Amplitude Modulation effect

Modulation Amplitude Effects On Signal Amplitude And Shape... 

More sophisticated pulse sequences have been developed to detect nuclear modulation effects. With a five-pulse sequence it is theoretically possible to obtain modulation amplitudes up to eight times greater than in a tlnee-pulse experunent, while at the same time the umnodulated component of the echo is kept close to zero. A four-pulse ESEEM experiment has been devised to greatly improve the resolution of sum-peak spectra. 

Fig. 5. Effective g assignment of the low-field S = IEPR signals in D. vulgaris Fepr protein [from 11)]. The spectrum was recorded at the optimEd temperature of 12 K, that is, at which the amplitude is maximal and lifetime broadening is not significEmt. EPR conditions microwave frequency, 9.33 GHz microwave power, 80 mW modulation amplitude, 0.8 mT.

Kitamaru, R., Horii, F., Hyon, S.-H. ACS Polym. Prepr. 17 (2), 549 (1976) revised results are used in this review, considering the effect of modulation amplitude... 

The line-shape of a true (undistorted) EPR spectrum should be independent of the acquisition parameters, and therefore to assess spectral distortion one can compare spectra acquired with different parameters. Figure 15.6 illustrates the effect of modulation amplitude on EPR line-shape. The central line-width (peak-to-peak width AHpp = 1.6 G) remains unchanged when the modulation amplitude is increased from 0.5 to 1 G while at a modulation amplitude of 10 G, distortion and line-broadening (AHpp = 6.4 G) can be clearly observed. The main sources of spectral distortions are modulation amplitude, microwave power, and scanning rate (speed). These are discussed in the following sections. 

Figure 15.6 Effects of modulation amplitude on cw-EPR line-shape. Spectra shown were obtained on a Bruker EMX spectrometer equipped with a high-sensitive cavity. The sample was an aqueous solution of tempol (4-hydroxy-2,2,6,6-tetramethylpiperidine-l-oxyl) (12.5 fiM, 5 fil) placed in a round glass capillary (0.6 mm ID, 0.8 mm OD Vitrocom, Inc., Mountain Lakes, NJ) sealed at one end. Acquisition parameters listed in Table 15.1 were used, except that the modulation amplitude (M.A.) was varied as respectively indicated for each spectrum. Note that these spectra have not been normalized, and differences in the amplitude of the spectrum are due to the different modulation amplitudes used.

The first term in curly brackets does not depend on t and represents the usual Hanle effect. The second term oscillates at modulation frequency fii, with the modulation amplitude growing in resonance fashion if Qojj> = fii holds (see Fig. 4.22). For u>j> T, when degeneracy is completely removed, we may write in the vicinity of resonance for Q > 0 ... 

One may include the effect of the modulation amplitude on the minimum detectable number of spins, for modulation amplitudes less than the linewidth, by multiplying the expression for by a factor (Poole, 1967)... 

Kazanskii, Pariiskii, and Voevodskii consider that the line arises from a bulk center because in their experiments it was unaffected by 2 (72), while Kohn, who observed the line to be destroyed by O2, assumes that it comes from a surface defect (72). Haul, Karra, and Turkevich (73) observed effects of O2 upon the spectrum near g = 2 but confirmed that the line at g = 2.0005 was little affected. These authors also showed that rather large changes in the character of the spectrum can be produced by varying the microwave power and modulation amplitude. Such effects might explain a disappearance of the line at g = 2.0005 in a particular experiment. Furthermore, if the line observed in one experiment came from a bulk F center and the one in the other from a surface counterpart, one would not expect so close an agreement in frequency (g = 2.0006 0.0004 vs 2.0005 0.0005). 

FIGURE 4.14 The effect of modulation amplitude on the recrystallization behavior of linear polyethylene. (From Scherrenberg, R., Mathot, V., and Van Hemelrijck, A., Thermochim. Acta, 330, 3, 1999. With permission.)... 

Most of the studies of methane oxidation in the literature utilize simplified feedstreams and fresh catalyst samples, generally in the form of powders or pellets. A recent paper [22] used a laboratory simulated NG vehicle exhaust to study the removal of methane, NO and CO using a Pd-only monolith catalyst. They found that optimum conversion of all three constituents occurred slightly rich of stoichiometry. These results appear to have been obtained over fresh catalyst samples. The present work utilizes monolith catalysts and laboratory simulated NG vehicle exhaust to study the effect of catalyst loading and space velocity, Ce02 addition and variations in hydrocarbon composition. The effect of modulation amplitude and frequency aroxmd the stoichiometric point was also... 

The effect of varying feedstream modulation amplitude and frequency on NG conversions was tested using aged 3.5g/L Pd on the proprietary base-metal washcoat, using two sets of A/F sweep tests at 500°C. During the first set of A/F sweeps, the frequency was held constant at 0.5 ffz, while the amplitude was varied from 0.25 to 1 A/F units. A second set of A/F sweep tests was run where the frequency of modulation of the feedstream was varied from 0.25 to 1 Hz at a constant amplitude of +/- 0.5 A/F units. 

Figure 5a illustrates the effect of changing amplitude at constant frequency on conversions over Pd/ base metal catalyst. Near the stoichiometric point (lambda = 0.995), decreasing the modulation amplitude from +/- 1 A/F ratio to 0.25 A/F ratio increases CH4 activity by a factor of three. CO and NO conversions are also improved by the decreasing amplitude. 

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