Modulation cycles

Electrodeposition of composition-modulated films was first performed by Brenner in 1939 (1) by employing two separate baths for the two components and a periodic immersion of the deposit in the two baths. This is too cumbersome a method to be adopted in practice. Deposition from a single bath with the presence of salts of the two components of the multilayer is what is desired, but there was a serious problem with the deposition of two metals from one bath. Specifically, whereas a layer of the more noble member can be deposited by choosing the potential to be between the reduction potentials of the two metals, one can expect that when the potential is set to a value appropriate for reduction of the less noble member, both will be deposited, resulting in an alloy layer rather than a pure metal. Thus, to nobody s surprise, even as recently as 1983, Cohen et al. (2), were able to deposit only a layered structure of alloys rather than pure metals. In addition they cast doubt on the possibility that a modulation cycle (the thickness of the basic layers, the periodic repetition of which... 

The actual deposition of the multilayered composite can be carried out by either current or potential control. Clearly, a pulsed polarization curve has to be constructed for the former case. The actual composition modulation cycle would be controlled coulometrically by fixing the amount of electric charge delivered while at point A in Figure 17.2 Qp and the amount of charge delivered while at point B in Figure 17.2 (2b) via suitable input to the unit regulating the pulsing. 

The measurement of vibrational optical activity requires the optimization of signal quality, since the experimental intensities are between three and six orders of magnitude smaller than the parent IR absorption or Raman scattering intensities. To date all successful measurements have employed the principles of modulation spectroscopy so as to overcome short-term instabilities and noise and thereby to measure VOA intensities accurately. In this approach, the polarization of the incident radiation is modulated between left and tight circular states and the difference intensity, averaged over many modulation cycles, is retained. In spite of this common basis, there are major differences in measurement technique and instrumentation between VCD and ROA consequently, the basic experimental methodology of these two techniques will be described separately. 

Fig. 15.4 A dual-jet cryogenic modulator, a Right-hand-side jet traps analytes eluted from the first column b right-hand-side jet switched off, cold spot heats up rapidly and analyte pulse is released into the second column simultaneously, left-hand-side jet switched on to prevent leakage of first-column material c next modulation cycle is started (adapted from [32])...

In addition, at that time, they cast doubt on the possibility that a modulation cycle (the thickness of the basic layers, the periodic repetition of which makes up the multilayer system) of less than 1000 A can be achieved by means of electrodeposition. 

In these WMS signals, the wavelength crosses the water absorption feature twice for each modulation cycle. Therefore, when water vapor is in the optical path, the amplitude modulation of the received laser power contains a periodic component having precisely twice the wavelength modulation frequency and fixed in phase relative to the wavelength modulation. 

Figure 12.2c shows the temporal variation of the instantaneous frequencies for the two modes. It is interesting to observe how the frequency of the fast mode is modulated in a fairly regular manner. With about 17 modulation cycles for fjast during the 500 s of observation time, we conclude that the frequency of the fast mode is modulated by the presence of the slow mode, indicating that the two modes interact with one another. If one compares the phase of the tubular pressure variations in Fig. 12.2a with the phase of the frequency modulation in Fig. 12.2c it appears that the maximum of ffast occurs about 60° after the maximum of Pt. It is important to note, however, that the various steps of our wavelet analysis may have introduced a certain phase lag. We are presently trying to correct for such effects in order to obtain a better understanding of the instantaneous relation between the two variables. 

The first successful ROA measurements were achieved with the use of an electro-optic modulator (EOM) operating in a square-wave modulation cycle. 

The most recent research efforts in FDCD have been concerned with developing analytical methods that are sensitive to fluorescent chiral systems. No doubt this will continue to be a driving force for experimental and conceptual advances in this field. One of the most interesting aspects of the technical improvements described in reference [21] for the measurement of time-resolved CPL is the fact that, by employing randomly spaced (in time) excitation pulses, the time scale for measurement has been decoupled from the 50 kHz PEM modulation cycle. In principle, this same approach could be used in FDCD measurements. Thus, it should be possible, for example, to measure time-resolved FDCD from fairly long-lived chromophores such as lanthanide (III) ions. 

Phase correction — synchronize the columns of data points with the modulation cycles. 

If the required phase correction is not an integer, two options are possible (1) round the phase correction to the nearest integer pixel index and accept a timing error of not more than one-half of the sample interval or (2) resample the data so that the resample point is precisely at the start of the modulation cycle. The first option typically is preferred because it maintains the original data, without introducing resampling errors, and is computationally simpler. 

In GCxGC data, the baseline usually can be observed at many points, for example, during the void time of each second-column separation, even if other regions of the data are crowded with peaks. This is an important attribute of GC xGC for accurate quantification because if the baseline cannot be estimated, then peak integration is less accurate. Typically, the baseline does not change significantly over the brief time of a few modulation cycles, so these observations are sufficient to reconstruct the baseline in a comprehensive fashion. 

Wraparound occurs when the retention time of a compound in the D column exceeds the modulation period. It can be identified in the 2D profile by the elution of the affected compound during the subsequent modulation cycle(s). Figure 9 shows the GCxGC-pECD contour plot obtained on ZB-5 x HT-8 for a... 

The saturation curves must be recorded under slow passage conditions, that is, conditions such that the time between successive field modulation cycles is sufficiently long for each spin packet to relax between cycles. The spin system is then continually in thermal equilibrium and the true line shape is observed. A convenient formulation of the slow passage condition is given by the expression (9.16) [79] ... 

The reason here is that the l N nucleus has a small magnetic moment, which is hard to drive by r.f. fields, the difficulty being compounded in the case of frozen solution samples by line broadening due to the l N nuclear quadrupolar coupling. Echo envelope modulation effects are, on the other hand, quite easy to see for nuclei with small moments. For the simple case of an I = 1/2 nucleus weakly coupled to an S = 1/2 electron, it can be shown that the modulation depth is independent of the nuclear moment and depends only on the ratio between the Zeeman field and the local field at the nucleus due to the electron.15 Breadth of the shfs line is, moreover, not a serious obstacle to detection, provided that at least one modulation cycle can be seen in the echo envelope. 

Next, a constant, irreversible thermal process with a latent heat is added to the modulation cycles, as is found on cold crystallization of PET (see Figs. 4.74 and 4.136-139). A latent heat does not change the temperatures of Fig. A. 13.1, so that the heat-flow rates need to be modified, as is shown in the upper graph of Fig. A. 13.2. The constant latent heat is indicated by the vertically shaded blocks and is chosen to compensate the effect of the underlying heating rate, so that the level of Ps is moved to zero. The reversing specific heat capacity is given by ... 

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