Electron impact excitation of the n = 3 levels of hydrogen

Abstract

This thesis is concerned with the study of the excitation of Hydrogen atoms to the n = 3 states from the ground state by electrons with incident energies ranging from just above the ionization threshold to energies where the first Born approximation is expected to be valid. The major physical effects in this region are exchange, the distortion of the wave describing the external electron, and the distortion of the atomic system. A model which includes these effects - the Distorted Wave Polarized Orbital (DWPO) approximation - is generalized for any excitation and used to investigate the excitation process for n = 3 in particular. Total (integrated) and differential cross sections, not previously calculated using this model, are presented and compared, where possible, with other theoretical and experimental work. Other sensitive indicators of the effects of the model are considered. These include the polarization of Balmer-a (Ha) radiation and the parameters which describe the orientation and alignment of the atomic system after collision and the coincidence rate for the observation of emitted photons with the ejected electrons (Fano-Macek and Macek-Jaecks parameters). Also studied is the asymmetry in the observed intensity of Ha radiation arising on sign reversal of an applied electric field along the interaction direction. There is a serious disparity between the results in this model or the Born approximation and the experimental observations. A number of reasons for this are discussed. The work here indicates a need for further theoretical and experimental study but that high levels of sensitivity are required in any experimental work particularly with regard to the polarization and asymmetry measurements. Addition any, this work illustrates a very serious failure in the DWPO model caused by the use of the adiabatic polarization potential rather than an energy dependent potential especially at higher energies in the 3d excitation where we found that for the total cross section, the results obtained by including full allowance for polarizationlie a factor of about 2.5 below the Bom result at 200 eV and do not approach the Born cross section even for impact energies measuredin keV. The most useful line of future research is expected to be the allowance for coupling to adjacent states by the unitarizaticn method and some preliminary work for this is included.