13

If I=1/2, is the difference between the number
of spins which are parallel or antiparallel to H₀. Since
n is even or odd, depending on whether the total number
is even or odd, the spacing between the
equidistant levels is γħH₀. The degeneracy of the level
in high , but is lifted by the interaction [term?] in
(2.24). Now the perturbation energy will be of the N order
. Although , the perturbation energy
for an assembly of many spins (N ≈ [1₀?]²²) will be
large compared with spacing of the unperturbed levels.
In view of the large number of non-degenerate levels
we can say that the energy levels belonging to (2.24)
are spread over a continuum. The density D of the
energy levels as a fⁿ of the energy has a sharp maximum,
the width being of the order N^1/2γħH₀. The average
occupation ρ of the coninuum is obtained by multiplying
by the Boltzmann factor exp(-H_0pkT). The regular spacing
γħH₀ has completely vanished from the picture. It seems
that as if the whole resonance phenomenon disappeared, for we
might expect transitions from any level p to to any level q in
the continuous distribution, when a rotating magnetic field
H₁ is applied with frequency ν = ([E_p - E_q?])/h. The probability
for such a field process per unit time is