Photoelectron Spectroscopy
A different, complementary line of molecular
spectroscopy was developed in the PCL following the election of David Turner as
Fellow and Tutor at Balliol in 1967 on the resignation of Ronnie Bell. With
W.C. Price, at King's College, London, Turner had invented the technique of
photoelectron spectroscopy (although there was independent work by groups in
Leningrad and in Uppsala), and his work brought new insights into molecular
orbital theories of chemical binding, and a new stimulus to the ab initio
calculation of molecular structures. There were eager converts to this brand of
spectroscopy in the early 1970's, and at one time there were no fewer than four
groups in Oxford, of which three were in the PCL, with Turner, with Thompson
and with Danby.
Turner had brought with him a small group of
three graduates, the last of the group that had helped him build up molecular
photoelectron spectroscopy after its birth at Imperial College. They were A.D.
Baker, C. Baker and C.R. Brundle. They brought with them also the early PES
instruments, notably the two `Paul Fund' machines, and enough other hardware
(spin decoupling nmr and grating monochromators) uncomfortably to fill the
space available.
In the first year or so, work on the PES
front concentrated on cataloguing the spectra of a wide range of relatively
volatile substances to lead to the publication of the first monograph, Molecular
Photoelectron Spectroscopy (Wiley, 1970) by Turner, Baker, Baker and
Brundle. Though its appearance had been delayed by the move from London, it
benefited considerably from contributions from inorganic chemists who were
quick to take advantage of the new facilities.
The group in the Inorganic Chemistry
Laboratory subsequently drew on the experience of David Turner to design and
construct a simple instrument which could be heated for the study of less
volatile substances. As well as reporting the spectra for a wide range of the
simpler molecules for which reasonably full MO descriptions could be attempted,
the group sought to show application to `big' molecules where some classic
questions of structure and spectra awaited solution, most notably the steric
inhibition of resonance that J.P. Maier explored in his D.Phil.
Before long the old machine was not up to it
(it ended in the Science Museum), the availability of commercial instruments
notably the Perkin Elmer He(I) PES and the VG XPS put chemical applications on
a routine footing and a new phase of instrumental refinement began and which
still continues.
The more physical aspects of PES have
included temperature dependent conformational equilibria and extensions into
the study of unstable species, and one PE spectrometer was modified to get at
flame fragments and another was built with a hemispherical analyser to study
species over a red-hot surface. This instrument was designed also to open the
way to electron angular dependence studies. There was a real need for
simultaneous mass analysis in these thermal experiments and Brehm and von
Puttkamer's success in showing photoelectron-ion coincidences prompted similar
experiments in which a routine method for obtaining ion time-of-flight mass
spectra within the now conventional PES deflection electron analyser emerged
and enabled the search for ion fluorescence emission accompanying the PES
experiment. This was done initially with an image intensifier and subsequently
with single photon counting in delayed coincidences. Ion optical emission in
corona discharges was also sought with the use of the image intensifier.
Important links with Swiss institutions were
forged enabling visiting post-graduates to reinforce the work on ion optical
emission - a link which led Maier eventually to become Professor at the
University of Basel, where many pioneering studies of ion structure by electron
and laser impact methods are being made.
Tim Softley has recently applied far-UV
laser radiation to a new method of photoelectron spectroscopy known as
zero-kinetic-energy (ZEKE) PES. In this technique, the yield of very low energy
photoelectrons (< 0.1 meV) is monitored as a function of laser wavelength; a
peak in the ZEKE photoelectron yield is observed at each threshold for
photoionisation, corresponding to an energy level difference between the
neutral molecule and the ion. The technique provides two orders of magnitude
better resolution compared to the original PES method as a consequence of the
improved energy discrimination which is possible with low-energy electrons.
This has allowed the observation of rotationally resolved photoelectron spectra
of small molecules such as H2 and N2, and detailed vibrational structure for more complex species. New
information is obtained concerning the structure of the molecules studied, as
well as a more detailed understanding of the photoionisation processes.
