Molecular Physics of Ions and Ionization
In the course of his opening address to the
first of the triennial conferences on Advances in Mass Spectrometry held in
London in 1958, Hinsh, then President of the Royal Society, commented on the
fundamental physics and chemistry of the processes by which ions are formed and
in which they fragment to give the characteristic patterns shown by the mass
spectra of complex molecules. He remarked `The whole sequence of events is
rather a mysterious one but it is of fundamental interest and indeed the
behaviour of molecules in this fragmentation is a new and self-contained
chapter of chemical kinetics.'
With mass spectrometry released from its
quantitative analytical role by the advent of gas chromatography, John Danby's
group turned its attention to this area, initially studying metastable ions
with a double-focussing instrument built in the laboratory. In the early
1970's, with John Eland, the technique of photoelectron-photoion coincidence
spectroscopy was developed which made possible the study of the fragmentation
of ions in precisely known initial energy states. The very simple photoelectron
energy analyser which was part of this instrument performed well and this led
to a programme of work in photoelectron spectroscopy which in turn led to John
Eland's book Photoelectron Spectroscopy (Butterworths, 1973). The
coincidence experiments showed that the dissociations of small molecules
frequently did not follow statistical predictions - indeed different excited
states of some ions dissociate along different pathways to give different
products.
In 1973 John Eland went to work in Brehm's
laboratory in Freiburg and later to the Laboratoire de Photophysique
Moléculaire in Paris. In the PCL, with Ivan Powis and Peter Mansell, a new
coincidence instrument, optimised for the determination of the energy released
into translation in ion dissociations, was built and a lot of interesting work
was done with it. On completing his D.Phil, Mansell went into medicine and is
now a Senior Registrar at Bath. On John Danby's retirement at the age limit in
1983, John Eland was appointed to his University Lecturership and Fellowship at
Worcester. Ivan Powis was appointed to a Lectureship in the Physical Chemistry
Department at Nottingham. By the middle 1980's the coincidence technique had
reached a certain maturity, having been taken up by some half-dozen other
laboratories world wide. The initiation in the PCL of another coincidence
technique, fated to follow a similar course occurred almost by accident. In a
perhaps foolish attempt to follow prevailing fashion, John Eland and his colleagues
were trying to use a CW laser to study spectra of molecular ions in the mass
spectrometer source. The available laser turned out to be barely intense enough
for the task, and competition from the United States turned out to be so
strong, that they looked for an alternative use of the existing apparatus. The
idea of ion-ion coincidence spectroscopy for the study of doubly-charged ions
was born, and was quickly successful. Despite the simultaneous and independent
invention of the same technique in Paris the Eland group gained a certain lead,
and more highly developed related techniques rapidly followed. The Paris
connection became a cooperative rather than a competitive one, to the great
benefit of advance in the subject. An unfortunate by-product of this activity
has been the coining of assonant acronyms. The 1970 technique was dubbed
PEPICO, the 1980's spawned PIPICO, PEPIPICO, PEPECO and the 1990's have brought
forth PEPEPICO and PEPEPIPICO which refuse to die despite an attempt to silence
them under a catch-all acronym CSMS, charge separation mass spectrometry. This
last does at least give a flavour of the main research area and makes contact
with the original starting point.
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A Reflectron
time-of-flight mass spectrometer in John Eland's laboratory |
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For a more detailed taste, the key to the
acronyms is P = photo-, I = ion, E = electron and CO = coincidence, with
spectroscopy understood. Of the newer techniques, PIPICO allows the study of
ion pairs from dissociations of doubly charged cations and gave a first means
of determining molecular double ionization energies when the species are
entirely unstable. PEPIPICO carries this idea further, and allows the dynamics
of the unimolecular dissociations of dications to be studied; it provides the
first direct method of distinguishing between concerted and sequential
mechanisms in three-body dissociation. EPECO was designed to be a spectroscopy
of doubly charged ions exactly as photoelectron spectroscopy caters for singly
charged ones, but this was not to be, as it turned out that the process of
double ionization by a single photon is itself often stepwise involving an
autoionization mechanism. The technique shows some spectra of both singly and
doubly charged ions but mainly reveals the dynamics of multiple electron
ejection. The other techniques add elaboration on these major themes; for very
simple physico-chemical reactions they discover vector correlations and, we
hope, femtosecond reaction dynamics without the use of lasers.
