Pulsed RF excited spectrometer having improved pulse width control

In a method of exciting and detecting resonance of a sample of matter under investigation in a radio frequency spectrometer, the steps of:

• generating and applying an excitation pulse train of radio frequency energy to the said sample of matter to excite resonance thereof, said pulse train including a train of composite pulses, each composite pulse being shaped to cause the nuclei of said sample to respond as if the composite pulse had substantially zero rise and fall times, each said composite pulse having a primary pulse component of radio frequency energy of a first phase, and an associated secondary pulse component of radio frequency energy of a second phase opposite to that of said first phase, said secondary pulse component being shorter than said primary pulse component, and further, said secondary and primary pulse components having substantially identical rise and fall characteristics whereby the excited nuclei of said sample combine the effects of the first and second phases of said composite pulse excitation so that the detectable nuclei free procession following said composite pulse substantially corresponds to the free precession response of the sample nuclei to a theoretical excitation pulse having a zero rise and fall time.