Reaction Kinetics: Gas Phase
The subject of gas kinetics remained alive
and well in the PCL after Hinsh's retirement: one year later, Richard Wayne was
appointed Student of Christ Church and University Lecturer. His interests have
centred on studies of processes of atmospheric interest, both in the
terrestrial environment and the atmospheres of other planets. The
photochemistry of ozone, carried out with I.T.N. Jones in the early 1960s,
addressed the still unanswered problem of the quantum yields of the
dissociation products in the ultraviolet region, and this work is now being
carried out in further studies using laser techniques in the OCAK
laboratory (OCAK = Oxford Centre for Applied Kinetics, see below). Development
of new and sensitive detection methods in kinetics and photochemistry has been
a feature of Wayne's research, from the construction of new germanium detectors
for the near-infrared region to the use of discharge-flow mass-spectrometric
techniques for the difficult task of measuring radical–radical reaction rate
constants. Recent experiments have concentrated on reactive species of interest
in the anthropologically polluted atmosphere such as NO3, ClO and OH radicals.
Mike Pilling's main interests centred on the
study of gas-phase reactions of free radicals. He used laser-flash photolysis
coupled with a range of detection techniques and concentrated on association
processes, such as radical + radical and atom + alkene reactions. His main aim
was to determine precisely the dependence of the rate constant on temperature
and pressure and then to develop models of the reactions or to test models
developed elsewhere. The CH3 + CH3 reaction was the subject of an extensive study, in collaboration with
both experimentalists and theoreticians from the U.S.A. and Germany. A new
technique was developed to study atom + radical reactions at moderately high
pressures and first applied to the CH3 + H reaction.
Isotope anomalies demonstrated that even this apparently simple reaction is
poorly understood. One application of these studies is the provision of data
for modelling combustion processes and this aspect has been further emphasised
in Pilling's work following his move to Leeds.
Gus Hancock's major interest has been in
reaction dynamics, the combination of gas kinetics with spectroscopy, with
reactions initiated and probed by laser techniques. Infrared multiple-photon
dissociation has been a feature of much of the work, the technique being used
both as a source of free radicals for further kinetic measurements, and the
physics of the process being studied by experiments involving shaped laser
pulses. Recent activity in reaction dynamics has exploited the fact that vector
properties of reactive events, reactants' and products' angular momenta and
velocities, can be controlled and measured by lasers, and this has been used to
investigate in detail the dynamics of the collision process. Gas-phase kinetics
can also play an important role in applied chemistry, and in this case laser
diagnostic methods have been used successfully to study reactive atoms and free
radicals in plasmas used for etching of semiconductors in the manufacture of
microcircuits.
The work of OCAK is described below. The
first publication resulting from its activities concerned the use of a simple
FTIR spectrometer developed in the Wayne group for studies of the continuous
emission in the near infrared in discharge-flow experiments. It was adapted for
time-resolved studies in Hancock's group, and is now being employed with
success by both members of OCAK in studies both of reaction dynamics and of gas
phase kinetics.
O.C.A.K. – The Oxford Centre for Applied Kinetics
The Oxford Centre for Applied Kinetics
(OCAK) is a collaborative research group formed in 1987 by Hancock, Pilling and
Wayne. Behind the concept of the centre lies the recognition that, with the
increasing sophistication and cost of experiments in the field of kinetics, the
competitive ability of individual research groups was becoming compromised. In
addition – and possibly more important – a symbiotic and productive interaction
follows from the different areas of expertise embodied in the individual
research groups.
OCAK was established with a substantial
grant from Oxford University in the expectation that, following its formation,
it would attract more funding. Indeed, further funding has been found from a
variety of sources including the national research councils (both SERC and
NERC) and the CEC. Much of the money has been used in purchasing capital
equipment such as lasers, computers and pumping systems. The variety of
techniques that it has been possible to develop over the last four years
include static, slow-flow and fast-flow systems, with detection of atomic,
radical and molecular species by laser-induced fluorescence, optical
absorption, mass spectrometry and resonance-enhanced multiphoton ionization
(REMPI). Other experimental techniques are currently under development.
From the outset, it was intended that a wide
range of kinetic systems would be investigated. The work performed already has
sampled a number of kinetically interesting areas, ranging from those with a
purely practical bias, such as atmospheric and combustion chemistry, to the
more theoretical, such as reaction dynamics. The specific systems studied so
far include the reaction and quenching of CH2(ă1A1), reactions of NO3, ClO
and OClO, and the photolytic generation of O2(a1Δg) from O3.
In 1990, Mike Pilling left to take up the
Chair of Physical Chemistry at Leeds; however, OCAK's activities have continued
unabated, with many of the experiments designed by him still being undertaken.
Indeed we hope that he will continue to take an interest in the activities of
OCAK. The experimentalists in OCAK have been supplied by the member groups, and
up to the present there have been three post-doctoral workers, three D.Phil.
students and a number of Part II students directly involved in the work.
However, the cooperation between the research groups at all levels following
its formation has made OCAK an extremely successful venture.
Questions of molecular dynamics have also
been taken up by Simpson and his colleagues. The thermal decompositions of
carbonyl and azo compounds have been studied to look at the states of CO and N2 as they are released, and of CO from the photochemical dissociation of
3-cyclopentanone and 7-norbornenone under collision free conditions. It seems
that these molecules dissociate by a concerted mechanism to give CO in very
high J levels – up to 10 000 cm-1 in
rotational energy.
