Academic literature on the topic 'Superelastic electron scattering'

This paper reports on the extension of the electron superelastic scattering technique to three new situations. The first considers scattering from the 32P3/2 level of Na that has been excited by two laser modes tuned, respectively, to the transitions from the two hyperfine states of the 32S1/2 ground level. Both coherent and noncoherent modes are treated in a full quantum electrodynamic model of the laser excitation. Under certain conditions, the time-averaged probability of finding an atom in the 32P3/2 level exceeds 0.5. The second situation is electron superelastic scattering from the 32D5/2 level of Na that has been resonantly excited from the ground level via a resonant intermediate level. With the first observation of superelastically scattered electrons from this higher lying level recently recorded, this paper considers the extension of the quantum electrodynamics (QED) model to describe the optical excitation process. Application of superelastic scattering to the 52S1/2–52P3/2 transition of Rb is the third situation considered. The superelastic scattering formalism is extended to allow for a nonzero spin flip cross section for this transition. The resulting optical pumping terms are calculated using the QED model and the method of their determination for the superelastic scattering experiment described. The experimental design necessary to measure all of the collision parameters for this transition is discussed.

Superelastic scattering experiment were performed on optically pumped calcium atoms at energies of 25.7 and 45 eV referred to the ground state. Orientation and alignment parameters derived from these experiments are compared with the predictions of several theories based on a distorted-wave formalism. The agreement between theory and experiment is unsatisfactory at the lower energy at all scattering angles. At the higher energy agreement improves at small scattering angles but is poor at middle angles. The results of our quantum electrodynamical calculation on optical pumping in lithium are compared with our observations. We find such good agreement between theory and experiment that we explore the possibility of superelastic scattering experiments on lithium atoms that are optically pumped with single-frequency laser light. A two-frequency pumping system is described and its use in the observation of superelastic scattering from lithium is discussed. Orientation and alignment parameters are presented at an equivalent energy of 21.8 eV for small angles. They are compared with those predicted by two close-coupling calculations. Excellent agreement is found between the present work and the convergent close-coupling theory of Bray.

A series of superelastic electron scattering experiments from lithium and from potassium is described in which the total polarisation parameter P+ is measured. We report significant departures from the coherence condition P+ = 1 for both targets. The structure observed in the parameter P+ can be interpreted by a qualitative wave mechanical model that had been introduced by our research group to explain similar structure in superelastic electron scattering experiments from sodium.

Three separate experiments are described which use laser radiation to probe the details of electron scattering processes from metal vapour targets. Results from superelastic scattering experiments from calcium are presented. The experiments were carried out at incident energies of 25�7 and 45 eV referred to the ground state. Scattering amplitudes derived from these experiments demonstrate that current theories are inadequate. The coherence of the excitation process has been studied by measuring the total polarisation. It is shown that the excitation process is coherent over the whole kinematic range. Preliminary results from a study of superelastic electron scattering from lithium are discussed where it is shown that a quantum electrodynamical model can be used to describe the optical pumping process in 6Li and 7Li. In addition the first superelastic electron spectrum is presented for experiments on lithium. A stepwise excitation technique is described with which cross sections for the electron impact excitation of the 3d9 4s2 2D state in copper can be measured. The experiments are complicated by the presence of D states in the incident copper beam. The origin of these D states is described as is a modification of the technique which leads to their removal.

This thesis describes an experimental study of superelastic electron scattering from the 6^2P_3/2 state of caesium. The present status of electron-atom collision studies is initially reviewed and the motivation behind the current work is then presented. A description of the theoretical framework is subsequently provided in the context of the present experimental study, followed by an overview of the several theoretical approaches for describing electron-atom interactions which are currently available. The apparatus and experimental setup used throughout the project are also described in detail. Technical specifications and data are provided, including diagrams (where appropriate) for a laser frequency locking system, electron gun and spectrometer, atomic beam source and data acquisition system. The experimental procedures are explained and discussed, including a detailed analysis of the optical pumping process required to excite the atomic target. A substantial component of this project was to address several potential sources of systematic error and to reduce these wherever possible. All of the errors and uncertainties relevant to the experiment are discussed in chapter 5. In chapter 6 the results of the present superelastic electron scattering experiments are reported for incident electron energies of 5.5eV, 8.5eV and 13.5eV, corresponding to superelastic electron energies of 7eV, 10eV and 15eV. These results are presented as three reduced Stokes parameters, P1, P2, P3 and a coherence parameter, P+ . For comparison, predictions from a number of currently available theories are presented alongside the experimental results. Finally, conclusions are drawn on this work in the context of the current status of electron-atom scattering from alkali-metals.

This thesis presents the results of a series of experiments in which electrons are superelastically scattered from various laser excited states of sodium. The atoms, once in the optically prepared state, are forced to relax via the superelastic collision with an electron. The rate of detection of superelastically scattered electrons was measured as a function of the laser polarisation which enabled pseudo Stokes parameters to be determined. These pseudo Stokes parameters are functions of both optical pumping parameters and atomic collision parameters. The optical pumping parameters describe the laser-atom interaction and the atomic collision parameters describe the electron-atom collision process. Three different laser excitation mechanisms were used to optically pump the atoms into various excited states. The first of these used a single laser tuned to the 32S 112(F'=2 hyperfine state)-~32P312 transition. The excited atoms underwent a superelastic collision with an electron leaving the atom in the ground state and pseudo Stokes parameters were measured as a function of both scattering angle and incident electron energy. The second superelastic experiment, utilised a folded step excitation mechanism which employed two lasers tuned from the two hypethne states of the 32S112 ground state respectively to the 32P312 excited state. Power broadening effects in the single laser experiment cause the atoms to be optically pumped into the F= 1 hyperfine ground state. The laser powers used were not great enough to power broaden the hyperfine ground states and as such the F'= 1 sublevel effectively acted as a sink. The folded step excitation method enabled the excited state population to be increased so that data at larger scattering angles could be obtained. Stokes parameters from both of these experiments which had an incident energy range of 10eV to 30eV and an angular range of 5°-25° were compared to three current electron-atom scattering theories and previous experimental data. Overall, fair to good agreement was found between theory and experiments for the individual Stokes parameters. Losses of coherence was observed at small scattering angles (50.200) at 20eV and 25eV incident electron energies which were poorly modelled by the three different theories. The third superelastic experiment involved the use of two lasers of specified polarisation to stepwise excite the atoms to the 32D512 excited state. Superelastic collisions with incident electron energies of 20eV from the 32D512-*32P312~312 collision were studied at three different scattering angles and pseudo Stokes parameters for the case where the polarisations of the radiation from the lasers were parallel were measured. The single step and folded step laser-atom interactions for it excitation were modelled using a full quantum electrodynamical treatment so that the optical pumping parameters from the single and folded step experiments could be investigated. Equations of motion were derived in the Heisenberg picture and it is shown that for the single laser case 59 equations of motion are required to fully model the interaction and for the folded step ease 78 equations of motion are required. The results of calculations demonstrated that the optical pumping parameters were sensitive to laser intensity, laser detuning and the Doppler width of the atomic beam. The theoretical quantum electrodynamical calculation results were in good agreement with the experimental results.

In the experiments detailed in this thesis, a series of scattering experiments were conducted in a versatile scattering chamber. In order to conduct these experiments, various electronic equipment was designed and built, including new computer controlled electron analyser power supplies. This new equipment was tested, adopted in this work, and is described in this thesis. The superelastic scattering technique was used on magnesium atoms to obtain a set of atomic collision parameters (ACPs), which describe the interaction. This was achieved by exciting a beam of magnesium atoms to the 3(1)P(1) excited state using resonant laser radiation around 285 nm, and using an electron beam with well defined momentum to de-excite the atoms. The momentum of the outgoing electrons was measured as the polarisation and scattering angle were varied, to obtain the ACPs. These measurements were carried out over an angular range of 30 degrees to 120 degrees and with incident energies equivalent to 35 eV, 40 eV, 45 eV, and 55 eV. A set of theoretical data was compared to the experimental results and found to be reasonably accurate at describing the interaction. Laser-aligned and ground-state (e,2e) ionisation measurements were taken from the 4(1)S(0) and 4(1)P(1) states of calcium. The measurements were taken with the energy of the scattered and ejected electrons set at 30 eV, and with one outgoing electron angle set to 45 degrees. The differential cross section was determined for a range of angles of the second electron, ranging from 30 degrees to 65 degrees. The incident and outgoing electron momenta were all defined in the same plane with the laser polarisation being in a plane perpendicular to the incident electron. The laser aligned (e,2e) measurements were compared to two theoretical models, one of which (a 3DW model) predicted an identically zero cross section when the laser polarisation was perpendicular to the scattering plane. The other model (a TDCC model) predicted a non-zero cross section, in agreement with the experiment. Simultaneous time-resolved two-colour photoionisation from the 5(2)P(3/2) and 6(2)P(3/2) states of rubidium was also conducted. These experiments investigated two pathways to creating 0.36 eV photoelectrons from rubidium. Photoelectrons were produced by either using laser radiation at ~780 nm to resonantly excite atoms to the 5(2)P(3/2) state followed by laser radiation at ~420 nm to ionise the atoms, or laser radiation at ~420 nm was used to resonantly excite atoms to the 6(2)P(3/2) state followed by radiation at ~780 nm which then ionised the atoms. Ionisation differential cross sections were measured over a full 360 degrees by rotating the laser polarisation vectors. By selectively detuning the laser beam so as to select individual ionisation pathways, and then by tuning both lasers to resonance, quantum interferences between the pathways that lead to ionisation were observed.