Multipulse experiments

S. Mukamel In general, multipulse experiments depend on a multitime correlation function of the dipole operator [1], The term x(n) depends on a combination of n + 1 time correlation functions. Their behavior for large n will depend on the model. In some cases (e.g., the accumulated photon echo used by Wiersma) the multiple-pulse sequence is simply used to accumulate a large signal and the higher... 

ITg. 12. Multipulse experiments with Pt/Si()2 and Pt/( oO,/Si02- (X)2 signal intensity (a) CO-precovered Pt catalyst at 327 K during O2 pulses (b) a CO-precovered Pt-CoO., catalyst at 313 K during Oj pulses [reproduced with permission from Mergler et al. (54)]. 

Since the bed is mostly covered with CO, one would expect that there would be no O left on the surface after the first peak of Fig. 36. Therefore, where does the oxygen come from to form the second peak at 1 s Apparently O and CO can coexist on the surface one explanation is that they may exist as separate islands, as most recently proposed by Hoebink et al. (214). Note that all the action for Fig. 36 occurs at the bed entrance downstream the surface is saturated with CO. The model (213) is supported by some experiments involving 02 and by a simulation of the results by a model based on the previous discussion and the classical sequence of steps. Multipulse experiments of CO over an 0-covered surface and of O2 over a CO-covered surface have also been performed. It would be interesting to know the effect of pulsing frequency on the results. No additional... 

TAP multipulse experiments have been done over polycrystaUine Pt at 72°C O2 is pulsed over a surface initially covered with CO (214). Figure 37 shows the usual induction period and then two well-separated peaks of CO2. Nijhuis et al. (213) did not find a double peak, but their pulse size was 20 times larger. It is difficult to explain Fig. 37, but a segregation of CO and O into islands on the surface has been used in a comphcated model to try to reproduce some of the results (214). However, the model was tested only on single-pulse data. In fact, the attractive feature of the TAP reactor is the use of single pulses, producing results that can be compared with those of models involving the kinetics of rapid elementary steps (35). Multipulse operation seems to be treated as a kind of titration, and no kinetics are available from the ensemble of the pulses. Indeed, the series of maxima seems to be taken to be the development of concentration with time (213, 215), or in some cases the total accumulated amounts are... 

Figure 2.20. The periods of a WHH4 multipulse experiment for narrowing homonuclear dipolar...

Conformational motions have been studied." Other line shape experiments include 13c 481,482 P in freeze-dried liposomes,DNA," " cytochrome" and spectra of lipids. The complex dynamics in lipid bilayers was studied by H-2D exchange." Dynamic decoupling found its application" as well as multipulse experiments." SLF experiments like LG-CP," DIPSHIFT" " and determine the amplitudes of fast... 

However, for nuclei that display a much greater frequency dispersion, such as C or F, this is often not the case, and resonance distortion and/or attenuation can occur, and spurious signals may arise in multipulse experiments as a result. One approach to overcoming these limitations is the use of clusters of pulses known as composite-pulses which aim to compensate for these (and other) defects see Chapter 9. 

Probe tuning is necessary for a number of reasons. Other than the fundamental requirement for maximising sensitivity, it ensures pulse-widths can be kept short which in turn reduces off-resonance effects and minimises the power required for broadband decoupling. A properly tuned probe is also required if previously calibrated pulse-widths are to be reproducible, an essential feature for the successful execution of multipulse experiments. 

Three different pulse techniques were used in this study a) single pulse, b) multipulse experiments, where a series of pulses is introduced, and c) pump-probe experiments, where two different pulses are alternately introduced at a user-specified time interval At. All single pulse and pump-probe experiments consisted of 40 pulse cycles of 3 seconds duration. Directly before the measurement 5 initial precycles were given. The responses of the 40 cycles were averaged to improve the signal/noise ratio. 

Figure 1. N2, NO and N2O responses during a NO multipulse experiment on a prereduced catalyst at 473 K.

Fig. 4 Maleic anhydride production at 437 C in TAP reactor multipulse experiments on PVO using...

Equation 22 is important and useful for determining the state-defining regime for a multipulse experiment. 

Computer developments. Increased computer capabilities have also greatly contributed to new applications. The modern NMR computer system and software can control complicated multipulse experiments where many factors such as delay times, decoupler levels, pulse widths or rf phase may be varied systematically. In fact, virtually all spectrometer functions are now set under computer control. 

MHz frequency as shown. A proton spectrum occurs over a chemical shift range of 10 ppm, which corresponds to 2.5 kHz at 500 MHz. As seen in Figure 3.22, all of the protons in the sample would see 98%-100% of the power of the 500 MHz radiation delivered and all would be excited simultaneously. A pulse programmer is used to control the timing and shape of the RF pulses used to excite the sample. Square wave pulses are commonly used, but multipulse experiments and 2D NMR experiments with other pulse shapes are performed. There are hundreds of pulse sequences and 2D experiments that have been developed, with curious names like attached proton test (APT), DEPT, INEPT, INADEQUATE, COSY, and many more, some of which will be discussed later in the chapter. Each pulse sequence provides specific and unique NMR responses that enable the analyst to sort out the NMR spectrum and deduce the chemical structure of a molecule. 

In the three-zone TAP reactor, it is difficult to maintain a uniform profile of the catalyst surface composition because of the gas concentration gradient, which is the driving force for the diffusion transport in the reactor. This nonuniformity of the catalyst surface composition becomes significant particularly in multipulse experiments. 

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