Free induction decay

From NMR Wiki

In magnetic resonance, free induction decay (or FID) is the temporal profile of induction of sample placed in the magnetic field in the absence of external radio-frequency (in NMR or microwave frequency in EPR) irradiation after some irradiation has been applied and then interrupted.

Also, spectrocopists often refer to FID as a recording of the actual FID signal sampled in time. FID is almost never sampled directly as larmor precession frequecies of nuclear and electron are too high to be sampled that way.

Oscillations in the recorded FID's occur at frequencies equal the difference between the larmor frequency and the radio- (or microwave) frequencies of the applied pulses.