For the creation of cold molecular ions, it is essential that the
nozzle and skimmer are aligned axially. To achieve such an alignment,
two techniques were used: a coarse alignment procedure was undertaken
with the source and flight region chambers at atmospheric pressure and
separated from each other and fine alignment achieved with the apparatus under vacuum.
A helium-neon laser was mounted on a 30 x 30 cm optical bench
consisting of two steering mirrors facing in perpendicular
directions. The optics were adjusted to steer laser radiation through
the gas line and exit from the nozzle. With the two chambers
separated, the nozzle was aligned to the skimmer using the nozzle
translational controls. When the laser spot could be observed passing
cleanly through the skimmer, the nozzle and skimmer were aligned. The
chambers were then reconnected and pumped down to vacuum.
Gas was then passed through the nozzle and fine adjustments made using the translational screws. Flight region pressure was then monitored, if the nozzle/skimmer alignment improved, the flight region pressure increased. If the alignment was poorer, the pressure fell. In this manner, the nozzle was correctly aligned to the skimmer, until a maximum was reached for all translations of the nozzle.