Design and performance of an apparatus to generate helium droplets
This thesis extensively outlines the design and performance of the fIrst helium droplet
apparatus to be built in the United Kingdom.
Helium clusters (6000 atoms) were generated: by adiabatic expansion of research grade
(99.9999%) 4He, at a stagnation pressure of 20bar, through a 5flm nozzle, cooled to 12K. The
helium droplet beam is collimated by a 0.5mm skimmer (Molecular Beam Dynamics) before
entering a 'pick-up' cell (lOcm, length) where the cluster beam interacts with a gaseous species of
spectroscopic interest. The droplets 'pick-up' singular or multiple molecules that release their
relative kinetic and potential energies to the droplet and become thermalised to the ultra cold
environment of the helium bath (0.38K). Helium droplets therefore provide a cold, largely noninteractive
medium in which to study the spectroscopy of molecular species.
Contained in this thesis is a description: of the experimental challenge to produce a cluster
source able to perform at the extreme conditions required to produce helium droplets; the
modifIcations of the 'pick-up' system such that a variety of species (gases, liquids and solids) can
be studied; the design and construction of a LIF region; the upgrade of a quadrupole mass
spectrometer for maximum utility for this application; a description of the laser system and data
acquisition methods; a review of cluster and helium droplet research; the theory associated with
cluster and especially helium droplet generation; characterisation of the helium droplet beam, 'pickup'
and fragmentation of neutral species upon detection; the preliminary results obtained from a
study of resonant photon absorption (UV) of benzene solvated by helium droplets and the fIrst
evidence of multiphoton ionisation of a molecule in a helium droplet