Free induction decay duration

Figure 1.26 Free induction decay and corresponding frequency-domain signals after Fourier transformations, (a) Short-duration FIDs result in broader peaks in the frequency domain, (b) Long-duration FIDs yield sharp signals in the frequency domain.

Fig. 3.4.1 Schematic description of the three-dimensional SPI technique. Gz, Cx and Gy are the phase encode magnetic field gradients and are amplitude cycled to locate each /(-space point. A single data point is acquired at a fixed encoding time tp after the rf excitation pulse from the free induction decay (FID). TR is the time between excitation (rf) pulses. Notice that the phase encode magnetic field gradients are turned on for the duration of the /(-space point acquisition.

Signal loss in the free induction decay (FID) can be partially refocused using a second application of with a duration and amplitude that produces an 180° flip angle of M (180° pulse). This has the effect of placing individual magnetic moments, which are no longer in phase with the central frequency, in a position to realign with spins at the central frequency. 

Figure 11 shows diagrammatically (Muller and Bloom, 1960) the arrangement of echoes and pulses which occur in a general type of three-pulse spin-echo experiment. In this diagram a pulse of duration 0 to t0 rotates the magnetization by the angle nj2. This pulse is followed by the free induction decay with time constant T determined by both extrinsic and intrinsic relaxation forces. At a time iq (iq > 11 t0) a pulse... 

The fact that the duration of transient signals after SSFP type sequences can exceed T (Tj is a time constant of the free induction decay) can be used to eliminate spurious signals. The SSS can replace conventional pulses in the multiple-pulse sequences with intervals between pulses longer than T2 while detecting explosives at frequencies higher than 3 MHz (RDX, HMX, tetryl), where the duration of... 

Selective excitation was accomplished by using a delays alternating with nutations for tailored excitation (DANTE) sequence of 20 short rf pulses of x =0.6 is duration [4]. The pulse separation used was 20 ps resulting in the total duration of the DANTE sequence of x,=400 [iS and the corresponding spectral excitation width A of the center band of 2.5 kHz. The excitation sidebands were separated from the centerband by (Xp) = 50 kHz and did not affect the NMR spectrum. After a recovery period,x of at least 35 ps a final 90° pulse was applied followed by the detection of the free induction decay. 

Studying moderately thin samples, the molecular time constants can be directly obtained from the experimental data without the need of computer simulations. This situation is termed nearly-free induction decay (NFID) which is related to the case of free induction decay in very thin samples. Some numerical results are depicted in Fig. 1. A specific experimental situation is considered with resonant, weak input pulses of Gaussian shape and duration tp (FWHM) and two values of the normalized propagation length, ai 0.2 and aJl 1. Here a denotes the conventional absorption coefficient at the maximum of the absorption band. The intensity of the transmitted pulse is evaluated from Maxwell-Bloch equations for homogeneous line broadening. 

In modem instruments called pulsed Fourier transform (FT) spectrometers, the magnetic field is held constant and an rf pulse of short duration excites all the protons simultaneously. The rf pulse covers a range of frequencies, so each nucleus can absorb the frequency it requires to come into resonance (flip its spin) and produce a signal—called a free induction decay (FID)—at a frequency corresponding to A . The intensity of the FID signal decays as the nuclei lose the energy they gained from the rf pulse. 

---