NNR IN RAPIDLY ROTATED METALS By - Nottingham eTheses ...

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phase can then be set so that the transverse magnetization is
completely nulled out. If, however, the reference frequency is
different from Vo the FID is modulated by beats at the frequency
difference and the null condition cannot be achieved. For asym-
metric spectra the spins fan out unevenly about any axis in the
rotating frame. As a result it is impossible to measure the
centre frequency by direct observation of the transient signals.
Experimentally the most homogeneous position in the magnet
gap was first found by using a suitable liquid specimen. A direct
comparison between the resonance frequencies of liquid samples
could be made by fine adjustment of the frequency synthesiser.
For weaker signals this operation was facilitated by a reduction
of the-receiver bandwidth and the use of the DL 102 signal averager.
The precise Knight shift measurements undertaken depended
upon the narrowing of the resonance spectrum by rapid sample rotat-
ion at the magic angle. Experimental Knight shifts are often
determined against liquid reference samples, but the conical rotors
used for this particular work would not rotate with liquid samples
of any reasonable volume. Therefore it was not possible to place
the metal and the reference specimen together in the same rotor
as has been done previously
ý67ý
.
Instead a different procedure
was adopted using the double rotor box shown in Figure 5.5. The
accuracy achieved was a significant improvement over previous
Knight shift estimates on the same metals. A short account of the
procedure adopted is included below.
A rotor was placed in one stator and the liquid reference
sample was put in an equivalent position in the second 'dummy'
stator. The assembly could be slid through the magnet pole gap