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

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provides further evidence that refinements to Van Kranendonk's
and Mieher's theories, other than those arising from simple anti-
shielding considerations alone, are needed to achieve quantitative
agreement with experiment.
One line of approach to the solution of this discrepancy is
the more recent development by Van Kranendonk and Walker.
Their detailed consideration of a second order anharmonic Raman
relaxation process has thrown doubt onto the absolute importance
of the direct Raman relaxation mechanism. The second order process
takes place via a combination of the direct single phonon spin-
lattice coupling and the cubic anharmonic lattice forces: in
effect the Raman change in phonon energy corresponding to a transit-
ion between nuclear spin states is preceeded by an intermediate
single phonon state. ' The theory as developed for the point charge
model of NaC1 type crystal lattices leads to the expression
2 ci + 4c2
WaR 27YG
d2 + 4d2
W1R
where WaR_and W1R are the anharmonic and first order Raman relax-
ation rates respectively; c1, c2, dl and d2 are spin-lattice coupl-
ing constants and YG is the Grüneisen constant. For the alkali
halides, the anharmonic relaxation mechanism given by equation (6.2)
predicts relaxation times a factor of 100 shorter than the first
order Raman process, which brings theory and experiment into much
closer accord. Although the appropriate values of c12 and d12 for
the more covalent zinc blende lattice are unknown, a relaxation
mechanism of similar magnitude would obviously secure a similar
agreement in the cuprous halides. The temperature dependence of
(21'
(6.2)