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

3.1 INTRODUCTION
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CHAPTER 3
NARROWING THE NUCLEAR RESONANCE SPECTRUM BY
MACROSCOPIC SAMPLE ROTATION
In Chapter 2 the various line-broadening interactions which
exist in solids, and metals in particular, have been considered.
In many liquids and solids the situation is different: motion of
the nuclear spins averages out the secular line broadening mechan-
ism. ' The internal motion may be described by a correlation funct-
ion f(v) with a mean correlation time Tc. If Tc is short and less
than T2 (the characteristic time describing the dephasing of spins
in the rotating frame) the spin vectors will execute a random walk
in the rotating frame and only slowly accumulate a phase difference
relative to the mean resonance frequency vo. In the extreme case
the secular interactions become so inefficient that the width of
the resonance spectrum is determined by the homogeneity of the
magnetic field across the sample and lifetime broadening.
The above discussion is concerned with microscopic motion with-
in a particular sample. Similar arguments apply when studying
the effect of a macroscopic rotation of the sample about a unique
axis, but there are a few important differences. Unlike random
internal motion, macroscopic rotation only causes averaging over
the plane of rotation and not over inter-nuclear distances. Macro-
scopic motion is a very slow process by molecular standards and
involves a single frequency of motion. Although Fourier analysis