Hand waving guide to Electron Spin Resonance
(ESR).
All electrons spin like tops. They also orbit around atoms and
molecules. These spinning motions make them behave like little "current
loops". And one of the basics of electromagetics is that current loops
produce magnetic fields.
If we apply an external H-field to some materials we can cause the spins of
some electrons to "line up" with the applied field. The spinning electrons
then behave a bit like a top spinning on the floor. When you watch a top you
can see that - unless it is perfectly upright - it will "precess". (This
means its axis of rotation moves around the upright direction at a steady
rate.) In effect the axis of rotation "wobbles around" the direction of the applied field at a steady rate.
In the case of a spinning electron, the applied H-field takes the place of
gravity. If we "kick" the spinning electron whilst applying a magetic field
the electron will precess. The rate of this precession depends on the field
the electron experiences and that is acutally a mixture of the external
field we're applying and the internal field produced by molecules inside the
material.
When the electron precesses the field produced by its own spin is being
waggled around. Again, basic EM theory tells us that a waggling field tends
to radiate waves at the frequency of the waggling. Hence when we kick the
electron into precessing it will radiate at the precession frequency.
In practice what we do is take a piece of gloop (that's engineer speak for
"a valuable sample of precious material"...) and stick it in a humungous
H-field. We then shine rf/microwave/mm-wave radiation on the sample and see
if it absorbs or re-radiates anything. If we can manage to illuminate it
at the right frequency (i.e. the precession frequency) the sample responds
by showing a resonance and abruptly changing it absorption/re-radiation
behavour.
Knowing the H-field we're applying, and the frequency of the
radiation we're putting onto the gloop, we can then deduce something about
the internal fields of the material and the way the electrons inside it are
orbiting. Hence we learn something about the gloop. We can therefore use an ESR system as a form of spectrometer to analyse the chemistry of samples. The resulting information is useful in applications from foresic science to the study of enzymes and the behaviour of solid-state displays.
Of course, it is more complex than that (what isn't?) but we can now sweep
the applied H-field and search out all the electrons in gloop to produce a
spectrum of resonances. If we wanted we could keep the H-field steady and
sweep the illuminating frequency, but it is usually easier to sweep the
field. Hence ESR spectra are normally plotted in terms of "output versus
applied H-field", not "output versus frequency". It all comes out the same
in the end, honest...
Downloaded from University of St. Andrews
Original content by: Jim Lesurf
(jcgl@st-and.ac.uk)
University of St. Andrews, St Andrews, Fife KY16 9SS, Scotland.