Lineshap defects

Figure 3.46. Lineshape defects that arise from inappropriate settings of various shims. These effects have been exaggerated for the purpose of illustration.

Further evidence for the unique nature of the shock-formed point defects is the dispersion in ESR lineshape characteristic of conductivity at temperatures above 30 K. In shock-modified powder the conductivity is constant down to 2 K, indicating that the electrons responsible for the conductivity are not trapped. These observations indicate that shock-modified rutile is in a physical defect state that has not been obtained in more conventional vacuum-reduction defect studies. 

Fig. 2 Mechanically oriented bilayer samples as a membrane model for ssNMR. (a) Illustration of the hydrated lipid bilayers with MAPs embedded, the glass supports, and the insulating wrapping, (b) A real sample consists of 15 stacked glass slides, (c) Schematic solid-state 19F-NMR lineshapes from an oriented CF3-labelled peptide (red), and the corresponding powder lineshape from a non-oriented sample (grey), (d) Illustration of typical orientational defects in real samples - the sources of powder contribution in the spectra...

Fig. 6.20 Ionization width vs electric field for the Na (20,19,0,0) level near its crossing with the (21,17,3,0) level from experiment (data points) and from WKB-quantum defect theory (solid line). The levels are specified as (n./q.ni.M) Because the lineshapes are quite asymmetric (except for very narrow lines), the width in this figure is taken to be the FWHM of the dominant feature corresponding to the (20,19,0,0) level in the photoionization cross section. For the narrowest line, experimental widths are limited by the 0.7 GHz laser linewidth. Error limits are asymmetric because of the peculiar fine shapes and because of uncertainties due to the overlapping m = 1 resonance (from ref. 37).

The spectra reported here and those found for trans-(CH) have comparable linewidths, see Table 2, and lineshapes. It may, therefore, be appropriate to extend the model developed for (CH) to polydiacetylenes. The model of a mobile bond-alternation defect has been used in general terms to explain the earlier observations on polydiacetylenes (, ). A detailed... 

An underlying question in many of the ESR measurements of the lineshape of the N-donor and its temperature dependence is the possible existence of an additional broad line at a similar g-value to the three-line spectrum. Most authors agree that the three lines in the isolated N-donor spectrum should have symmetric lineshapes and, since there is a slight asymmetry to the full spectrum, this leads to the conclusion that there is another relatively broad ESR line shifted slightly from the three-line spectrum. This could be due to a second donor, a conduction electron spin resonance or a structural defect. A weak signal, possibly due to a Si vacancy, is also observed at higher temperatures (T > 50 K) in some samples. 

The considerations above apply to fast dynamic processes in the sense that the amplitude of the modulation of the resonance frequency Amq (induced by modulation of the local field) multiplied with the correlation time Tc is much smaller than unity. This Redfield regime [27] is usually attained in solutions with low viscosity [2], but may also apply to small-amplitude libration in solids [28]. For slower reorientation in solutions with high viscosity or in soft matter above the glass transition temperature (slow tumbling), spectral lineshapes are directly influenced by exchange between different orientations of the molecule (Section 4.1). Relaxation times in solids outside the Redfield regime carmot be predicted from first principles except for a few crystalline systems with very simple structure and few defects [29]. In such systems, qualitative or semi-quantitative analysis of relaxation data can still provide some information on dynamics. 

Due to the nature of the electron wave fimctions involved in this case, p (p) is not periodic, but its shape carries information on the electronic structure of the defect. This can be measured by 2D-ACAR and lineshape spectroscopies. Another important quantity to consider in this case is the total annihilation rate A ... 

The trapping of positrons by lattice defects is clearly established in Tl2Ba2CaCu20g by the occurrence of multi components in the lifetime spectra (Sundar et al. 1990a, Jean et al. 1989a) and in the lineshape (Pujari et al. 1989), and by deviations from the lifetime calculated for the defect-free structure (Barbiellini et al. 1994). 

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