Thin Films, Surfaces and Interfaces Probed with Positrons
- Chief Supervisor
- Professor J.F. Williams (University of Western Australia)
- Supervisor
- Dr. S. Samarin (University of Western Australia)
- Centre Node
- University of Western Australia
The proposed positron studies are complementary to those using electron beams as probes for which UWA has an international facility comprising spin-polarised electron-pair (e,2e) spectroscopy in addition to most single electron spectroscopies, such as LEED, Auger, EELS, XPS, UPS, SPLEED as well as an optical ellipsometer and Crystallography and Microscopy facilities. The combined use of these techniques enables their overlapping and complementary information to establish a better understanding of many surface phenomena.
Our facilities and achievements leads naturally to approach positron studies from the 'time scale' for their interactions relative to their mean vacuum lifetime of a few hundred picoseconds and for binding to an electron to form positronium with an energy of ~6.8 eV. When a positron enters a solid, it loses energy to thermalize in about 10 psec, scattering between Bloch states to diffuse through the solid; it is essentially delocalised and strongly correlated with conduction electrons until it annihilates in about 100 psec. Inelastic conduction-electron scattering, and specifically plasmon excitation, dominates the slowing down of positron around energies near the Fermi energy. The possibility of Ps localisation at the surface is a measure of the "correlated positron and electron" which infers the experimental momentum distribution is representative of electrons at the surface.
The most striking advantages of the positron beam technique follows from the narrow energy width, controllable low energy, pulsed and polarised beam. The difference in charge and absence of the exchange interaction, enables positron studies to complement electron studies. Usually only one positron is present at any one time whereas there are many electrons which can transfer their energy to others in the solid.
The observables and the detectables in these studies are positrons, characteristic X-rays, secondary or Auger electrons, or UV photons and hence they complement traditional studies observing gamma rays resulting from annihilation of positrons in delocalised, trapped positronium states. Variable energy Ps offer promise for studying surfaces because it is neutral and is the simplest "hydrogenic system" available with a mass of three orders of magnitude less than hydrogen.
Our studies concern the scattering variable-energy thermalised positrons from thin films, surfaces and interfaces for metals, semiconductors an insulators, to study (a) the interaction processes and (b) the surface potential, (c) oxygen (and later, biomolecules) adsorbates and (d) the preferential surface sensitivity for positronium [Ps] formation, together with a polarised positron beam, to study surface magnetism. The new physics will come out of the unforeseen.
The interaction process studies will explain (i) elastic re-emission into the vacuum (ii) localisation of the positron in a surface state, (iii) Ps emission by pickup of a near-surface electron, (iv) excitation of a bound positron out of a surface state, and (v) reflection of the positron at the surface.
The surface potential studies will give information on the electrostatic dipole potential barrier, surface state localization, surface defects and fine structure.
The oxygen, (and later, carbon monoxide and biomolecules) adsorbates using Re-Emittted Positron Energy-Loss Spectroscopy (REPELS), which is similar to EELS when inelastically emitted positrons lose energy to create electron-hole pairs and bulk or surface plasmons or phonons. The roles of elastic and inelastic scattering will be clarified and, for example, vibrational and excitation modes of adsorbate molecules will be studied.
The preferential surface sensitivity for Ps formation, together with a polarised positron beam, will study surface magnetism. The normal ratio of 3:1 for random Ps spins can be changed significantly by a preferential alignment of electron spins at the surface to form triplet o-Ps (m=1) and p-Ps (m=0). The dynamics of Ps formation in Ni and Fe will be measured from the energy distribution of emitted Ps as a function of incident positron energy.
