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

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samples by direct observation of the spinning beats on the FID.
5.7 FITTING THE DEAD-TIME POINTS
Before computation it was necessary to define that portion
of the transient nuclear signal that was lost during the intrumental
dead time. The dead time after the
.
rf pulse was typically 10 ps
for the Bruker probe system, but slightly longer for the single
coil circuits. The characteristic FIDs from the spinning samples
were long, so of the full decay shapes recorded in the store of
the signal averager only the first one or two data points were lost.
These could be estimated fairly accurately from the subsequent
behaviour of the FID. The dead time points of the signals recorded
off resonance were estimated by extrapolating the beats of the
detected decay shapes back to the centre of the rf pulse. The
phase of the first data point of the correct full decay signal was
then set empirically so as to give a flat baseline on the trans-
formed lineshape.
The attainment of reliable second moment values for the alumin-
ium samples depended upon the accurate prediction of the form of
the FID near the time origin. To assist in this task the behav-
iour of the transverse magnetization near zero time was investigated
using the solid echo pulse sequence and a three pulse sequence
b Jeener and Brockaert(69).
proposed Y The procedure adopted consist-
ed of plotting out the first few detected points of the FIDs: those
points lost were then computed so that the initial part of the
decay curve mirrored the decay envelopes obtained from the two pulse
sequences as described below.