Free Induction Decay (FID)
Definition and meaning of Free Induction Decay (FID) in chemistry.
Free induction decay (FID) is the time-domain signal detected in nuclear magnetic resonance (NMR) spectroscopy as the net transverse nuclear magnetization precesses and decays back toward equilibrium after a radiofrequency (RF) pulse.
In more detail
Immediately after an RF pulse tips the bulk nuclear magnetization into the plane transverse to the applied magnetic field, individual spins precess at their Larmor frequencies but gradually lose phase coherence through spin-spin (T2) relaxation and magnetic field inhomogeneities, so the detected coil signal decays exponentially while oscillating. Because this raw waveform contains information from all resonating nuclei at once, a computer applies a Fourier transform to convert it from the time domain into the familiar frequency-domain spectrum. This principle underlies modern pulsed Fourier-transform NMR (FT-NMR), which is far faster and more sensitive than older continuous-wave scanning methods.
Key facts
| Field | Analytical Chemistry |
|---|---|
| Signal domain | Time domain (decaying, oscillating waveform) |
| Governed by | T2 (spin-spin) relaxation and field inhomogeneity |
| Converted to spectrum via | Fourier transform |
In proton NMR of ethanol, a single 90-degree RF pulse produces an FID that oscillates and decays over milliseconds to seconds; Fourier transforming this FID yields the familiar spectrum showing separate peaks for the CH3, CH2, and OH protons.
Frequently asked questions
Why does the FID signal decay over time?
Individual nuclear spins precess at slightly different frequencies and lose phase coherence (dephase) due to T2 relaxation and local magnetic field inhomogeneities, so the net transverse magnetization shrinks toward zero.
Why is the FID important for modern NMR?
It lets an entire spectrum be excited and recorded from one short RF pulse instead of scanning frequencies sequentially, making Fourier-transform NMR much faster and more sensitive than older continuous-wave instruments.