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

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quadrupole moments to produce transitions within the spin levels
of the nuclei. The vibrational modes in a solid may be assumed
to occupy a Debye spectrum, the Debye frequency being defined by
hvD - keD. There are three suggested phonon relaxation processes,
(i) Single Phonon. Transitions induced between the nuclear
spin states correspond to the creation or destruction of a single
phonon. Only these phonons with energy hvp = hV0 can participate.
However V0 « VD, so there are very few phonons which fulfil this
condition and such a process is not an efficient relaxation
mechanism. As phonons obey Bose-Einstein statistics and hv0 « kT
the number of phonons with energy hV0 is proportional to the absolute
temperature. The relaxation probability depends upon the number
2
of vibrational modes with frequency V0. Hence l/T1 a v0T.
(ii) Four Phonon. A second order process has been suggested (20)
by which two phonons are created and two destroyed, the energy
difference being made up by the lattice transition. This mechanism
predicts aT4 dependence of T1 but although it was expected to
dominate at high temperatures it has never been observed experi-
mentally. Indeed, a more recent calculation
(21) has shown that
such a process does not in fact give rise to any net transition
between nuclear spin levels, and consequently does not contribute
to the overall
Tl.
(iii) Two Phonon (Raman). This process is far more efficient
than the single phonon because any phonon in the Debye spectrum
can participate. A phonon is scattered inelastically
at a nucleus
so an induced transition between spin states is accompanied by a
corresponding change in the phonon energy. Although the relaxation