Free induction decay signal origin

When the pulse is switched off, the excited nuclei return slowly to their original undisturbed state, giving up the energy they had acquired by excitation. This process is known as relaxation. The detector is switched on in order to record the decreasing signal in the form of the FID (free induction decay). You can observe the FID on the spectrometer s computer monitor, but although it actually contains all the information about the NMR spectrum we wish to obtain, it appears completely unintelligible as it contains this information as a function of time, whereas we need it as a function of frequency. 

The signal (FID, free induction decay) resulting from an NMR experiment contains the original data which are stored in the computer, and after the Fourier transformation (FT) we obtain the NMR spectrum itself. 

When the pulse is discontinued, the excited nuclei begin to lose their excitation energy and return to their original spin state, or relax. As each excited nucleus relaxes, it emits electromagnetic radiation. Since the molecule contains many different nuclei, many different frequencies of electromagnetic radiation are emitted simultaneously. This enaission is called a free-induction decay (FID) signal (Fig. 3.15). Notice that the intensity of the FID decays with time as aU of the nuclei evenmaUy lose their excitation. The FID is a superimposed combination of all the frequencies emitted and can be quite complex. We usually extract the individual frequencies due to different nuclei by using a computer and a mathematical method called a Fourier transform (FT) analysis, which is described later in this section. 

Figure 9. Apodizations of chi experimental FT-NMR signal. (A) Fourier transform of the original unweighted free induction decay (F.I.D.) time-domain response to a 90 -pulse excitation. (B) Signal-to-noise enhancement F.I.D. weighted by the factor, exp(- r>LB-t), with LB = 3.0 Hz, before F.T.

Both measurements show a fast rising beat structure at negative delaytimes. The origin of this signal results from optical coherence generated by the linearly polarized probe pulse and corresponds to a first-order Free-Induction-Decay. An interpretation of this signal is found in ref. [2]. 

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