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

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near the time origin. By using the solid echo (and Jeener and
Brockaert) pulse sequence it was shown that the decay curves for
sample (a) were Gaussian right back to the time origin. However
this was not found to be the case for the other samples. Both
the FIDs and the solid echo responses became more exponential
than Gaussian close to zero time. The anomalously high values of
second moment measured by other workers in samples similar to (c)
are regarded generally as indicating the presence of a nuclear
quadrupole interaction. If this is so, the validity of the data
points predicted by the solid echo sequence is brought into quest-
ion. The response following the 90-T-9090 sequence has been calc-
ulated for those cases where the internal spin interactions arise
either-from the dipolar coupling between spins
(71) or from the
first order quadrupole broadening in an imperfect cubic crystal
but not a combination of the two. Nevertheless for times less
than T2 the observed solid echoes were found to follow the shape
of the FIDs. ' Consequently the FID dead time points were estimated
so that the preliminary part of the decay envelope resembled the
outline of the solid echo maximum.
Experimental values of the second and fourth moments are recorded
in Table 7.1. The large variation in the second moment values of
the different specimens is indicative of the relative Lorentzian
character of the experimental lineshapes. The errors quoted refer
to the spread of measured values about the mean. The large error
values given for samples (b) and (c) arise out of the variation in
the tails of the transformed lineshapes caused by the uncertainty
in fitting the dead time points. These estimates do not however
include any consideration of the contribution to the second moment