Relaxation phenomena time resolution

As noted above, step-scan FT-IR can provide a better time resolution than PA-IR spectroscopy for time-resolved studies, as well as full spectra at the desired resolution. On the other hand, its major limitation is that the phenomenon under study must be perfectly repeatable-information which often is not available before an experiment is carried out. Another problematic aspect to consider is that sufficient relaxation time must be allocated for the sample to return to its initial state between consecutive perturbations. Unfortunately, this parameter is also often not known a priori before the experiment is performed, and may risk artifacts appearing in the data. In contrast, a single perturbation is required in a PA-IR experiment to record the time-resolved data, eliminating the requirements of repeatability and an a priori knowledge of the relaxation time. PA-IR spectroscopy was used to assess directly the repeatability of the orientation/reorientation cycles for 5CB [27]. Table 13.1 shows the switch-on and switch-off time constants determined individually for a series of 300 consecutive reorientation cycles. As expected for this well-studied LC, the time constants did not evolve systematically as a function of the number of cycles. In this case, however, the repeatability was demonstrated experimentally and not only assumed, as is often necessary in step-scan studies. 

This phenomenon of super-radiance is used in the photon-echo technique for high-resolution spectroscopy to measure population and phase decay times, expressed by the longitudinal and transverse relaxation times Ti and T2, see (7.1). This technique is analogous to the spin-echo method in nuclear magnetic resonance (NMR) [904]. Its basic principle may be understood in a simple model, transferred from NMR to the optical region [905]. 

Enhanced longitudinal and transverse relaxation (PRE) of the nuclear spins surrounding unpaired electrons is often seen as a barrier to resolution and sensitivity solid-state NMR. At the same time, this phenomenon provides a number of advantages for structure determination that have been recognized early in the study of paramagnetic systems. 

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