Absorption tail

Because it is the UV-B radiation (280-320 nm) that causes the degradation, the absorption spectra of the UV-absorber must coincide with these wavelengths. UV-A (320-400 nm) does not cause damage (it is not energetic enough) and UV-C (wavelength less than 280 nm) does not reach the troposphere (it is filtered out by ozone in the stratosphere). The problem is to find an additive that absorbs UV-B but does not have an absorption "tail" in the UV-A and visible wavelengths, and therefore would have a yellow appearance. 

Quantitative data on local structure can be obtained via an analysis of the decaying slope next to the absorption edge. The absorption of an X-ray photon boosts a core electron up into an unoccupied band of the material which, in a metal, is the conduction band above the Fermi level. Electrons in such a band behave as if nearly free and no fine structure would be expected on the absorption tail . However, fine structure is observed up to 500 to 1000eV above the edge (see Figure 2.73(b)). The ripples are known as the Kronig fine structure or extended X-ray absorption fine structure (EX AFS). 

Often the UV absorption tails into the visible due to light scattering by the microparticles, although the Amax is usually very close to that in solution. The aggregate CD spectrum, however, is markedly different from the solution-state spectrum, showing an intense bisigned Cotton effect coincident with the UV due the Davidov coupling. 

The experimental observations of red shifts of the UV absorptions tails with increase in silicon dimensionality were corroborated by ZINDO-calculated spectra comparing linear polysilane, network polysilyne, crystalline cluster, and amorphous cluster structures, which showed respectively lowest absorption transition energies of 5.38 eV (230.4 nm), 4.60eV (269.5nm), 4.57eV (271.2nm), and 2.46eV (503.9nm), as shown in Figure 57.362... 

Fig. 10.4 Absorption spectra of dissolved C60. The main absorbance peaks are clearly in the blue although the long absorption tail allows longer wavelength light to be used to stimulate C60 (Levi et al., 2006)...

The initial solution was deep blue and had to be diluted by a factor of ten before the spectrum was recorded, curve (a). It shows a peak in the spectrum at Xmax = 597 nm. After the passage of 564 C, the color changed to deep brown and it was diluted by a factor of ten before the spectrum was recorded, curve (b). The spectrum recorded after 1224 C was yellowish brown and the solution was diluted by a factor of ten, as shown in curve (c). On continuing the electrolysis by the passage of 2200 C, the solution has become pale yellow. After the passage of 5856 C the solution is effectively colorless and the spectrum (see curve d) shows only an absorption tail into the UV. 

Figure 3.29, for example, shows measurements of the photolysis rate of 03, J(03), made at the Mauna Loa Observatory on two different days, compared to model calculations of the photolysis rate constant (Shetter et al., 1996). The two model calculations use different assumptions regarding the quantum yield for 03 photolysis in the absorption tail beyond 310 nm (see Chapter 4.B). The measurements are in excellent agreement for the second day but somewhat smaller than the model calculations on the first. 

Fig. 14. Vibrational analysis of the lAlg - iAlg absorption band of the more intense component of the trans-[Co(NH3)4(CN)2]Cl polarized spectrum. Dotted line experiment, solid line superposition of propagated fundamental spectral pattern with (upper part) and without (lower part) considering the absorption tail from the higher transition into lEg...

Dominated by Iron Absorption. Discernable increases in absorbance (reflectance decreases) are produced by increasing iron content in two wavelength regions - the absorption tail diminishing throughout the uv-visible portion of the of the spectrum (sampled at the short wavelength limit of the spectra) and the broad peak at 970nm. These features are attributable to a series of d-d transitions of ferric and ferrous iron Q4.). ... 

Kaino and co-workers [255] have investigated the effect of polymer polydis-persity on electro-optic materials properties. No dependence on polydispersity was observed for guest host materials but for Disperse Red chromophores covalently attached to monodisperse polystyrene weaker absorption tails were observed. This result suggests that chromophore-chromophore interactions are modified by the polymer host. 

Further complication in semiconductor band shape analysis concerns the spectral region near the fundemental absorption onset. Ideal semiconductor crystal at 0 K should not absorb any photons with energies lower than Eg. Real systems, however, show pronounced absorption tails at energies lower than the bandgap energy (Figure 7.7). The absorption profile within the tail region can be very well approximated by the empirical Urbach s rule [23-26] ... 

Most distibines appear yellow, while dibismuthines are red, either in their liquid phases or in solution in organic solvents. A series of tetravinyldisti-bines (7) and dibismuthines (24) show absorption maxima in the range of 200-330 nm. 2,2, 5,5 -Tetramethylbistibole (30) shows a maximum at 346 nm (22), while the corresponding bibismole (31) absorbs at 320 nm (34). In all cases, the distibines show a low intensity, featureless absorption tail extending out to about 400 nm, while the tail extends to approximately 800 nm for the disbismuthines. Presumably, this feature corresponds to the observed yellow and red colors. No spectral assignments have been made. 

In PDA crystals, PDS shows that an absorption tail extends on the low-energy side of the intense 2-eV absorption [133]. In poly-pTS, for instance,... 

Because of their special polymerization process [136], PDA crystals should not contain any dopant or associated residual polaron or bipolaron concentration. These negative results give some constraints on other low-lying states, especially exciton states. Triplet-state absorption from the ground state would probably be too strongly spin-forbidden to have absorption coefficients as high as 1 cm-1 [129]. As for g states, the absorption coefficient depends sensitively on how far from the intense absorption they are (for the polyene case, see Ref. 127). That they are not found in these experiments means that they are either above the main transition at 2 eV or not far below it and buried in its tail. Indeed, evidence of a weak absorption = 0.1 eV below the main transition has recently been found at low temperature [118] it would be buried in the absorption tail at higher temperatures. 

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