Pulse, length adjustment

The observed CT-VPP-REDOR data confirms the expected general behaviour and can be best simulated assuming a second moment of 5.8 X lO rad s. Interestingly, in CT-VPDP-REDOR, this oscillatory behaviour can be observed even at low maximum AS / So values, which can be adjusted via the 0-pulse length. This is shown in Figure 12, in which the slices taken from a CT-VPDP-REDOR experiment on the glycine sample for five different 6 pulse lengths are collected. 

The switching time for the three-way valve is At = 20 ms and the pulse length is adjustable by a commercial electronic pulse generator. The pulse length can thus be varied from 50 ms to 10 s. A sequence of pulses can also be delivered with... 

The 27A1 nutation spectra were measured at 104.26 MHz using a Bruker MSL-400 multinuclear NMR spectrometer with a high-power static probe-head and a 5 mm diameter horizontal solenoidal coil. Using an aluminium nitrate solution the amplitude of the rf pulse (corf/27c) was adjusted and kept at a constant value of 70 5 kHz unless otherwise stated. Nutation experiments were performed on the same amount of each sample and the same number of transients was accumulated for samples in the same series. The rf pulse length was increased in ln.s increments from 2 M-s to 65 is. The spectral width was 125 kHz, the recycle delay 0.2 s and the number of transients accumulated in each measurement was between 2000 to 4000. 

Fig. 3.26 Plots of the magnetization produced by a pulse as a function of the offset. The pulse length has been adjusted so that on resonance the flip angle is 90°. The horizontal axes of the plots is the offset expressed as a ratio of the RF field strength, a>i the equilibrium magnetization has been assumed to be of size 1.

A shaped pulse must fulfil two main criteria it must be selective and it must generate the required tilt angle e.g. 90° or 180°. The selectivity of a shaped pulse, which is related to the excitation range, is inversely proportional to the pulse length. Selectivity also depends upon the shape of the pulse. Unlike hard pulses, the pulse length and the pulse shape are pre-determined by the desired excitation profile. Thus, the pulse power must be adjusted to the desired title angle of the selective pulse. The relationship between these parameters is complex but may be broken down into three steps and examined using either a spectral representation or the tools of the Bloch module of NMR-SIM. 

In our previous study, we calculated one-dimensional wave packet dynamics for the prolate-oblate hg( 1) mode and reported that vibrational energy deposited into hg(l) can be controlled by adjusting the pulse length Tj, [24, 45, 46]. Large vibrational... 

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