Measurements of the Integrated Stokes parameters

Dr L Pravica, Prof. JF Williams, Dr Danica Cvejanovic, Stuart Napier

This study uses spin-polarised electrons to impact target atoms and transfer angular momentum and energy to them. In this process the electrons in the target atom can be excited and/or ionised and after a short time (which is usually between 10^-8 and 10^-15 s) they can decay to an energetically favourable lower state by emission of a photon. These photons carry information about the collision process which can be quantified in terms of integrated Stokes parameters P₁, P₂ and P₃ (integrated as scattered/ejected electrons are not observed). They are proportional to the orientation and alignment of the electron charge cloud distribution, i.e. proportional to expectation values of components of angular momentum.

Source of polarised electrons. The polarised electrons are created by impact of circularly polarised light on a specially prepared surface of GaAs crystal and then transferred to the interaction region via electron optics. The main parts are:

• Laser, linear polariser and a liquid crystal variable retarder which convert linearly polarised laser light into circularly polarised light.
• GaAs crystal which surface has been carefully prepared by application of oxygen and cesium
• Electron optics which is used to extract, focus and tranfer polarised electrons to the interaction region.

Source of target atoms which can be a simple gas jet for gaseous targets or a specially designed metal vapour oven for metalic targets.

Photon polarisation analysis and detection system consisting of an optical lens used to collect light, a polarisatation analysis system (liquid crystal variable retarder and a linear polariser), photon wavelength/energy selector (interference filter or an acousto-optic tunable filter) and a photon detection system (photo-multiplier tube).

Spin-polarised incident electrons have provided the significant competitive experimental advantage to enhance the understanding of atomic structure and scattering processes. In particular, control of the electron spin has enabled the study of different interactions, such as the attractive Coulomb interaction with the nucleus, the repulsive electron-electron interaction, as well as the spin-dependent electron exchange and the spin-orbit interactions.

Selected references:

• Pravica L, Cvejanovic D, Napier S and Williams JF, Spin and Electron Correlation Effects in Excitation of 3d Metal Atoms, Hadzievski Lj, Marinkovic BP and Simonovic N (eds), The Physics of Ionized Gases, AIP Conf. Proc. 876 (2006), pp.72-79 Abstract

• Pravica L and Williams JF, Polarised electron inner-shell and outer-shell excitation and ionisation of zinc atoms, Published in XVI Australian Institute of Physics Congress Proceedings 132 PTH34 (2005)

• Yu DH, Pravica L and Williams JF, Polarized electron inner-shell ionization-with-excitation of zinc atoms, J. Phys. B: At. Mol. Opt. Phys. 34 3899-3908 (2001) Abstract