Dipole loss peak

Figure 11 a illustrates the frequency dependence of e for Eq. (3-6). Note that e is midway between eu and er when co = l/id. The corresponding plots for s" are more complex, because one must assess the relative contributions of a and the dipole loss. The simplest case is for cr = 0 (Fig. lib), where the characteristic dipolar loss peak of amplitude (sr — eu)/2 is observed at frequency co = l/td. For non-zero ct, however, the 1/co dependence of e" greatly distorts the e" curve from the ideal Debye peak. Log-log scales are helpful, as illustrated in Fig. 12. The ct = 0 case is replotted from Fig. lib also plotted are the frequency dependences of e" for CTTd/Eo having various values relative to er — eu. Asct increases, it becomes increasingly difficult to discern the dipole loss peak. Roughly speaking, for CTTd/Eo greater than about three times er, the observed e" is entirely dominated by ct. (Ideally, even when cr dominates the dipolar contribution to e", it should still be possible to observe the dipolar contribution to e however, when o is large, electrode polarization effects tend to dominate the e measurement as well. See Sec. 3.2.1).

Fig. 26. Time-temperature-trans-formation diagram for the system EPON 825/DDS showing times to reach dipole loss peaks at 10 and 10,000 Hz. L-R denotes gelation R-G denotes vitrification. (Reprinted from Ref. 47) with permission of the Society for the Advancement of Material and Process Engineering)...

Figure Bl.25.12. Excitation mechanisms in electron energy loss spectroscopy for a simple adsorbate system Dipole scattering excites only the vibration perpendicular to the surface (v ) in which a dipole moment nonnal to the surface changes the electron wave is reflected by the surface into the specular direction. Impact scattering excites also the bending mode v- in which the atom moves parallel to the surface electrons are scattered over a wide range of angles. The EELS spectra show the higlily intense elastic peak and the relatively weak loss peaks. Off-specular loss peaks are in general one to two orders of magnitude weaker than specular loss peaks.

L of CO was adsorbed at a pressure of 1 x 10 mbar and T= 200 K. At zero energy loss one observes the highly intense elastic peak. The other peaks in the spectrum are loss peaks. At high energy, loss peaks due to dipole scattering are visible. In this case they are caused by CO vibration perpendicular to the surface. The... 

A number of other investigations of the electrical properties of lipid mono- and multilayers were published recently. It is obvious from studies of the conductivity of thin Langmuir films that the electrical properties of metal-organic layer-metal structures can be described by well known concepts from solid state physics, like Schottky injection of electrons from the metal into the lipid film (45, 46, 47). Measurements of dielectric losses in calcium stearate and behenate indicate the presence of movements of dipoles in the organic molecules, and loss peaks connected with the amorphous and crystalline parts of the layers were identified (48). 

Change of g influences insignificantly the resonance line pertinent to the ensemble of rigid dipoles reorienting in a hat-like potential well, since the resonance absorption/loss peaks are usually placed at much higher frequencies than the Debye loss peak. 

The adsorption of iron(O) pentacarbonyl was recently studied on a Si(lll)-(7 x 7) surface. The interest in Fe(CO)5 lies in its availability as a source gas for the chemical vapor deposition of FeSi2, a critical microelectronics material. Even at temperatures as low as lOOK, Fe(CO)5 already underwent dissociative adsorption to yield a linear iron monocarbonyl (FeCO) surface complex. The prominent loss peaks that appeared at 53meV (428 cm ), 81meV (653 cm ), and 255 meV (2056 cm ) were assigned to the Si-COFe, Fe-CO, and C=0 stretch modes, respectively. These peaks were shown to arise only via dipole scattering which, because of the dipole selection rule, indicates that the adsorbed FeCO is oriented vertically with the CO moiety bonded to the Si surface. 

Localised phonon excitations are in principle best studied by neutral-atom scattering, or off specular HREELS, in order to reduce the strong dipole excitation of Fuchs-Kliewer modes. Two off specular HREELS measurements on MgO(lOO) have been reported [25, 68], however there is some disagreement concerning the energy and assignment of the substrate derived loss peaks. Since the microscopic surface modes are expected to be sensitive to the surface structure, it has been suggested [9] that the differences may be associated with differences in surface preparation. 

This state describes (i) dielectric response arising from libration of a permanent dipole p in a hat-like potential well [the relevant librational band is located near the border of the infrared region (at 700 cm-1)] and (ii) the nonresonance relaxation band, whose loss peak is located at microwaves. The lifetime Tor of the LIB state is much less than a picosecond. 

In view of Table II the main difference of the parameters, fitted for HW, from those, fitted for OW, concerns (i) some increase of the libration amplitude / , (ii) decrease of the form factor /, (iii) decrease of the frequency vq (the center frequency of the T-band) and increase of the moment nq, responsible for this band, and (iv) decrease of the intensity factor gj, which strongly influences the THz band. Comparison of curves 3 in Figs. 4h and 5h shows that the partial dielectric loss peak g"max of HW, located at v near 150 cm-1 and stipulated by harmonic longitudinal vibration of HB molecules, substantially exceeds such a peak of OW, since the elastic dipole moment / (D20) 8.8 D exceeds the moment / (H20) 3.5 D. 

The loss and absorption peaks at v 700 cm-1, located near the border of the IR region, arise due to mechanism a—that is, due to reorientation of a rigid (permanent) dipole in the hat well. This mechanism is also responsible for the microwave loss peak located between the frequencies 0.1 and 1cm-1. The complex permittivity s of the corresponding relaxation band is actually governed by Debye theory, which is involved formally in our calculation scheme. 

The dielectric strength. As, which is proportional to the area under the loss peak, is much lower for the secondary processes, relative to the a relaxation analysed in the next section. This is a common pattern foimd in both polymer materials and glass formers. The P secondary process is even more depleted in linear polymers that contain the dipole moment rigidly attached to the m chmn, such as polycarbonate [78-80] and poly(vinyl chloride) (the behaviour of this polymer was revisited in ref [81] where the secondary relaxation motions are considered as precursors of the a-relaxation motions). Polymers with flexible polar side-groups, like poly(n-alkyl methacrylate)s, constitute a special class where the P relaxation is rather intense due to some coupling vnth main chain motions. 

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