaving the ion in the ground state. Measurements were performed for both symmetric- and asymmetric-energy-sharing kinematics. By presenting results as a ratio, a direct comparison can be made between theoretical and experimental predictions without recourse to normalization. The experimental data are compared to theoretical predictions employing various first-order models and a second-order hybrid distorted-wave + convergent R matrix with pseudostates (close-coupling) approach. All the first-order models fail in predicting even the approximate size of the cross-section ratios. The second-order calculations are found to describe the experimental data for asymmetric-energy-sharing with reasonable fidelity, although significant disparities are evident for the symmetric-energy-sharing cases. These comparisons demonstrate the need for further theoretical developments, in which all four charged particles are treated on an equal footing