Symmetrical extinction

Parallel or symmetrical extinction. Can be isotropic if viewed down the c axis, but then a six-sided silhouette should be observed. 

All crystals will show parallel or symmetrical extinction. 

The solvent used was 5 %v/v ethyl acetate in n-hexane at a flow rate of 0.5 ml/min. Each solute was dissolved in the mobile phase at a concentration appropriate to its extinction coefficient. Each determination was carried out in triplicate and, if any individual measurement differed by more than 3% from either or both replicates, then further replicate samples were injected. All peaks were symmetrical (i.e., the asymmetry ratio was less than 1.1). The efficiency of each solute peak was taken as four times the square of the ratio of the retention time in seconds to the peak width in seconds measured at 0.6065 of the peak height. The diffusivities obtained for 69 different solutes are included with other physical and chromatographic properties in table 1. The diffusivity values are included here as they can be useful in many theoretical studies and there is a dearth of such data available in the literature (particularly for the type of solutes and solvents commonly used in LC separations). 

Fig. 3 Schematic representations of extinction character (a) isotropic, (b) parallel, (c) symmetrical, and (d) oblique extinction.

In reflection, the intensity of the X-ray wavefield inside the crystal falls off very rapidly away from the surface, due to transfer of energy to the diffracted beam. Absorption also becomes important at low incident angles to the surface. By choosing the radiation and the reflection (inclnding its symmetry), the penetration may be varied between about 0.05 and 10 micrometres. This is ideally matched to device stmctures. This is quantified by the extinction distance g, defined as the depth at which the incident intensity has decreased to 1/e of its value at the surface. This may be calculated from diffraction theory, and some examples, for GaAs with CuK radiation, are shown in Table 3.2. It is assumed that the wafer surface is (001), hence the 004 reflection is symmetric and the others asymmetric. 

Card, J. M., and F. A. Williams. 1992. Asymptotic analysis of the structure and extinction of spherically symmetrical n-heptane diffusion flames. Combustion Science Technology 84 91-119. 

In trisannelated benzenes, where the symmetry is restored, there is little effect of ring strain on the extinction coefficient values for e are similar to those of hexamethylbenzene where a drop in e to ca. 300 is seen (Table 11). The symmetric... 

Gaussian curves (normal distribution functions) can sometimes be used to describe the shape of the overall envelope of the many vibrationally induced subbands that make up one electronic absorption band, e.g., for the absorption spectrum of the copper-containing blue protein of Pseudomonas (Fig. 23-8) Gaussian bands are appropriate. They permit resolution of the spectrum into components representing individual electronic transitions. Each transition is described by a peak position, height (molar extinction coefficient), and width (as measured at the halfheight, in cm-1). However, most absorption bands of organic compounds are not symmetric but are skewed... 

Experimentally, S02 shows a region of absorption from ca. 2400 to 1800 A ca. 2000 A). Its maximum molecule extinction coefficient is several times that of the 3400— 2600-A system. Duchesne and Rosen (J. Chem. Phys., 1947, 15, 631) have shown, by vibrational analysis, that at least two and probably three electronic transitions arc involved in the region. The first of these has its origin at 42,170 cm.-1 (2371 A), the vibrational frequencies appearing in the upper state being 963 (symmetrical stretching)... 

Squarylium dyes such as (83) [75] have probably received less attention than cyanine dyes due to the fact that the majority of syntheses furnish symmetrical species which are difficult to monofunctionalize in reactions such as the formation of peptide conjugates. The unsymmetrical types have been reported but seem to suffer from about 50% decrease in extinction coefficient. Squaiyliums are also more difficult to handle due to their low solubility. Very few water-soluble systems have been reported. These compounds are also used exclusively as fluorophores, but quantum yields are highly dependent on substituents and environment. 

Fig. 5.4a,b. Susceptibility artifacts due to the vicinity of air in the sphenoid sinus the flow signal of ICA is lowered symmetrically on the TOF images (a), when a fat-signal suppressing water excitation pulse sequence was used. The conventional TOF/ TONE sequence shows no false vessel extinction, but a higher background signal from fat tissue (b)... 

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