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

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lattice and T2 is the spin-spin relaxation time describing the
decay of any transverse magnetization. Bloch's equation predicts
a Lorentzian lineshape function
f (c)) aT21 1+ w2 T2
which usually provides a good approximation of the experimental
lineshape obtained from liquid samples. Implicit in equation (1.10)
is the assumption that both the transverse and longitudinal magnet-
izations decay exponentially. Although the decay of transverse
magnetization in solids does not usually obey such a relationship,
the two relaxation processes introduced in equation (1.10) are
generally valid. The absorption lineshapes obtained from solids
can often be represented satisfactorily by a Gaussian-shape funct-
ion
fýý)
1e W2/2A2
. 2. i
where A is the half-width between points of maximum slope of f(W)
and 1/02ir is a normalizing factor.
At this point we consider the development of the nuclear
magnetization in the rotating frame following a 900 rf pulse
applied along the x* axis. Immediately after the pulse the
nuclear magnetization will lie along the y* axis. In solids the
spins experience finite local internal magnetic fields (described
in detail in the next chapter) which give rise to a spread of -
precession frequencies about wo As a result the spins begin
to fan out about y* in thex-y plane, and the transverse magnet-
ization decays with a characteristic time T2. Any mechanism