Multi-pulse technique

Table 4.1. The principal applications of the multi-pulse techniques described in this chapter...

Abstract Experimental and theoretical aspects of using direct multi-pulse techniques... 

Besides various detection mechanisms (e.g. stimulated emission or ionization), there exist moreover numerous possible detection schemes. For example, we may either directly detect the emitted polarization (oc PP, so-called homodyne detection), thus measuring the decay of the electronic coherence via the photon-echo effect, or we may employ a heterodyne detection scheme (oc EP ), thus monitoring the time evolution of the electronic populations In the ground and excited electronic states via resonance Raman and stimulated emission processes. Furthermore, one may use polarization-sensitive detection techniques (transient birefringence and dichroism spectroscopy ), employ frequency-integrated (see, e.g. Ref. 53) or dispersed (see, e.g. Ref. 54) detection of the emission, and use laser fields with definite phase relation. On top of that, there are modern coherent multi-pulse techniques, which combine several of the above mentioned options. For example, phase-locked heterodyne-detected four-pulse photon-echo experiments make it possible to monitor all three time evolutions inherent to the third-order polarization, namely, the electronic coherence decay induced by the pump field, the djmamics of the system occurring after the preparation by the pump, and the electronic coherence decay induced by the probe field. For a theoretical survey of the various spectroscopic detection schemes, see Ref. 10. 

Felix Bloch and Edward Purcell, who jointly received the Physics Nobel Prize in 1952. The development of complex NMR methods has had enormous impact on the development of chemistry, and the technique has become a universal tool for organic, inorganic and biochemists aUke. Of particular relevance to inorganic chemists are multi-pulse techniques and two-dimensional spectroscopy, for which Richard Ernst was awarded a Nobel Prize in 1991. 

Among the double pulse techniques, DDPV is very attractive for the characterization of multi-electron transfer processes. Besides the reduction of undesirable effects, this technique gives well-resolved peak-shaped signals which are much more advantageous for the elucidation of these processes than the sigmoidal voltammograms obtained in Normal Pulse Voltammetry and discussed in Sect. 3.3. 

The electrochemical characterization of multi-electron electrochemical reactions involves the determination of the formal potentials of the different steps, as these indicate the thermodynamic stability of the different oxidation states. For this purpose, subtractive multipulse techniques are very valuable since they combine the advantages of differential pulse techniques and scanning voltammetric ones [6, 19, 45-52]. All these techniques lead to peak-shaped voltammograms, even under steady-state conditions. 

Until MAS, multi-pulse, and Fourier techniques became available on a commercial basis, wideline NMR was the most frequently used method of solid-state NMR spectroscopy. 

To measure high-resolution spectra of disordered samples, the anisotropy of the chemical shift must be eliminated in addition to the homonuclear dipole-dipole interaction. This can be achieved for abundant nuclei by combined rotation and multi-pulse spectroscopy (CRAMPS), which is the combination of homonuclear multi-pulse spectroscopy and MAS [Bur2, Ger2, Sch6]. In Fig. 3.3.13, H spectra of monoethyl fumarate are compared for different NMR techniques [Brol]. Only the combination of MAS and multi-pulse excitation produces a high-resolution spectrum of the polycrystalline powder. 

Fig. 3.3.13 H NMR spectra of CH3CH202CCH=CHC02H at 187MHz for different excitation techniques, (a) Single-pulse excitation of the nonspinning sample, (b) Single-pulse excitation and MAS. (c) Multi-pulse excitation of the nonspinning sample with BR24 sequence, (d) Combination of BR24 and MAS. Adapted from [BrolJ. Copyright 1988 American Chemical Society.

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