Extinction, interpretation

Absorption Spectra, of Aqueous Ions. The absorption spectra of Pu(III) [22541-70 ] Pu(IV) [22541 4-2] Pu(V) [22541-69-1] and Pu(VI) [22541-41-9] in mineral acids, ie, HCIO and HNO, have been measured (78—81). The Pu(VII) [39611-88-61] spectmm, which can be measured only in strong alkaU hydroxide solution, also has been reported (82). As for rare-earth ion spectra, the spectra of plutonium ions exhibit sharp lines, but have larger extinction coefficients than those of most lanthanide ions (see Lanthanides). The visible spectra in dilute acid solution are shown in Figure 4 and the spectmm of Pu(VII) in base is shown in Figure 5. The spectra of ions of plutonium have been interpreted in relation to all of the ions of the bf elements (83). 

Helf White (Ref 2) interpret the above behavior of the nitrocompds in inhibiting the scintillation process as one of simple light absorption rather than as a true chemical quenching (ae-excitation process). To substantiate this, the UV and near-visible spectrum of each of the light compds in toluene—PPO soln was measured using the 50% extinction concn for each nitrocompd (as determined from Fig 1). 

The various conflicting interpretations are attributed in part to the non-specific nature of the spectrum of lignin and more so to the fact that the. molecular weight of lignin is, as yet, unknown. Recently, Aulin-Erdtman (4) has successfully used the methoxyl content of various lignins instead of molecular weights in the determination of extinction coefficients. 

Therefore, Cext is a well-defined observable quantity we measure U with and without the particle interposed between source and detector. Because Cext is inherently positive, the effect of the particle is to reduce the detector area by Cext this, then, is the interpretation of Cext as an area. In the language of geometrical optics we would say that the particle casts a shadow of area Cext However, as stated previously, this shadow can be considerably greater—or much less—than the particle s geometrical shadow. We note from (3.33) that Cext is the maximum observable extinction. The scattering term fi(D) X =0/k2 cannot be greater than Csca and is positive therefore, the observed extinction Cext lies within the limits... 

In previous chapters we have always taken particles to be in a nonabsorbing medium. We now briefly remove this restriction. The notion of extinction by particles in an absorbing medium is not devoid of controversy more than one interpretation is possible. But Bohren and Gilra (1979) showed that if the extinction cross section is interpreted as the reduction in area of a detector because of the presence of a particle [see Section 3.4, particularly the development leading up to (3.34)], then the optical theorem for a spherical particle in an absorbing medium is formally similar to that for a nonabsorbing medium ... 

Measurements of extinction by small particles are easier to interpret and to compare with theory if the particles are segregated somehow into a population with sufficiently small sizes. The reason for this will become clear, we hope, from inspection of Fig. 12.12, where normalized cross sections using Mie theory and bulk optical constants of MgO, Si02, and SiC are shown as functions of radius the normahzation factor is the cross section in the Rayleigh limit. It is the maximum infrared cross section, the position of which can shift appreciably with radius, that is shown. The most important conclusion to be drawn from these curves is that the mass attenuation coefficient (cross section per unit particle mass) is independent of size below a radius that depends on the material (between about 0.5 and 1.0 fim for the materials considered here). This provides a strong incentive for deahng only with small particles provided that the total particle mass is accurately measured, comparison between theory and experiment can be made without worrying about size distributions or arbitrary normalization. 

Low pressure burning behavior gives information concerning the detailed structure of the flame zone. It is known that the fuel-oxidant reaction zone becomes very weak at very low pressures. Thus, the nature of any remaining exothermic reactions occurring at or near the propellant surface is more obvious in the over-all propellant burning behavior. Burning rates and extinction behavior have been measured for a number of propellant systems and are reported below. These results are then interpreted in terms of the theoretical predictions made previously. 

Pearson (66) found that hot solid surfaces drastically accelerate the ignition of these vapors. In line with our identification of the A/PA reaction zone as the major heat source, it is expected that both burning rate and extinction pressure depend on the total surface area of AP particles exposed to the A/PA reaction occurring in the pores of the ash. Thus, as observed experimentally, both burning rate and extinction pressure depend upon oxidizer particle size. However, this interpretation is obscured by the fact that combustion inefficiency, an important parameter, is also expected to be particle size dependent. [Total AP surface area exposed to A/PA reaction zone = A = na, where n = (number of AP particles exposed), (volume of each AP particle)"1 d 3, a = (exposed surface area of each AP particle) — dr. Therefore, A — d1.]... 

When the residence time is varied so that we approach an ignition or extinction point in the stationary-state locus, then the flow and reaction curves L and R become tangential. The condition for tangency is R = L and 8R/da = SL/da. Thus the difference between the slopes of R and L decreases to zero. From eqn (8.17) we see that the tangency condition also causes the value of the eigenvalue A to tend to zero. An alternative interpretation, in... 

In methanol, the extinction of the absorption at 241 nm is less than 40 % of the value in isopentane with little change on lowering the temperature. In isopentane, the extinction drops significantly to the value found in methanol when the temperature is lowered. The data on the temperature-dependent CD in methanol can be interpreted on the basis of a temperature-dependent equilibrium between two chiral species. The change of the rotatory strength appears to be AG° = —2.0 KJ mol-1. This phenomenon is interpreted by the assumption that the cis-conformer about the C(6)-C(7) bond is favored in methanol and, at lower temperatures, in isopentane. 

Circular dichroism arises from the same optically active transitions responsible for the Cotton effects observed in ORD curves, but unlike ORD it is an absorption, not a dispersion, phenomenon. Hence, the CD effect is restricted to the region of the transition and can be interpreted more straightforwardly. Both ORD and CD can best be understood if one imagines the incident plane-polarized beam resolved into two in-phase circularly polarized beams whose vectors rotate in opposite directions. A difference in index of refraction between the left and right circularly polarized beams results in rotation of the transmitted plane polarized beam while differential absorption of the two circularly polarized beams results in depolarization of the transmitted beam, so that an incident plane-polarized beam whose frequency is within that of an optically active absorption band becomes both rotated and elliptically polarized upon passage through the sample. This depolarization effect is CD, and the measured parameter is (et — er), the difference in extinction coefficient between the left and right circularly polarized beams. The data is usually recorded as the specific ellipticity, defined as ... 

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