Free induction decay - Wikipedia, the free encyclopedia: "In Fourier Transform NMR, a free induction decay (FID) is the observable NMR signal generated by non-equilibrium nuclear spin magnetisation precessing about the magnetic field (conventionally along z). This non-equilibrium magnetisation is generally created by applying a pulse of resonant radio-frequency close to the Larmor frequency of the nuclear spins.
If the magnetisation vector has a non-zero component in the xy plane, then the precessing magnetisation will induce a corresponding oscillating voltage in a detection coil surrounding the sample. This time-domain signal is typically digitised and then Fourier transformed in order to obtain a frequency spectrum of the NMR signal i.e. the NMR spectrum.
The duration of the NMR signal is ultimately limited by T2 relaxation, but mutual interference of the different NMR frequencies present also cause the signal to be damped more quickly. When NMR frequencies are well-resolved, as is typically the case in the NMR of dissolved samples (solution-state NMR), the overall decay of the FID is relaxation-limited and the FID is approximately exponential (with a time constant T2 or more accurately T2*). FID durations will then be of the order of seconds for nuclei such as 1H. If NMR lineshapes are not relaxation-limited (as is commonly the case in solid-state NMR), then the NMR will generally decay much more quickly e.g. microseco"
Thursday, May 10, 2007
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