MM-Wave ESR page2
Pix of equipment and mugshots. 
In the St. Andrews instrument the input mm-wave illumination 'looks down' on
the sample along the H-field axis. This means it can essentially see the
electrons spinning around the line of view. The
absorbtion/re-radiation interaction interaction between the electrons and
the input mm-wave power means that the 'reflected' signal has a different
polarisation state to the input. Since we use optical methods we can make
use of this polarisation change. (Conventional ESR's use waveguide which
will only carry one polarisation, hence they can't see this effect.)
We are able to exploit this polarisation change by arranging that our
detectors only respond to returned signals whose polarisation is
perpendicular to the signal we're using to illuminate the sample. As a
result, in an ideal system we won't see any output unless we've
found an ESR resonance in the material. Conventional systems tend to have
their sensitivity limited by 'stray' reflections and unwanted spurious
resonances in the instrument. Our approach means we can reduce these effects
and get a much clearer measurement.
The instrument uses an InSb mixer which is much more sensitive than a
conventional mixer diode, and can be used at any frequency from below 100
GHz to well above 200 GHz.
Examples of results
Spectrum of a thin metal film.
This is an example of
a measurement which is simply impossible at lower fields and
frequencies. The fields required to induce these resonances are very
high and the resulting resonant frequencies are also high. The spectrum
is also spread out over a very wide range of field levels - well beyond
the capability of a conventional ESR.
This measurement is an early result. Since it was made the sensitivity
of the instrument has been improved by over 50dB!
Spectrum of DPPH + Fluroenthene at 90 GHz.
Note the very narrow H-field scale (in Gauss, not Teslas). At 10GHz the two
lines for the different materials would overlap and make it impossible to
detect the presence of a small amount of one material in the presence of the
other. The result therefore indicates the improved resolving power of the
mm-wave instrument.
Ni Complex
This graph shows the spectrum of a monomolecular Ni complex with a very high zero field splitting. Due to the high zero field this spectrum cannot be seen using commercial 10 GHz or 35GHz systems.
Ferritin
This shows the very broad Ferritin spectrum using a sample of only a few hundred micrograms. This sample would have been undetectable using conventional instruments.
Perylene Radical at 170K
This shows the very narrow linewidth of a single Perylene crystal at 170K.
Perylene Radical at 140K
At around 160K the Perylene undergoes a phase change and the line splits. The ability to resolve these very narrow, closely spaced lines is indicative of the stability and resolution of the St. Andrews ESR Instrument. It demonstrates milligauss resolution at field strengths of many Teslas. (i.e. resolutions of one part in a million or better.)
Content and pages maintained by: Jim Lesurf (jcgl@st-and.ac.uk)
using HTMLEdit3 on a StrongARM powered RISCOS machine.
University of St. Andrews, St Andrews, Fife KY16 9SS, Scotland.
Pix of equipment and mugshots. 