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July 9, '49

N. Bloembergen, Nuclear Magnetic Relaxation p. 29
(2.4) Dipole-dipole interaction

Looking for other types of interaction besides the very
small radiation damping, the contact of the nuclear spin
system with the outside world appears to be very limited.
Electric forces, which act during atomic & electronic collisions
& play e.g. an important role in the mechanism of arcs,
readily establish an equilibrium for electronic states. But
they do not perturb the nuclear spin. Only a gradient
of an electric field can interact with a nuclear quadripole
moments.

We shall neglect the possibility of interaction by exchange
forces. For electronic states with overlapping wave functions
this exchange is sometimes considerable (ferromagnetism
or anti ferromagnetism). But for the nuclei in crystals &
liquids at room temperature exchange is not likely to occur.

So we come back to a magnetic type of interaction, by
means of the magnetic moment associated with the nuclear
spin. This interaction will, of course, be much smaller than
the corresponding magnetic interaction of electronic moments,
as the nuclear magneton is so much smaller than the Bohr
magneton. So for the magnetic field acting on a nucleus
was taken to consist only of the externally applied field