Free induction delay

At the very beginning of our discussion in 1.1.1, we mentioned that any pulse experiment begins with a delay period. This is necessary so that the spins can return to equilibrium before they are excited. After excitation (when the pulse is turned off) we observe the FID, the free induction decay What decays The induced magnetization of the spins, and this process is known as relaxation. It may be slow or fast, as we shall see, and can also occur via a number of processes, which are discussed in detail in the monographs we have recommended for further reading. We will only treat relaxation very briefly here. 

The standardized pulse program for a proton decoupled 13C spectrum is shown in Figure 4.2a. The sequence is relaxation delay (Rd) (see Section 4.2.3), rf pulse (6), and signal acquisition (t2). The proton channel has the decoupler on to remove the H—13C coupling, while a short, powerful rf pulse (of the order of a few microseconds) excites all the 13C nuclei simultaneously. Since the carrier frequency is slightly off resonance FID (free induction decay), for all the 13C frequencies, each 13C nucleus shows a FID, which is an exponentially decaying sine wave. 

It should be noted that when using these sequences there is a dead time between the last pulse and the start of the acquisition (due to instrumental limitations). This can, in some cases, lead to baseline distortions as some of the initial points of the free induction decay (FID) may be lost. Such distortions can effectively be removed by (1) introducing a short delay before the final 180° pulse, (2) incorporating a spin echo at the end of the sequence,25 or (3) spectral processing methods.26... 

Pulse delay Acquisition time Spectral width Data points/spectrum Free Induction Decays (FID s) accumulated ... 

I rf 1 (Free Induction i Delay Pulse Decay) if used... 

FIGURE 5.5. Gated broadband proton decoupling pulse sequence. The 13C pulse is on the order of several microseconds. The acquisition time is from about 0.1 s to 1.0 s the delay period is usually slightly longer. The stippled areas represent the periods during which the broadband proton decoupler is gated on. The acquisition period is represented by the free induction decay (FID). The sequence is pulse, acquisition, and delay periods are not shown in proportion. 

In case the effective laser linewidth is less than the hyperfine splitting(s) excitation will prepare a two-level system. The effect of spin-flips on the coherence in this system will then manifest itself as a 7 ,-type process. No beats are expected in the decay of the optical free induction. With broadband excitation that spans some of the hyperfine splittings spin-flips will be monitored as 7 2-type processes and quantum beats are expected in the photon-echo intensity vs probe delay. Burland et al. also demonstrated the feasibility of optical nutation in this system from which in principle, as from the OFID, the transition dipole could be calculated. 

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