Spectroscopy and Molecular Structure
Leslie Sutton was primarily interested in
problems of structure, conformation and molecular interactions in systems which
were then too large to be handled by spectroscopic techniques. He made direct
determinations of structure – for example of molecules such as the acetyl
halides, carbon suboxide and sulphur and selenium tetra- and hexa-fluorides –
by electron diffraction in the gas phase. But information was also sought in
other ways, particularly through the use of thermochemical measurements and
from measurements of dipole moments. These results were important in the
development of current theories of valence. Interest in dipole moments led to
further work on the measurement of dielectric constants and of dielectric loss,
measurements which were used to evaluate atomic polarizations and to study
molecular association.
At the end of World War II, although
microwave spectroscopy was being developed, spectroscopy at other wavelengths
was still limited by the resolution of spectrometers and by the quality of
detectors: very little was known of the region between about 4000 and 10000 cm-1, and even when high resolving power was available,
there seemed to be no way of penetrating the Doppler limit. Thompson and his
students had carried out many analyses of the rotational structure of species
such as carbonyl sulphide, with J.H. Callomon, diazomethane, with I.M. Mills,
methyl iodide, with E. Wynn Jones, and allene with J. Overend. G.L. Caldow had
worked on solvent effects and band intensities in infrared spectra. For much of
this active period, Richard Popplewell was an able and effective lieutenant in
the group. By the mid-1970s, classical infrared grating spectroscopy had been
taken as far as it could go, and the study, by Bob Thomas, of the hydrogen
bonding in the 1:1 complex between water and hydrogen fluoride, published in
1975, marked the end of an era.
Barrow and his students had meanwhile been
using electric discharges to study the electronic spectra of species like HF
and OH, whose energies of dissociation were of interest. The spectrum of HF is
an unusual, many-line, affair resulting from a transition between an ionic
state with a large internuclear distance and the ground state. Analysis was
difficult, and it was a sign of his skills that John Johns in the Long Vacation
between Part II and D.Phil., unravelled the analogous spectrum of DF. John has
subsequently had a distinguished career at what is now the Herzberg Institute
of Astrophysics at Ottawa. Later, when higher resolution became available, Neil
Travis oversaw the construction of a high temperature King furnace which was to
prove valuable as source of absorption and thermal emission spectra which were
used in the attempt to sort out the patterns of the electronic states of
transition metal diatomics such as ScF and CeO – a subject taken up so
fruitfully by another student of the 1960's, Tony Merer, now Professor at the
University of British Columbia. One of the results of a close collaboration
with Albin Lagerqvist at the University of Stockholm was the realization that
perturbations could be analysed to get rather detailed information about
perturbing states: examples were BaO and CS, and, much later, with Stewart
Harris and with R.W. Field at M.I.T., SiS.
Meanwhile, the growing power of computers
began to make possible quite precise ab initio calculations on small
molecules, and Graham Richards, who had done his D.Phil. with Barrow, having
spent some time in Paris learning the business with Carl Moser and Mme
Lefebvre-Brion, began in the late 1960's with calculations on the energy states
of species like BeO and MgO, and first with Tim Walker and later with David
Cooper attacked the problem of the estimation of spin-orbit coupling constants
and of λ-doubling in astrophysically important species. A decade later it
could be claimed that some spectroscopic frequencies could be calculated more
accurately than laboratory experiments can measure them. David Cooper left on
appointment to a Lectureship at the University of Liverpool in 1985, and with
his departure the work of the Richards group became directed exclusively to the
problems of conformation, structure a nd reactivity of large molecules –
particularly those showing (beneficial) drug activity.
Also in the late 1960s, M.S. Child was
working on his semi-classical treatment of linewidths in the predissociation of
diatomics, work which was to be important in interpreting the spectra of some
of the interhalogens and alkali halides.
Within a relatively short time, new
developments changed this world dramatically. Molecular beam techniques,
developments in the technology of lasers and of detectors, the availability of
relatively cheap computers which enabled Fourier transform spectrometers to be
built – all these altered and are still changing the face of spectroscopic
studies, and there is no sign that this process has come to an end.
In 1975, Brian Howard came to the PCL from
Southampton, where he had been a Royal Society Pickering Research Fellow, first
to stand in for John White on the latter's secondment as Director of the
Institut Laue Langevin at Grenoble. Brian had worked with Professor Klemperer
at Harvard, and he introduced the molecular beam electric resonance experiment
to the PCL, using it to study the potential energy surfaces of van der Waals
complexes such as ArHCl. The original MBER experiments probe the potentials
only near their minima: more recently diode lasers have been used successfully
to obtain the first infrared spectra of complexes generated in beams and to get
information about potentials higher up the wells. A new microwave Fourier
transform spectrometer has recently been built to study rotational spectra of
transient species. It has been used successfully on several van der Waals
molecules and hydrogen bonded dimers. It is planned to use laser photolysis in
a jet to produce cold radicals, ions and cluster ions whose microwave spectra
will provide structural and other information on these species.
Schematic of the LMR
system used by John Brown.
When high power argon ion and krypton ion
lasers became available, Barrow began a collaboration with colleagues in Lyon
and with Jean Vergès of the Laboratoire Aimé Cotton at Orsay on high
resolution, Fourier transform studies of laser induced infrared fluorescence,
with the aim of getting information about electronic states inaccessible from
the ground state, such as the gerade states of Na2, Te2 and Bi2. Additional
double resonance experiments on Na2 led, in
collaboration with David Cooper, to a rather detailed description of the double
minimum state (2)Σu+. When he retired in 1983, the PCL welcomed John
Brown, like Brian Howard from Southampton, where he had been Reader in
Chemistry. Interested in the insights that can be given by nuclear hyperfine
structure, he brought to the PCL a new technique, laser magnetic resonance,
which he had developed, partly in collaboration with Ken Evenson at Boulder.
This high resolution technique is also of high sensitivity, and he has for
example used it to determine the structure of HO2. Currently he is also using the laser facilities at the Rutherford
Laboratory to study the resolved laser-induced fluorescence of transition metal
halides.
Tim Softley, who joined the PCL in 1990 from
Cambridge, is extending the theme of laser excitation and laser fluorescence
spectroscopy to higher energies in the region of the far ultraviolet. Here
there are vast gaps in knowledge and understanding, and present work is
concentrated on apparently simple species like H2 and H2O: in fact, because there are strong perturbations
between overlapping Rydberg states, many of the spectra in this region turn out
to be of daunting complexity (although recently Mark Child has made great
progress in the interpretation of the Rydberg spectrum of water).
|
The laboratory of Tim
Softley in 1991 |
|
For a few years, the PCL had the good
fortune to entertain Alan Carrington, Royal Society Professor (with whom Tim
Softley had earlier done his Ph.D. at Southampton). Carrington's work at Oxford
centred around the study of the last bound levels of the ground state of HD+. The experiment involves tuning the energy levels of the ion into
resonance with an infrared laser by the Doppler effect. The study of the
hyperfine structure of these levels near dissociation gives information about
the early stages of chemical binding at very large internuclear distances and
about the spatial distribution of the single electron.
The technique of laser excited fluorescence
can also be used to detect the presence of species in reacting systems and fast
Fourier transform spectrometry has been used in kinetic studies by Hancock and
his colleagues. Gus Hancock joined the PCL in 1976 from the University of
Bielefeld. His interests span the range of kinetics and laser spectroscopy, and
subjects of present interest include infrared multiple photon dissociation,
kinetic studies of reactive free radicals, the plasma etching of semiconductors
and the study of the energy state distribution of reaction products by tunable
uv laser induced fluorescence.
