Extinction effects

Many polymer films, eg, polyethylene and polyacrylonitrile, are permeable to carbon tetrachloride vapor (1). Carbon tetrachloride vapor affects the explosion limits of several gaseous mixtures, eg, air-hydrogen and air-methane. The extinctive effect that carbon tetrachloride has on a flame, mainly because of its cooling action, is derived from its high thermal capacity (2). 

The other possible sources of error are primary and secondary extinction effects, inadequate sample thickness, and finally, instrument-related errors. These issues are addressed in the literature [1]. 

Weiner, 1., Tarrasch, R., Hasson, O., et al. (1994) The effects of chronic administration of ceronapiil on the partial reinforcement extinction effect and latent inhibition in rats. Behav. Pharmacol., 5, 306-314. 

In inorganic and organometallic solids, the average electron concentration tends to be high. This means that absorption and extinction effects can be severe, and that the use of hard radiation and very small crystals is frequently essential. Needless to say that the advent of synchrotron radiation has been most helpful in this respect. The weaker contribution of valence electrons compared with the scattering of first-row-atom-only solids implies that great care must be taken during data collection in order to obtain reliable information on the valence electron distribution. 

C urves of extinction as a function of size parameter show a wealth of features, even when calculated with uninteresting constant optical constants, as has often been done. When realistic optical constants are used in calculations the different types of extinction effects become even more numerous. In this section we incorporate optical constants of the three illustrative materials of... 

Chapter 10 with Mie theory in a survey of extinction effects which we shall discuss in more detail in subsequent sections. 

Both primary and secondary extinction effects may usually be avoided by powrdering a crystal. For this and other reasons the intensities of the arcs on powder photographs are likely to be more reliable than those of other types of photograph but in practice, in structure determination it is only possible to use powder intensities alone for very simple structures for complex crystals reflections from different planes overlap seriously. 

In setting out to discover the relative positions of the atoms in a crystal, it is best, when the unit cell dimensions have been determined and the intensities of the reflections measured, to calculate F for each reflection. (See Chapter VII.) Absolute values of F, derived from intensities in relation to that of the primary beam, form the ideal experimental materisi, though very many structures have been determined from a set of relative F s. The reliability of the set-of figures depends on the success with which the corrections for thermal vibrations, absorption, and extinction effects have been estimated. 

The slight increase m oxygen noticeable on ascending the methane series of hydrocarbons is probably due to the highly extinctive effect of the increasing proportion of carbon dioxide, to which reference has already been made. The low oxygen content in the case of hydrogen is noteworthy. 

Ettenberg A, Camp CH (1986a) Haloperidol induces a partial reinforcement extinction effect in rats implications for a dopamine involvement in food reward. Pharmacol Biochem Behav 23 813-821. 

Feldon J, Weiner I (1991) Effects of haloperidol on the multitrial partial reinforcement extinction effect (PREE) evidence for neuroleptic drug action on nonreinforcement but not on reinforcement. Psychopharmacology (Berl) 705 407 114. 

Equation (3.19) is of course only correct in the small crystal limit where extinction effects and other beam attenuation processes are small. 

Zacharicisen, W. H. The secondary extinction effect. Acta Cryst. 16, 1139-1144... 

Extinction effects, which are dynamical in nature, may be noticeable in diffraction from nearly perfect and/or large mosaic crystals. Two types of extinction are generally recognized primary, which occurs within the same crystallite, and secondary, which originates from multiple crystallites. Primary extinction is caused by back-reflection of the scattered wave into the crystal and it decreases the measured scattered intensity Figure 2.51, left). Furthermore, the re-reflected wave is usually out of phase with the incident wave and thus, the intensity of the latter is lowered due to destructive interference. Therefore, primary extinction lowers the observed intensity of very strong reflections from perfect crystals. Especially in powder diffraction, primary extinction effects are often smaller than experimental errors however, when necessary they may be included in Eq. 2.65 as ... 

Figure 2.51. The illustration of primary (left) and secondary (right) extinction effects, which reduce intensity of strong reflections from perfect crystals and ideally mosaic crystals, respectively. The solid lines indicate actual reflections paths. The dashed lines indicate the expected paths, which are partially suppressed by dynamical effects. The shaded rectangles on the right indicate two different blocks of mosaic with identical orientations.

The extinction effect can operate, not only in single-crystal specimens, but also in the individual grains of polycrystalline specimens. Extinction may be assumed to be absent in ground or filed powders and is usually negligible in finegrained polycrystalline specimens. If its presence is suspected in the latter, the specimen can always be reduced to powder by grinding or filing. 

The effective-thickness factor Z(6) in Equation (15) accounts for the dependence of the penetration depth of the primary X-ray field (extinction effect) in conjunction with the escape depth, A, of the out-going secondary fluorescence X-rays. For atoms at the crystal surface (e g., adsorbates) or at a depth much smaller than the extinction depth, the effective-thickness factor is constant at Z(6) = 1. For atoms evenly distributed throughout the semi-infinite crystal... 

The structure factors of InP were determined using samples compacted by pressures of 2500 kfg/cm but the intensities of the 220 and 440 reflections were corrected for the preferential orientation. These corrections were found by comparing the relative intensities obtained for samples which were not compressed with the intensities found for the compacted samples. Corrections were also made for the thermal diffuse scattering [4]. The extinction effects were slight because we used very fine powders. The linear absorption coefficient jtt was assumed to be 993 cm [5]. The polarization factor was foimd from the expression [6]... 

There are three experimental methods (Figure 8). The back-reflection Berg-Barrett camera is useful in examining imperfections near the crystal surface at depths of 10-100 um. A well-collimated monochromatic X-ray beam incident on the crystal is diffracted by planes [hkl) and recorded on film. Because of extinction effects, the diffracted intensity from imperfect regions exceeds that of the perfect regions. Dislocations appear as bands of contrasting darkness, 5-50 pro. wide. Film and crystal are moved in synchronization to map out... 

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