Advances in photostop

Abstract

This thesis is an expansion on previous work using the photostop technique for
the production of near-zero velocity atoms and molecules. The goal is to produce
stopped SH molecules and trap them in a permanent magnetic trap and the aim of
this project was to construct a new experimental apparatus to accomplish this.
During initial tests of the apparatus, the Rayleigh scattering cross-section of N2 was
measured to provide a reference point for future experiments. The uncertainty and
systematic errors in the measurements was such that denitive quantitative results
of this were not be obtained at this stage.
The emerging technique of cavity-enhanced laser-induced
uorescence (CELIF) was
used to perform absolute number density measurements of a molecular beam of
SO2. CELIF was then applied to measuring the photostop of SD/SH. This showed
that CELIF would not have the required sensitivity to measure the trapped SD/SH
molecules due to issues of stray light from the lasers. As a result of this we elected
to use resonance-enhanced multi-photon ionisation (REMPI) as an alternative.
We devised and constructed a novel ion extraction system for use in performing
REMPI, which was based on a time-of-
ight mass spectroscopy system, but utilising
the magnets themselves as electrodes, as well as some ion lensing components.
This was initially tested using Xe, showing a strong signal and good mass resolution.
Using this, the photostop of SH and S was measured showing that the detection apparatus
is able to distinguish signal over a range of 9 orders of magnitude. However,
despite this sensitivity, the trapping of these stopped molecules could not initially
be demonstrated as the signal from these stopped molecules was obscured by signal
from the inadvertent dissociation of the background parent molecules by the probe
laser. More recent measurements in the group have directly addressed this issue
with background subtraction and the results have now demonstrated the trapping
of SH.
Signicant headway has been made in the demonstration of the trapping of SH
produced by photostop. From the results produced using REMPI the detection limit
has improved signicantly over the prior experiments and very recent measurements
have successfully demonstrated the trapping of SH.