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Title: Progress towards a source of cold, slow molecules for tests of fundamental physics
Author: Bumby, James
ISNI:       0000 0004 6348 6191
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2017
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This thesis reports on two beam sources of ytterbium fluoride (YbF), and assesses their viability for use in precision measurements of the electron electric dipole moment. The first source is a single-stage cryogenic buffer-gas source optimised for high extraction efficiency and low forward velocity. The thermalisation of the YbF rotational and translational temperatures with the buffer gas were measured through laser-induced fluorescence spectroscopy of molecules within the beam. The velocity distributions and fluxes of beams produced by this source were measured for buffer gas flow rates between 10 and 40 SCCM. We found that the mean beam velocities were between 190 and 210 m/s, and characteristic molecular fluxes were of the order 2*10^9 /sr/pulse. The second source is a two-stage cryogenic buffer-gas source, where the first stage is supposed to produce a high flux and the second stage is meant to slow the beam to low velocity. We found optimal behaviour running at lower pressures than the single-stage source, and measured YbF beams with a peak velocity of 70 m/s. Although not fully optimised, the source shows clear evidence of deceleration in the second stage, and drastically outperforms even the best measured fluxes from the single-stage source in terms of molecules with velocities below 100 m/s. The characteristic flux from this second source is of the order 7*10^7 /sr/pulse. As part of our parametrisation procedure, we developed a novel experimental technique to measure velocity distributions in the limit of low signal-to-noise ratios. We optically bleached population from a target hyperfine level of certain Doppler classes within the molecular beam. By integrating over all bleached molecules, we measured the number of YbF molecules with a given velocity. This technique was experimentally implemented, and was found to be in good agreement with results obtained using more traditional measurement schemes.
Supervisor: Tarbutt, Mike ; Hinds, Edward Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral