Spectral sensitization, effect

As in chemical sensitization, spectral sensitization is usually done after precipitation but before coating, and usually is achieved by adsorbing certain organic dyes to the silver haUde surfaces (47,48,212—229). Once the dye molecule is adsorbed to the crystal surface, the effects of electromagnetic radiation absorbed by the dye can be transferred to the crystal. As a result of this transfer, mobile electrons are produced in the conduction band of the silver haUde grain. Once in the conduction band, the electrons are available to initiate latent-image formation. 

Improvement of spectral sensitization can be accompHshed by dye combinations. The effect has been found to often be greater than the predicted additive sensitivity increase. This phenomenon is called supetsensitization (94), which is appHed most effectively to polymethine aggregates (96). 

The detection of spectral sensitizing action often depends on amplification methods such as photographic or electrophotographic development or, alternatively, on chemical or biochemical detection of reaction products. Separation of the photosensitization reaction from the detection step or the chemical reaction allows selection of the most effective spectral sensitizers. Prime considerations for spectral sensitizing dyes include the range of wavelengths needed for sensitization and the absolute efficiency of the spectrally sensitized process. Because both sensitization wavelength and efficiency are important, optimum sensitizers vary considerably in their stmctures and properties. 

Fig. 6. Effect of added supersensitizers on a J-aggregated spectral sensitizing dye, l,l -dieth5i-2,2 -quinocyariine chloride [2402-42-8] (1), for which...

Photopolymerization reactions are widely used for printing and photoresist appHcations (55). Spectral sensitization of cationic polymerization has utilized electron transfer from heteroaromatics, ketones, or dyes to initiators like iodonium or sulfonium salts (60). However, sensitized free-radical polymerization has been the main technology of choice (55). Spectral sensitizers over the wavelength region 300—700 nm are effective. AcryUc monomer polymerization, for example, is sensitized by xanthene, thiazine, acridine, cyanine, and merocyanine dyes. The required free-radical formation via these dyes may be achieved by hydrogen atom-transfer, electron-transfer, or exciplex formation with other initiator components of the photopolymer system. 

The absolute and spectral sensitivities can often vary by up to 100% within a few millimeters on the surface of the photocathode [49]. Figure 19 illustrates this effect for a sideways and vertical adjustment of a photomultiplier, in addition slight maladjustment of the light entrance can lead to zero hne runaway as a result of thermal effects. 

Phosphorus-containing pesticides la 254 Phosphorus insecticides lb 83 Phosphorus pesticides lb 32 Photochemical activation lb 13 Photochemical reactions lb 15,17 Photodiodes la 24,29 Photo effect, external la 24 -, internal la 24, 29 Photo element la 24,29 Photography, exposure times la 137 -, instmmentation la 137 Photomultiplier la 25ff -, disadvantages la 27 -, energy distribution la 26 -, head on la 27 -, maximum sensitivity la 28 -, side on la 27 -, spectral sensitivity la 28 -, window material la 28 Photocells la 25 Phloxime lb 116... 

Self-assembly of functionalized carboxylate-core dendrons around Er +, Tb +, or Eu + ions leads to the formation of dendrimers [19]. Experiments carried out in toluene solution showed that UV excitation of the chromophoric groups contained in the branches caused the sensitized emission of the lanthanide ion, presumably by an energy transfer Forster mechanism. The much lower sensitization effect found for Eu + compared with Tb + was ascribed to a weaker spectral overlap, but it could be related to the fact that Eu + can quench the donor excited state by electron transfer [20]. 

Matrix effects are typically divided into spectral (isobaric) and non-spectral types. The spectral or isobaric effects include 1) elemental isobaric interferences such as Cr at " Fe, 2) molecular interferences such as Ca O at Fe and Ar N at Fe, 3) double charge interferences such as Ca at Mg. Non-spectral matrix effects are largely associated with changes in the sensitivity of an analyte due to the presence of other elements (Olivares and Houk 1986). Changes in sensitivity correspond to a change in instrumental mass bias, and therefore non-spectral matrix effects can have a significant impact on the accuracy of isotope measurements. 

The current trend in photoreceptor design is to use multilayer structures of various Se-based compositions, hi particular, certain additives, e.g.. As, Te, and Bi, have been shown to extend the spectral sensitivity of selenium to longer wavelengths [17, 20, 21]. Arsenic has been particularly effective in providing increased thermal stability and resistance to crystalhzation. 

There seems no doubt that organic photoconductor systems will permit the development of new reproduction processes based on the electrophotographic principle 10>. Dyes play an important role in this development on the one hand they act as spectral sensitizers of the organic photoconductors (e.g. poly-N-vinyl-carbazole 161> and on the other hand, the photoelectric effects of dyes are important for... 

RESIST STRUCTURE OF REPEAT UNIT EFFECTIVE SPECTRAL SENSITIVITY RANGE (nm) RELATIVE SENSITIVITY REF. ... 

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