Anisotropy in Electron-Atom Collisions. A Study of the Alignment and Orientation of Helium States Excited by Electron Impact

摘要

Most of the work described in this thesis deals with studies using coincidence experiments, particularly for investigating the electron impact excitation of the 2 exp 1 P and 3 exp 1 D states in helium. A peculiarity is that in the 3 exp 1 D studies the directly emitted 3 exp 1 D implies 2 exp 1 P photons are not observed but the 2 exp 1 P implies 1 exp 1 S photons resulting from the 3 exp 1 D implies 2 exp 1 P implies 1 exp 1 S cascade instead. Another interesting point is the choice of the quantisation axis. The author demonstrates that it is of great advantage to take the quantisation axis perpendicular to the scattering plane rather than in the direction of the incident beam, as was done (on historical grounds) in previously reported electron-photon coincidence experiments. Contrary to the incident beam direction the axis perpendicular to the scattering plane really represents an axis of symmetry in the coincidence experiment. In Chapter II the so-called 'parity unfavoured' excitation of the (2p exp 2 ) exp 3 P state of helium by electrons is studied. In chapter III the anisotropy parameters for the electron impact excitation of the 2 exp 1 P state of helium in the energy range from 26.6 to 40 eV and in the angular range from 30 exp 0 to 110 exp 0 are determined. Chapter IV contains a description of a scattered electron cascaded-photon coincidence experiment on the electron impact excitation of helium's 3 exp 1 D state. The measurement of complex scattering amplitudes for electron impact excitation of the 3 exp 1 D and 3 exp 1 P states of helium is discussed in Chapter V. (Atomindex citation 14:765454)