Two techniques have been used to study low energy electron scattering processes in the alkali metals. A transmission experiment was used to search for resonance structure in the total scattering cross section of sodium, potassium, rubidium, and cesium. A shape resonance was observed in sodium at 0.08 (+OR-) 0.03 eV. In rubidium, evidence was found for the existence of a shape resonance below 0.05 eV. No shape resonance structure was observed in potassium or cesium for energies above 0.1 eV. For energies near the first excited state to near the lowest ionization potential, a progression of Feshbach resonances was observed in each atom. The graphical analysis technique of Spence was used to group most of these resonances into seven families. An analysis is given of the mechanisms affecting the rejection of scattered electrons in the transmission spectrometer. A quantitative description of two modes of operation of the spectrometer is presented. The second portion of this dissertation concerns the measurement of the near-threshold excitation and ionization cross sections in sodium, potassium, and rubidium. A trapped-electron experiment was used to make a relative measurement of these cross sections. The shapes of the excitation cross sections are, in general, somewhat different from other experimental work but are in good agreement with the close-coupling calculations of Moores and Norcross. The shapes of the ionization cross sections are in reasonable agreement with previous work. The ratio of the excitation cross section at 2.5 eV in sodium, 2.0 eV in potassium, and 1.8 eV in rubidium to the respective ionization cross section at the first peak was measured and found to be 3.9 (+OR-) 13% in Na, 5.8 (+OR-) 10% in K, and 6.4 (+OR-) 13% in Rb. These ratios are generally larger than those derived from independent values of the individual cross sections. An analysis of several systematic effects associated with the measurement of the cross section ratio was made. Results from this analysis were used to correct the measured excitation cross section for effects of the elastically scattered electrons.

Jan 1, 19830 citations

Superelastic electron scattering from the laser excited 52P3/2 state of rubidium

Brenton Victor Hall

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.

Jan 1, 19980 citations