24

The mean square local field from term A is [?] by

The mean square deviation in frequency by

To this we have to add the contribution of term B. To
B corresponds the simultaneous flopping of two antiparallel
spins ( or vice versa. This process
is energetically possible & is caused by the proession of the
nuclei around H₀ with the Larmor frequency. So they
produce an oscillating field of resonance frequency at
the position of their neighbors, resulting in reciprocal transitions.
Classically we might say that this process limits the life
time of the spin in a given state & thair therefore [?]
the spectrum. To (2.35) we have to add a numerical
factor to take the effect of B into account. The proper
factor has been calculated by Van Vleck in a rigorous
manner with the diagonal sum method, using formula
(2.32) For H_op we take the terms of type A & B of the
perturbation The component of the magnetic moment,