Superelastic electron scattering measurements are presented from rubidium atoms excited by laser radiation to the 52P states at around 780 nm. The incident energy of the electrons was 18.4 eV corresponding to 20 eV incident electrons for the excitation process 52S-52P. The measurements were conducted over a range of scattering angles from 5࠴hrough to 125A complete set of atomic collision parameters for the interaction process is presented together with the associated pseudo-Stokes parameters obtained from the measurements. A comparison with three sophisticated theoretical models indicates that none of the models completely describes the interaction process at this energy, and that further experimental and theoretical work is needed.
Superelastic electron scattering measurements are presented from rubidium atoms excited by laser radiation to the 52P states at around 780 nm. The incident energy of the electrons was 18.4 eV corresponding to 20 eV incident electrons for the excitation process 52S-52P. The measurements were conducted over a range of scattering angles from 5࠴hrough to 125A complete set of atomic collision parameters for the interaction process is presented together with the associated pseudo-Stokes parameters obtained from the measurements. A comparison with three sophisticated theoretical models indicates that none of the models completely describes the interaction process at this energy, and that further experimental and theoretical work is needed.
Contained within this thesis are the collective theoretical and experimental methodologies required for the investigation of superelastic electron scattering from the laser excited 52P312 level of atomic rubidium. Superelastic electron scattering is the process by which an incident electron collides with an excited atom, in this case optically prepared rubidium, subsequently de -exciting the atom and gaining the relevant kinetic energy. By observing the different superelastic electron scattering rates as a function of laser polarisation, in conjunction with performing pseudo Stokes parameter analysis, the alignment and orientation of the rubidium 52P state caused by electron impact excitation is deduced. As a consequence of this experimental study, rubidium is at present the heaviest alkali to which the superelastic electron scattering technique has been applied. The motivation for performing an electron scattering experiment with a rubidium target, pertains to the possible relativistic effects that arise due to the heavy nucleus of the target atom. Recent fully relativistic first order distorted wave Born approximation (RDW) scattering calculations, for electron impact excitation have shown that rubidium has exhibited strong relativistic effects. As this calculation predicts radically different behaviour when compared to a conventional non relativistic first order distorted wave Born approximation (DWBA) calculation, an experimental investigation to determine the validity of each theory is justified A continuous wave Titanium Sapphire ring laser was used to provide the coherent single mode laser radiation that was resonantly tuned to excited the 52S1/2 - 52P3/2 rubidium transition. The laser excitation process, for both the 85Rb and 87Rb isotopes, was theoretically modelled using a full quantum electrodynamic (QED) model. This modelling of the rubidium atom within the influence of a resonant laser field was used to calculate values for the optical pumping parameters K, K' and K". These parameters are necessary as they provide information about the depolarisation of the excited 2P state caused by the laser. An unpolarised electron beam with an incident electron energy of 18.4eV was used to collisionally de -excite the optically prepared rubidium. Superelastic electron scattering measurements were made for an angular range of 5�-120�, allowing experimental determination of the Stokes parameters, the natural frame parameters and the collision frame parameters associated with electron impact excitation of the 52P state of rubidium. At small scattering angles the DWBA and RDW theories predict similar values for the aforementioned parameters. One of the key aims of this study was the collection of electron scattering data beyond 25�, an experimental limitation previously encountered on this superelastic apparatus. This aim was achieved and experimental data has been collected to maximum angle of 120�. A comparison between the experimental data and the DWBA and RDW scattering theories, reveals that while the RDW theory consistently outperforms the DWBA, there are areas where both theories do not agree with experiment.
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.
Recent data from two methods in which high resolution laser radiation is used to assist in determining electron-atom collision parameters are presented. The electron superelastic method has yielded the first measurement of Stokes parameters for electron de-excitation of the 32D5/2–32P3/2,1/2 transition of atomic Na, the upper level having been optically prepared by resonant, stepwise excitation from the 32S1/2 ground level via the 32P3/2 level using two single mode lasers. As well, we report on the development of a model to determine the optical pumping parameters for superelastic scattering from the 32P3/2 level when it is prepared by two lasers exciting from the F = 1 and F = 2 states respectively of the 32S1/ 2 ground level. Data are also presented for collision parameters for the excitation of the 61So–61 PI transition of the I = 0 isotope of Hg by electrons of 50 eV incident energy. The technique employed for these measurements is the stepwise electron–laser excitation coincidence method, in which the electron excited atom is further excited by resonant laser radiation, and fluorescence photons emitted by relaxation from the laser excited state are detected in coincidence with the scattered electron.
The atomic collision parameters ?, ?, ?, ?, P, |P| and |�| were determined from measurements of the pseudo Stokes parameters for superelastic scattering of 20 eV incident electrons at angles of 10� and 15� from the 32P312 excited state of sodium. Two depolarising factors K and K' for the pseudo Stokes parameters are defined for it and a laser excitation respectively to account for the depolarising influence due to optical pumping in the hyperfine structure of the 32P3t2 excited state. A theoretical model of the laser excitation of the sodium transition is developed to determine K and K'. The theory developed is a quantum -electrodynamic description of the resonant interaction of monochromatic light with the 32S in(F'=2) <-> 32P3a(F=3,2,1) hyperfine transitions of the sodium D2 line formualated in terms of the Heisenberg atomic operator. Off diagonal state coherences and all relaxation terms are included. It is found that the equations for the populations, optical coherences and certain state coherences form a closed subset of the total system of equations. This subset is sufficiently small to enable numerical computation. As well as K and K', the model is used to calculate the probability of occupation of the various atomic states and the polarisation of the sideways scattered fluorescence from the laser excited atoms. The results of this model are compared with the predictions of four other models, a rate equation model which includes only the F=3 excited state, a semiclassical model based on the density operator, a low light intensity approximation to the semiclassical model and a high intensity model which uses a transformed basis for the states. For both it and a laser excitation it is shown that the QED calculated values converge to the values predicted by the high and low intensity approximations in the appropriate limits.
A series of superelastic scattering experiments have been carried out on optically pumped sodium atoms in the 3 2 P 3/2 state. The total polarization P + has been observed at electron energies of 12.1 eV and 22.1 eV referred to the ground state, over a wide range of electron scattering angles. The connections between this parameter, the coherence of the excitation process, and exchange are discussed. Significant departures from P + =1 are observed. A qualitative explanation of this phenomenon is described which provides some insight into the collision process.
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.