文档介绍：4. The Molecular Beam
The Supersonic Expansion
The investigation of small plexes has recently e an im-
portant source of information on intermolecular forces. These small systems
are at the limit of what can be accurately described by theoretical (ab initio)
models. A description of larger systems, such as surface interactions or fluids,
is beyond the scope of what can be achieved at the moment. -
plexes have therefore turned out to be small, theoretically tractable systems,
which are excellently suited for the investigation of intermolecular forces. The
experimental techniques which are used for the study of plexes
are scattering experiments and spectroscopy. In both cases, efficient produc-
tion methods are required. Since the van der Waals interaction is quite weak,
plexes are not bound at room temperature. An efficient production
method therefore requires a decrease in temperature in order to achieve a
thermal energy which is less than the binding energy (typically 150 K). Even
in the case of strongly plexes, such as hydrogen-plexes,
a decrease in temperature will increase the number of pared with
monomers. In order to produce very low temperatures, one can either cool
the gas in a cell surrounded by a liquid at low temperature (. liquid ni-
trogen) or use molecular beams. Since the molecular-beam technique is the
method used in our laboratory and by most other groups, the discussion will
be restricted to the production plexes in molecular beams. A very de-
tailed introduction to molecular beams is given in [176]. A short introduction
is given here, which summarizes Chap. 2 of [176] and an article by Hagena
about the nucleation and growth of clusters in expanding nozzle flows [68].
In a molecular beam, the gas is expanded through a nozzle into a vac-
uum. The situation is sketched in Fig. . The gas starts from a temperature
T0 and a pressure P0. The chamber is evacuated by a pump (a mechanical
pump or an oil diffusion pump) and is