Pulse

When a burst, or pulse of rf is sent into the NMR probe, the effect on the nuclei of the sample depends on the power level, duration, and direction of the pulse. It is not easy to vary the pulse power level, so, in practice, the needed variations are generated by changing the pulse length (or pulse width) and phase. The magnetic moment associated with an excitable nucleus is aligned with the external magnetic field, B₀, along the +z axis. After the pulse, the moment tips away from this axis, down toward the x-y plane. The angle between this new position and the +z axis is called the tip angle or the pulse angle. The pulse width and tip angle are directly proportional. If a 3 msec pulse moves the magnetic moment through 45^o, a 6 msec pulse will tip it 90^o, onto the x-y plane. the pulse phase determines the direction in which the magnetic moment moves. If a pulse is sent in on the -y axis, a 90o pulse will place the moment along +x; a +x pulse tips the moment onto +y.