Chemistry Close to Absolute Zero
This ambitious project, led by Bertrand Rowe in Rennes and Ian Smith in Birmingham, was to pioneer the study of chemical reactions down to extremely low temperatures.
To carry out
such measurements on reactions in
the gas-phase, it is necessary to
keep the species from solidifying
at temperatures well below their freezing
points. This formidable challenge
is met by cooling the gas by expansion
through a specially shaped nozzle.
The resultant jet of gas moves at
supersonic speed. Its temperature
depends on details of the nozzle design
but can be as low as a few degrees
above absolute zero. Because the gas
strikes no surfaces and condensation
within the gas is slow compared with
the flow, there is no condensation.
Chemical reaction is initiated within
this jet of very cold gas, usually
by creating a reactive species such
as a free radical using a pulsed laser.
A rich variety
of molecular processes were studied
in the unique facilities in Birmingham
and Rennes, especially many reactions
between electrically charged ions
and molecules and between electrically
neutral radicals and molecules. The
results set theoreticians a real challenge.
What factors control the rates of
these rapid reactions that go still
faster as the temperature is lowered?
Because of their
fundamental significance, the Birmingham-Rennes
results have generated much interest
and excitement. However, their importance
extends further - they have had a
major impact on the understanding
of how molecules might form in the
deep recesses of space. About 120
molecules have now been identified
in huge astronomical aggregates known
as interstellar clouds where the molecules
are found in close association with
particulate matter - 'interstellar
dust'. The temperature and densities
are very low in these clouds and it
is a fascinating to ask what chemistry
synthesises the observed molecules.
interstellar clouds are the birth-place
of stars, as dust and molecules are
pulled together by gravity. Understanding
molecular formation will help astronomers
understand this process of collapse
and star formation. The Birmingham-Rennes
work at low temperatures has shown
that a rich chemistry occurs in interstellar
clouds and provides, directly and
indirectly, information on what appropriate
chemical models should be constructed.
Much has been
achieved in this collaboration but
much exciting work in low temperature
chemistry lies ahead! Chemistry Close
to Absolute Zero: Movie