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

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sequence may be repeated. In practice the PGI was used to measure
the signal amplitude at a fixed time after the second pulse. Once
a constant value was recorded the pulse separation T was changed
and the procedure repeated.
5.8.2 DIPOLAR RELAXATION TIME (TID)
Measurements of T1D (or more accurately Tis) were under-
taken using the 90-T-4590-t'-4590 pulse sequence of Jeener and
(69)
Brockaert. In this sequence two rf pulses 900 out of phase
are used to transfer Zeeman order into observable amounts of dipolar
order. We include below a simple explanation as to the effect
of this pulse sequence on the nuclear spin system.
The first 900 pulse aligns the spins along the y* axis in the
rotating frame. After a time t ti T2 the spins have dephased so
that the fast spins point along the +x* axis direction and the
slow spins along the -x* direction. The second pulse along y*
turns the fast spins along the -z direction and the slow spins
along the +z direction. Once the remaining transverse magnet-
ization has decayed to zero this corresponds to a state of dipolar
order, i. e. an absorption spectrum which is absorptive above vo
and emissive below. Although this explanation is based on two
90° pulses the transfer of order is most efficient when the second
pulse length is reduced to 450. A third pulse, again 45 0, is
used to detect the amount of dipolar order after a time t'.
Experimentally the sequence was set up in the same manner as
described by Tunstall and Brown (72) and will not be reiterated
here. The delay unit described in Section 5.2 was used to step