Modulation amplitude change

Figure 6. A schematic representation of a modulated excitation wave form (solid line) and the time delayed modulated emission waveform (dashed line). The parameters Atp and AA represent the change in phase and the change in modulation amplitude of the two wave forms.

Illustration of modulation of two pulse electron spin echo decay envelope. Microwave pulses 1 and 2 separated by the time x produce the echo signal at time x after pulse 2. As x is increased the echo amplitude changes and traces out an echo envelope which may be modulated. 

Researchers have reported that, for impedance measurements on human skin under fixed-amplitude galvanostatic modulation, significant changes in skin properties were observed that could be attributed to the impedance measurement. The magnitude of the skin impedance varied from about 10 Ocm at high frequency to 100 fcOcm at low frequency. The perturbation amplitude was 0.1 mA on an exposed skin sample of 1 cm area. Explain the reasons for their observation and suggest an improved experimental protocol. 

Figure 5.16 Force modulation. The tip is kept in contact with the sample. Differences in local properties of the sample will be reflected in the amplitude change of the cantilever. (Reproduced with permission from P.C. Braga and D. Ricci (eds), Atomic Force Microscopy, Humana Press. 2004 Humana Press.)...

When the amplitude of modulation is small, i.e., A(B/B/ < /s, see Fig. 7 (i)(b), the time dependent change in the resistance SR under photoexcitation at frequency / shown in Fig. 7 (i)(a), reflects mostly the time variation of the magnetic field within a phase factor. This situation changes dramatically, however, when the modulation amplitude matches the period of the radiation induced resistance oscillations, see Fig. 6(c), and Fig. 7(ii)(a) and (b). Here, in Fig. 7(ii)(a), the time response of the specimen, i.e., Sl (t), exhibits a strong harmonic component, which is evident both in the Fourier transform (inset. Fig. 7(ii)) and the harmonic band-pass filtered portion of Si (t) (see Fig. 7(ii)(a)). A further increase in the modulation amplitude such that it corresponds to two periods of the radiation induced resistance oscillations (Figs. 6(d) and 7(iii)), leads to the disappearance of the 3 harmonic component, as a 5 harmonic component takes its place, see inset Fig. 7(iii). 

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). 

Perdeutero nitroxides will yield lines that are about three times narrower than protonated nitroxides, which, assuming optimum field modulation amplitude is employed, will increase the signal by about three times. Because the lineshape may change from mostly Gaussian to mostly Lorentzian in character, this procedure does not always yield the expected improvement. 

Fig. 15 (78). EPR spectra of bovine erythrocuprein (0.15 mM) at different pH values. 0 pH 6.7 pH 11.8 readjusted to pH 8.0. From pH 2—8 no measurable changes were observed. Spectra were recorded at 77 °K. Microwave frequency 9.169 GHz modulation amplitude 2G microwave power 5 mW...

The structural role of iron to be discussed here is, in general, somewhat broader than that already described in the literature for Ca2+, Mg2+, and the light alkali metal ions (9—11). These cations appear to function mainly as effectors of enzyme activity, either by modulating conformational changes of small amplitude that regulate the affinity of the substrate for the active site or, more directly, by bridging the substrate to the enzyme. These proteins possibly are already in a native state, even in the absence of the metal ion. By contrast, and with the possible exception of Mn2+, in most cases heavy metal ions are necessary constituents to hold the structure of the metalloprotein in a conformation that is close, if not identical, to that of the active species. This role is in addition to whatever action the heavy ion may play in the catalytic event itself. 

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. 

Fig. 16. Changes in the EPR spectra recorded during the reaction of NO with the Mn(III) form of 1 (200pM) (T=25°C, 50mM phosphate buffer at pH 7.4). A synthetic NO donor (DEA-NONOate) was used as a source of NO (leq=100pM). Instrument settings microwave frequency 9.51GHz power lOmW modulation amplitude 2G gain 2x10. Taken from Eilipovic et al. (43).

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