Laser excitation of positronium produced in various materials at a range of temperatures

• Main Author: Cooper, Ben
• Other Authors: Cassidy, D.
• Published: University College London (University of London) 2018
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.747414

Methods and techniques in the production, manipulation, and accumulation of positrons in a `Surko-type' buffer-gas trap are presented. The pulsed output of the trap is optimised for laser excitation of positronium (Ps). The techniques of positron time compression and single-shot positron annihilation lifetime spectroscopy (SSPALS) are discussed in detail and used throughout the study. A small array or Ps converter materials have been characterised at various temperatures. Ps cooling as a function of positron implantation is presented and discussed. Direct laser irradiation of the samples, and the effect on the Ps yield has also been studied. In some cases, particularly at cryogenic temperatures, laser induced paramagnetic centres are formed and this has a detrimental effect on the Ps yield as they cause triplet Ps to convert to the shorter-lived singlet state. Whereas in single crystal semiconductors, the laser has an enhancing effect due to an exciton-like positron-electron surface state. Many of the Ps converter materials studied here are formed from porous structures. The long term confinement of Ps within isolated cavities has also been observed, where a lifetime measurement technique using an excitation laser is presented. General methods of Ps-laser spectroscopy are also presented and the applications of which are discussed including laser-enhanced time-of-flight spectroscopy (LEPTOF), and the production, and electrostatic guiding of highly excited Rydberg Ps. Using these techniques a crossed beam scattering experiment involving Rydberg Ps, electrons, and Argon ions was carried out. This work presents important considerations for experiments to create a laser-cooled Bose-Einstein condensate (BEC) of Ps in an engineered porous material at cryogenic temperatures.