Absorption center

A typical absorption curve for vitreous siUca containing metallic impurities after x-ray irradiation is shown in Eigure 12. As shown, the primary absorption centers are at 550, 300, and between 220 and 215 nm. The 550-nm band results from a center consisting of an interstitial alkah cation associated with a network substituent of lower valency than siUcon, eg, aluminum (205). Only alkaUes contribute to the coloration at 550 nm. Lithium is more effective than sodium, and sodium more effective than potassium. Pure siUca doped with aluminum alone shows virtually no coloration after irradiation. The intensity of the band is deterrnined by the component that is present in lower concentration. The presence of hydrogen does not appear to contribute to the 550-nm color-center production (209). 

Fig. 1. (a) Phase matched second harmonic generation (2cJ = 0.49 fiTo) at cj = 0.98 where = refractive index by ordinary rays and = by extraordinary rays, (b) Hypothetical anomalous dispersion phase matching at 850 nm in similar a crystal having a Lorent2ian absorption centered at 650... 

Using the same reaction, Cole and Wan later photogenerated mcta-quinone methides 110-112 from the appropriate m-hydroxy-a-phenylstyrenes.78 The three were characterized by LFP, with 110 and 111 showing strong and sharp absorptions at 430 and 450 nm, respectively, similar to those observed for 104 and 105. The dimethoxy-susbstituted 112 showed a much broader absorption centered at 420 nm. Addition of the electron-donating methoxy groups stabilized the quinone methides the lifetimes of 110-112 in 1 1 aqueous acetonitrile were 5-200 times longer than that of 104. 

Neutralization with KOH in aqueous THF gave the desired poly(styrene-b-potassium methacrylate) (S-b-MA.K). The carbonyl band in the IR spectrum is replaced with a strong, broad carboxylate absorption centered near 1566 cm 1 (Figure 2c). The carboxylate and potassium contents were assayed by non-aqueous titration and ICP, respectively. The resulting values of 0.91 meq COj /g and 0.98 meq K/g indicate essentially quantitative conversion to the potassium methacrylate. S-b-MA.K obtained in this manner is easily dissolved in solvents such as THF and dichloromethane, in contrast to the... 

The dynamics of photoinduced charge separation, kcs, and charge recombination, kcr (Fig. 2a), have been studied in several families of hairpins containing an Sa linker and a single G C base pair by means of femtosecond time-resolved transient absorption spectroscopy [27, 28]. Both the singlet state and anion radical of Sa have strong transient absorption centered at 575 nm. The difference in the independently determined band shapes for Sa ... 

C results in the appearance of an absorption centered at 497 nm due to an intermediate called lumirhodopsin. Above — 40°C this changes to metarhodopsin I (Amax = 487 nm) and above — 15°C metarhodopsin I establishes an equilibrium with another form, absorbing at 380 nm, called metarhodopsin II. Finally, above 0°C the absorbance due to all-/ra/w-retinal appears (Ama I = 387 nm). These results are summarized in Figure 12.4 and Table 12.3. 

US patent 6,756,381, Compositions and formulations of 9-nitrocamptothecin polymorphs and methods of use thereof [112]. This invention discloses a polymorphic form of 9-nitrocamptothecin, the polymorph being characterized as having an infrared spectrum with an absorption centered between 3625 and 3675 cm-1 and containing more than a trace of water. 

Figure 9.12 Interaction of light with a solid. Some absorption centers can re-emit light as fluorescence or luminescence. Scattering centers can reflect or diffract light depending upon size.

The interaction of light with an absorption center involves a change in energy. The absorption of energy by an absorption center will cause it to move from a... 

When energy is lost from an absorption center, it moves from the excited state back to the ground state. The energy can be released directly by passing from Ex to Eq (Fig. 9.14a), with an energy output given by... 

Absorption bleaching, 10 808-809 Absorption centers, in vitreous silica, 22 435-436... 

The spectrum of any octahedral c/6 complex can be assigned with the help of Fig. 11.42. For high spin species such as [CoF6]3, the only spin allowed transition is - SEI1 and only one absorption should be observed. Indeed Ihe blue color of this complex results from an absorption centered at 13,100 cm-1.44 For low spin Co3+ complexes there are two spin allowed transitions at relatively low energies A g — r, and T, There are additional spin allowed transitions at higher energies. 

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