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

- 16 -
From a classical point of view the paired s electrons in the
filled shells cannot contribute to the contact term. However they
become exchange polarized by the polarized conduction electrons.
The magnitude and sign of the resulting hyperfine field depends
upon the relative s, p, d, etc. character of the conduction electron
wave functions. Generally this term appears to enhance the Knight
(6)
shift; in some cases by as much as 30%. Similarly, for metals
and alloys with unfilled electron shells the applied field produces
a net polarization. The unfilled shells exchange polarize the
core s electrons resulting in a negative effective field at the
nucleus(l). The negative Knight shifts recorded for platinum(8) and
palladium(9) have been accounted for in this way.
Most transition metals have large Knight shifts. This is
due in part to the large density of states at the Fermi surface of
the narrow conduction bands, but there is also a large second order
paramagnetic orbital contribution arising from the small energy
separation between the occupied and unoccupied levelsl1)
The p and d character of the conduction electron wave function
couples with the nucleus through a classical dipolar interaction
between spins. In metals with cubic symmetry such an interaction
is zero, but for non-cubic metals it gives rise to anisotropic
Knight shifts. Polycrystalline metal samples are normally used
in NMR measurements. This anisotropy then becomes apparent as a
field-dependent broadening of the resonance spectrum. Following
Winter (p. 53), assuming axial symmetry the resonance frequency
at a nuclear site is given by
va vs + la(3 cos20 - 1)
(10,