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

- 42 -
observations are inconsistent with all of the simple relationships
predicted. Undoubtedly the precise form of the dependence is
determined by the line-broadening interactions present in the
. sample and hence the shape of the static absorption spectrum.
Generally the experimental lineshape narrows slowly until the rot-
ation frequency is about equal to the static half-width of the
absorption curve. It then narrows rapidly and later more slowly
until a rotationally invariant limit is reached.
We remark at this point that it was shown in Section 3.4 that
first order quadrupole broadening can be removed by rotation about
the magic angle. However, the rotation rates required for this
effect are normally considerably in excess of those which can be
achieved experimentally. If the quadrupole interaction is due
to imperfections in otherwise cubic solids it results in long tails
on the frequency spectrum which do not appreciably affect the line-
width. Dipolar broadening may then still be narrowed by magic
angle rotation, even though the quadrupolar broadening remains
unaffected.
In the limit any residual linewidth besides depending upon . ýt°
is subject to the normal field inhomogeneity and T1 broadening
mechanisms. Rapid rotation serves to average out part of the
contribution from field inhomogeneities but only in a plane perp-
endicular to the axis of rotation. The rotor systems employed
here could only be accommodated in a large magnet pole gap so the
field gradients along the axis of rotation could be significant in
determining the width of the narrowed central lineshape.