Reeder, Samuel Harry;
(2025)
Spectroscopic studies of positronium and helium in excited states.
Doctoral thesis (Ph.D), UCL (University College London).
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Abstract
This thesis presents the laser, microwave and millimetre-wave (THz) spectroscopy of positronium (Ps) and helium in excited states. A pulsed beam of positrons from a 22Na source is implanted into a mesoporous silica target to produce ground state Ps which is subsequently excited to the 23S1 state via single-photon laser excitation in an electric field. This excitation scheme is studied and found to be optimum within the current experimental parameters. Two measurements of the 23S1 → 2 3P2 (ν2) fine structure transition have been performed. Free-space microwave spectroscopy of the transition revealed that uncontrollable reflection effects were causing linear shifts to the measured ν2 transition frequency. A new precision measurement performed in a waveguide structure designed to mitigate these effects yields the result of ν2 = 8627.94 ± 0.95 MHz, which amounts to a 1.3σ deviation from theory. However, the precision is limited by a large systematic effect linked to the unknown microwave radiation behaviour inside the waveguide. A pulsed supersonic beam of metastable helium (He*) atoms excited to Rydberg states with a UV laser was used to probe the stray electric fields within the waveguide via microwave spectroscopy of the 37P → 37D transitions, which are in a similar frequency range to that of the Ps measurements. Stray fields are found to not be in excess of 400 mV/cm, which would correspond to Stark shifts in the Ps ν2 measurement on the order of ∼ 10 kHz and are far too small to explain the Ps systematics. New potential experiments are discussed, and THz spectroscopy of He* is performed to test a commercial frequency multiplication source of THz radiation in the context of performing such a measurement with Ps in the future.
| Type: | Thesis (Doctoral) |
|---|---|
| Qualification: | Ph.D |
| Title: | Spectroscopic studies of positronium and helium in excited states |
| Language: | English |
| Additional information: | Copyright © The Author 2025. Original content in this thesis is licensed under the terms of the Creative Commons Attribution-NonCommercial 4.0 International (CC BY-NC 4.0) Licence (https://creativecommons.org/licenses/by-nc/4.0/). Any third-party copyright material present remains the property of its respective owner(s) and is licensed under its existing terms. Access may initially be restricted at the author’s request. |
| UCL classification: | UCL UCL > Provost and Vice Provost Offices > UCL BEAMS UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Physics and Astronomy |
| URI: | https://discovery.ucl.ac.uk/id/eprint/10211728 |
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