If a spin system has come to equilibrium in a fixed magnetic field B, how does one push the system away from equilibrium?
The approach employed in modern instruments is to apply a small magnetic field, Bex, perpendicular to the primary static magnetic field B. This additional magnetic field is very weak: Bex << B. Given the weakness of Bex, one might wonder how it can make any difference at all.
In almost all circumstances, the effect of the very small Bex would be a very slight wobble in the bulk magnetization, too little to be useful. But there is one circumstance in which Bex can have a profound impact. The trick is to make Bex an oscillating magnetic field and to make it oscillate at the Larmor frequency of the sample nuclei. If Bex rotates at the same frequency as M precesses, then the influence of Bex builds over time. The result is that M will precess about Bex in the rotating frame.
In addition to detector coils along the x and y axes, NMR spectrometers also have RF coils along these axes to generate Bex for the purpose of manipulating the orientation of M. The RF coils are only turned on for very brief periods of time (usually < 50 μsec). For the vast majority of time during an experiment, Bex is zero and precession is solely about the z axis and B.
The most basic NMR experiment is called a single-pulse experiment. The experiment begins with the system at equilibrium and thus M is oriented along the z axis. The first step is a 90ox pulse, which means an excitation field Bex is applied along the x axis by one of the RF coils. The bulk magnetization precesses about the x axis in response to Bex. Instead of characterizing this precession by a frequency, the time required for M to rotate through a particular angle is used. For a 90o pulse, Bex is applied just long enough for M to rotate 90o. Thus the effect of the 90ox pulse is to rotate M from the z axis to the y axis.
After the pulse has been applied, the detection coils are monitored and the FID is acquired. Notice that detection is solely in the xy plane, because that is the plane containing the detection coils. Two detection coils are used in tandem to provide phase-sensitive detection, known as quadrature detection, which allows one to distinguish between positive and negative frequencies. That is, one can tell which direction M is precessing.
The net pulse sequence is often written: 90ox - FID
The simulation below begins with the system at equilibrium and allows one to step through the individual stages of the single-pulse experiment.
Be aware that the time scale of the experiment is different during the pulse and FID acquisition stages. In both cases the motion of the bulk magnetization has been slowed drastically to permit visualization. Look at the graph to see the relative timings. In particular, note how the duration of the 90o pulse is MUCH shorter than the duration of the FID.
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