Spin Polarisation of Electrons Scattered Superelastically by Laser-excited Na Atoms

We have measured the spin polarisation of electrons scattered superelastically from laser-excited Na atoms at a collision energy of 20 eV and a scattering angle of 30’. The measured spin polarisation is 0.095 zt0.048 for scattering from the 3p zPl/z excited state of Na. The polarisation changes sign if the laser is tuned to the 3p 2P3,z state, as is expected on formal grounds. We also present calculations of the spin polarisation using theoretical scattering amplitudes from Moores and from Kennedy er al. It is well known that electron exchange cannot lead to spin polarisation effects, unless electrons or atoms are initially polarised, if one assumes a collision time much shorter than the typical relaxation times associated with angular momentum motions of the excited target electrons (see e.g. Hanne 1976, Kessler 1976). This approximation is certainly valid for light target atoms when fine-structure states are not resolved experimentally. In this picture the orbital and spin angular momenta of the excited electrons are considered to be constant during the collision process and thus a total angular momentum J’ of the excited target is not defined during scattering. The situation is different, however, if the scattering takes place between target states with well defined total angular momenta, i.e. if fine-structure splitting is resolved. Such an experimental condition may be realised by high-resolution spectrometry of scattered electrons, by electron-photon coincidence studies with sufficient optical resolution or by preparing well defined angular momentum states by laser excitation (Hertel and Stoll 1977). Collisional transitions between target states with well defined quantum numbers J, J’ are governed by the selection rule